I.
INTRODUCTION
A.
Taxonomic Considerations
Dogs are mammals in the order Carnivora, suborder Cani-formia (or superfamily Arctoidea),
and family Canidae. The domesticated dog has been designated as Canis familiaris.
Other members of the genus Canis include four species of jackals, the coyote (C. latrans),
the red wolf (C. rufus), and the gray wolf (C. lupus). Canis familiaris is subdivided
into approximately 400 breeds, ranging in size and shape from the teacup chihuahua
to the large Irish wolfhound. The domesticated dog is thought to have descended from
a Eurasian subspecies of the gray wolf, possibly either the Indian wolf (C. lupus
pallipes) or the Chinese wolf (C. lupus chanco). Wolf–dog hybrids have been produced
when the species are brought together in captivity (Nowak, 1999).
B.
Use in Research
1.
Historical Use of Dogs in Research
The dog played an important role as a laboratory animal in the early history of biomedical
research, primarily because of its
Status as a cooperative companion animal of reasonable size. Dogs were used in the
mid-1600s by William Harvey to study cardiac movement, by Marcello Malpighi to understand
basic lung anatomy and function, and by Sir Christopher Wren to demonstrate the feasibility
of intravenous delivery of medications (Gay, 1984). The use of dogs continued as biomedical
research advanced, and they were featured in many noteworthy studies, including those
by Pavlov to observe and document the conditioned reflex response and by Banting and
Best to identify the role of insulin in diabetes mellitus. For a comprehensive but
concise review of the use of the dog as a research subject, the readers are directed
to the manuscript by Gay (1984).
2.
Current Use of Dogs in Research
The breed of dog most commonly bred for use in biomedical research is the beagle.
Some commercial facilities also breed foxhounds or other larger-breed dogs for use
in surgical research studies. Some specific breeds with congenital or spontaneous
disorders are also maintained by research institutions (see specific examples below).
Random-source dogs used in research are most frequently mongrels or larger-breed dogs
(e.g., German shepherd, Doberman pinscher, Labrador and golden retrievers) that are
used for surgical research and/or training.
According to a computerized literature search for beagle for the years 1998–1999,
approximately 40% of the biomedical scientific publications identified were in the
fields of pharmacology or toxicology. Especially common were studies focusing on pharmacokinetics,
alternative drug delivery systems, and cardiovascular pharmacology. The next most
common areas of research using beagles were dental and periodontal disease and surgery
(12% of publications), orthopedic surgery and skeletal physiology (7%), and radiation
oncology (4%). Other research areas that utilized beagles included canine infectious
disease, surgery, imaging, prostatic urology, and ophthalmology.
Most large-sized dogs (either purpose-bred or random-source) are used in biomedical
research because of their suitability for surgical procedures. Anesthetic protocols
and systems for dogs are well established, and the organs of larger-breed dogs are
often an appropriate size for trials of potential pediatric surgical procedures. Surgical
canine models have been used extensively in cardiovascular, orthopedic, and transplantation
research.
There are also some unique spontaneous conditions for which dogs have proven to be
valuable animal models. A colony of gray collies is maintained at the University of
Washington (Seattle) for the study of cyclic hematopoiesis. This condition is manifested
by periodic fluctuations of the cellular components of blood, most notably the neutrophil
population. These dogs are used to study the basic regulatory mechanisms involved
with hematopoiesis, as well as possible treatments for both the human and the canine
conditions (Brabb et al., 1995). Golden retrievers affected with muscular dystrophy
have been used as models of Duchenne muscular dystrophy in human children. Duchenne
muscular dystrophy is caused by an absence of the muscle protein dystrophin, inherited
in an X-linked recessive manner. The dystrophy in golden retrievers is caused by absence
of the same protein and is inherited in the same way. The clinical signs (such as
debilitating limb contracture) are also similar between the canine and human conditions
(Kornegay et al., 1994). Bedlington terriers have been used to study copper storage
diseases (such as Wilson's disease), and the development of spontaneous diabetes mellitus
and hypothyroidism in a variety of dogs has also been studied for comparisons with
the human conditions.
3.
Decline in Numbers Used
Although historically the dog has been a common laboratory animal, the use of dogs
in research has been waning over the past few years. According to the U.S. Department
of Agriculture (1998), the number of dogs used in research has declined from a high
use of 211,104 in 1979 to only 75,429 in 1997 (Fig. 1
). This decrease was caused by a variety of factors, including (but not limited to)
increased cost, decreased availability, local restrictive regulations, conversion
to other animal models (such as livestock or rodents), and shift in scientific interest
from pathophysiology to molecular biology and genetics.
Fig. 1
Number of dogs used by research, 1973–1997, according to the u.s. Department of Agriculture.
(Data from U.S. Department of Agriculture, 1998).
© 2002
2002
Since January 2020 Elsevier has created a COVID-19 resource centre with free information
in English and Mandarin on the novel coronavirus COVID-19. The COVID-19 resource centre
is hosted on Elsevier Connect, the company's public news and information website.
Elsevier hereby grants permission to make all its COVID-19-related research that is
available on the COVID-19 resource centre - including this research content - immediately
available in PubMed Central and other publicly funded repositories, such as the WHO
COVID database with rights for unrestricted research re-use and analyses in any form
or by any means with acknowledgement of the original source. These permissions are
granted for free by Elsevier for as long as the COVID-19 resource centre remains active.
C.
Availability and Sources
Dogs used for research are generally segregated into two classes: purpose-bred and
random-source. Purpose-bred dogs are those produced specifically for use in biomedical
research; they are intended for use in long-term research projects and/or pharmacologic
studies in which illness or medication would require removal from the study. Usually
these dogs are either beagles or mongrel foxhounds, although other breeds may be available.
Purpose-bred dogs typically receive veterinary care throughout their stay at the breeding
facility. They are usually vaccinated against canine distemper virus, parvovirus,
adenovirus type 2, parainfluenza virus, Leptospira serovars canicola and icterohemorrhagiae,
and Bordetella bronchiseptica. Rabies virus vaccination may also be included. Purpose-bred
dogs are also usually treated prophylactically for helminths and ectoparasites, intestinal
coccidia, and bacterial ear infections (R. Scipioni and J. Ball, personal communication,
1999).
Random-source dogs are not bred specifically for use in research. They may be dogs
bred for another purpose (e.g., hunting), retired racing dogs, or stray dogs collected
at pounds or shelters. The health status of these dogs can be the same quality as
purpose-bred dogs, or it can be an unknown entity. Random-source dogs that have been
treated and vaccinated in preparation for use in research are termed conditioned dogs.
These dogs are then suitable for long-term studies or terminal preparations that require
unperturbed physiologic parameters. Conditioned dogs are often tested for heartworm
antigen because of the implications that infestations can have on cardiovascular status
and surgical risk. Nonconditioned random-source dogs are useful only in a limited
number of research studies, such as nonsurvival surgical training preparations.
Options for procurement of dogs for biomedical research typically include purchase
from a U.S. Department of Agriculture–designated Class A or Class B licensed dealer
or directly from a municipal pound. The requirements for USDA licensure are detailed
in Code of Federal Regulations (CFR), Title 9, Chapter 1 (1-1-92 edition), Subchapter
A, Animal Welfare, 1.1, Definitions, and 2.1, Requirements and Application. Briefly,
Class A licensees are breeders who raise all animals on their premises from a closed
colony (suppliers of purpose-bred dogs are typically Class A dealers). Class B licensees
purchase the dogs from other individuals (including unadopted animals from municipal
pounds) and then resell them to research facilities. There are additional regulations
that apply to Class B dealers (such as holding periods and record-keeping documentation)
because of the public concern that stolen pets could enter biomedical research facilities
in this manner. Regulations regarding the sale of pound dogs to research facilities
or Class B dealers vary from state to state and include some bans on this practice.
The best resource for identification of possible vendors is the “Buyer's Guide” issue
of the periodical Lab Animal. Typically the last issue of each year, the “Buyer's
Guide” lists sources for both purpose-bred and random-source dogs and denotes such
features as pathogen-free status, documentation of health status, and availability
of specific breeds and timed pregnant females. Some suppliers also have separate advertisements
within that issue of the journal.
D.
Laboratory Management and Husbandry
Federal regulations promulgated by the Animal and Plant Health Inspection Service,
USDA, in response to the Animal
Welfare Act (7 CFR 2.17, 2.51, and 371.2[g]) are described in 9 CFR Chapter 1 (1-1-92
edition), Subchapter A, Animal Welfare. Regulations pertaining specifically to the
care of dogs used in research are found in Subpart A, Specifications for the Humane
Handling, Care, Treatment, and Transportation of Dogs and Cats, of Part 3 (Standards)
of Subchapter A. Particular attention should be paid to Section 3.6c (Primary Enclosures—Additional
Requirements for Dogs), because the space required for housing dogs is calculated
using the length of the dog rather than the body weight (which is used for other species
and also for dogs, according to National Research Council (NRC) guidelines). Section
3.8 (Exercise for Dogs) describes the requirements that dealers, exhibitors, and facilities
must follow in order to provide dogs with sufficient exercise.
The Institute of Laboratory Animal Research (ILAR) has written the “Guide for the
Care and Use of Laboratory Animals” (Seventh edition, 1996). The “Guide” is the primary
document used by institutional animal research programs to develop and design their
programs, as well as by the Association for Assessment and Accreditation of Laboratory
Animal Care International (AAALAC International) and other animal care evaluation
groups to facilitate site visits and inspections. The ILAR committee on dogs has also
written “Dogs: Laboratory Animal Management” (1994). This publication describes “features
of housing, management, and care that are related to the expanded use of dogs as models
of human diseases” and includes “an interpretive summary of the Animal Welfare Regulations
and the requirements of the Public Health Service Policy on Humane Care and Use of
Laboratory Animals.” The reader is encouraged to use these publications to obtain
further information on care and husbandry of dogs in the biomedical research setting.
II.
BIOLOGY
A.
Normal Values
Growth data for beagles from a purpose-bred dog breeding facility are provided in
Table I
. Table II
features hematology data from beagles from the same commercial facility. Table III
lists serum and urine chemical data for beagles. Normal physiologic data for dogs
(no breed specified) are provided in Table IV
. The information presented in the tables represents a range of normal values that
can vary, depending on the analytical method and equipment used as well as the age,
breed, gender, and reproductive status of the animal.
Table I
Beagle Growth Chart from 3 Months to 1 Year of Agea
Beagle weight data (in kg)
Male
Female
Age (months)
Mean
S.D.b
Cases
Mean
S.D.
Cases
3
4.8
1.01
108
4.7
0.78
155
4
6.2
1.03
8018
5.3
0.86
7187
5
7.1
1.11
3850
6.0
0.97
3182
6
8.1
1.29
2412
6.7
1.06
1485
7
8.8
1.51
2252
7.3
1.26
1365
8
8.9
1.59
1746
7.8
1.46
1220
9
8.9
1.67
618
8.2
1.47
501
10
9.6
2.11
455
8.3
1.55
430
11
9.3
1.73
391
8.5
1.64
351
12
9.7
1.78
394
8.5
1.61
463
a
Data from R. Scipioni and J. Ball of Marshall Farms USA, Inc., North Rose, New York.
b
S.D., standard deviation.
Table II
Hematology Data from Purpose-Bred Beaglesa
Beagles tested for period 2/28/99 through 9/01/99
Male
Female
Testb
Units
Cases
Mean value
S.D.b
Mean age (days)
S.D.b age (days)
Cases
Mean value
S.D.b
Mean age (days)
S.D.b age (days)
WBC
×103μl
4746
13.50
2.90
133
26.3
4744
13.61
2.85
139
34.6
RBC
×103μl
4746
6.27
0.46
133
26.3
4744
6.47
0.55
139
34.6
HGB
gm/dl
4746
13.77
1.05
133
26.3
4744
14.34
1.34
139
34.6
HCT
%
4746
43.76
3.16
133
26.3
4744
45.47
3.94
139
34.6
MCV
fL
4746
69.86
2.74
133
26.3
4744
70.39
2.72
139
34.6
MCH
pgm
4746
21.97
0.88
133
26.3
4744
22.19
0.92
139
34.6
MCHC
gm/dl
4746
31.46
0.82
133
26.3
4744
31.54
0.78
139
34.6
RDW
%
4746
15.30
0.90
133
26.3
4744
15.27
0.90
139
34.6
HDW
gm/dl
4746
1.45
0.12
133
26.3
4744
1.47
0.13
139
34.6
PLT
×103μl
4746
390.28
105.41
133
26.3
4744
386.47
106.99
139
34.6
MPV
fL
4746
6.72
1.03
133
26.3
4744
6.86
1.01
139
34.6
NEUT— Absolute
×103μl
4746
7.60
2.17
133
26.3
4744
7.55
2.07
139
34.6
LYMP— Absolute
×103μl
4746
4.87
1.11
133
26.3
4744
4.98
1.17
139
34.6
MONO— Absolute
×103μl
4746
0.58
0.35
133
26.3
4744
0.64
0.31
139
34.6
EOS— Absolute
×103μl
4746
0.30
0.13
133
26.3
4744
0.26
0.13
139
34.6
BASO— Absolute
×103μl
4746
0.08
0.03
133
26.3
4744
0.09
0.03
139
34.6
LUC— Absolute
×103μl
4746
0.07
0.04
133
26.3
4744
0.07
0.04
139
34.6
NEUT—%
%
4746
55.83
6.74
133
26.3
4744
55.14
6.73
139
34.6
LYMPH—%
%
4746
35.56
6.54
133
26.3
4744
37.01
6.59
139
34.6
MONO—%
%
4746
4.28
2.32
133
26.3
4744
4.71
1.93
139
34.6
EOS—%
%
4746
2.24
0.98
133
26.3
4744
1.95
0.92
139
34.6
BASO—%
%
4746
0.58
0.25
133
26.3
4744
0.65
0.28
139
34.6
LUC—%
%
4746
0.52
0.28
133
26.3
4744
0.55
0.31
139
34.6
LI
Index
4746
2.05
0.27
133
26.3
4744
2.03
0.27
139
34.6
MPXI
Index
4746
1.10
3.82
133
26.3
4744
2.00
3.71
139
34.6
Prothrombin time
sec
4374
6.05
0.12
122
10.4
4302
6.07
0.13
122
11.6
Reticulocyte
%
262
1.03
0.37
176
38.4
845
1.04
0.37
201
28.0
a
Data graciously provided by R. Scipioni and J. Ball of Marshall Farms USA, Inc., North
Rose, New York. Beagles tested for period 2/28/99–9/01/99.
b
S.D., standard deviation; WBC, white blood cells; RBC, red blood cells; HGB, hemoglobin;
HCT, hematocrit; MCV, mean corpuscular volume; MCH, mean corpuscular hemoglobin; MCHC,
mean corpuscular hemoglobin concentration; RDW, red cell distribution width; HDW,
hemoglobin distribution width; PLT, platelets; MPV, mean platelet volume; NEUT, neutrophils;
LYMP, lymphocytes; MONO, monocytes; EOS, eosinophils; BASO, basophils; LUC, large
unstained cells; LI, lobularity index; MPXI, mean peroxidase activity index
Table III
Clinical Chemistry Data for the Beaglea
Male
Female
Analyte
Units
<1 year
>1 year
<1 year
>1 year
Serum
Glucose
mg/dl
98.0 ± 10.3b
95.0 ± 9.2
98.0 ± 7.9
95.0 ± 10.1
Urea nitrogen
mg/dl
22.0 ± 7.1
18.0 ± 4.0
17.0 ± 4.3
19.0 ± 5.3
Creatinine
mg/dl
0.80 ± 0.13
0.80 ± 0.14
0.80 ± 0.11
0.80 ± 0.15
Sodium
mEq/1
150.0 ± 3.40
149.0 ± 4.20
152.0 ± 2.60
150.0 ± 3.40
Potassium
mEq/1
4.90 ± 0.38
4.70 ± 0.35
4.80 ± 0.33
4.70 ± 0.35
Chloride
mEq/1
113.0 ± 2.00
113.0 ± 4.00
114.0 + 2.80
113.0 ± 3.50
Bicarbonate
mEq/1
21.0 ± 1.5
Calcium
mg/dl
11.2 ± 0.62
10.7 ± 0.71
11.1 ± 0.47
10.8 ± 0.55
Phosphorus
mg/dl
5.60 ± 0.85
3.70 ± 0.79
4.80 ± 0.83
3.80 ± 1.18
Magnesium
mg/dl
2.10 ± 0.30
2.20 ± 0.28
Iron
μg/dl
105 ± 37.5c
,
d
Total Iron Binding Capacity
μg/dl
359.6 d
Haptoglobin
mg/dl
125 ± 37.5
62.5 ± 18.75d
Alanine aminotransferase
IU/liter
23.0 ± 7.4
29.0 ± 11.4
25.0 ± 9.4
26.0 ± 15.0
Aspartate aminotransferase
IU/liter
19.0 ± 5.1
21.0 ± 4.6
20.0 ± 4.4
20.0 ± 4.5
Alkaline phosphatase
IU/liter
63 ± 21.1
41 ± 18.7
60 ± 21.7
43 ± 17.0
Lactate dehydrogenase
IU/liter
52 ± 28.4
53 ± 23.9
49 ± 28.0
58 ± 38.0
Sorbitol dehydrogenase
IU/liter
4.5 ± 1.9c
,
d
γ-Glutamyl transpeptidase
IU/liter
3.5 ± 1.8c
,
d
Creatinine kinase
IU/liter
106 ± 71.1
78 ± 36.7
118 ± 60.5
81 ± 45.3
Protein, total
gm/liter
59 ± 3.5
62 ± 4.4
58 ± 2.9
61 ± 3.7
Albumin
gm/liter
30 ± 2.5
36 ± 4.5
Cholesterol
mg/dl
175 ± 38.0
151 ± 31.5
183 ± 38.6
176 ± 41.9
Triglycerides
mg/dl
62 ± 22.7
58 ± 19.0
55 ± 19.6
65 ± 22.9
Bilirubin, total
mg/dl
0.5 ± 0.49
0.4 ± 0.38
0.4 ± 0.36
0.4 ± 0.35
Bile acids
μmol/l
2.6 ± 0.40d
Uric acid
mg/dl
1.15 ± 1.43c
1.06 ± 1.45c
Luteinizing hormone
ng/ml
0.2–10c
2.0 (basal)
Prolactin
ng/ml
9a
12.6 (anestrus)
Growth hormone
ng/ml
2.0c
2.0c
Thyroid stimulating hormone
ng/ml
<0.5c
<0.5c
Adrenocorticotropic hormone
pg/ml
20–100c
20–100c
Vasopressin
pg/ml
2.5–10c
,
d
Oxytocin
pg/ml
0–30c
4–73c
Thyroxine, total
μg/dl
1.5–3.0c
,
d
Triiodothyronine, total
ng/dl
100–200c
,
d
Thyroxine, free
ng/dl
0.8–2.0c
,
d
Parathyroid hormone
pg/ml
18–122c
,
d
1,25-Dihydroxy vitamin D
pg/ml
88 ± 13
35 ± 7.0c
,
d
Cortisol
μg/dl
1.8–4.0c
,
d
Aldosterone
ng/dl
6–31c
5–11
Epinephrine
pg/dl
100–400c
,
d
Norepinephrine
pg/dl
68–526c
,
d
Insulin
μIU/ml
5–25c
,
d
Progesterone
ng/ml
<0.2c
<1c(proestrus, estrus)
>10c (diestrus, pregnancy)
Estradiol
pg/ml
<20c
25–50c(proestrus, estrus)
<20c (pregnancy)
Testosterone
ng/ml
1–7c
<0.2
Urine
Volume
ml/22 hr
186 ± 159
191 ± 156
Specific gravity
gm/liter
1.036 ± 0.014
pH
7.5 ± 0.14
Osmolality
mOsm/kg
1013 ± 539
1019 ± 517
Chloride
mmol/liter
126 ± 68
126 ± 68
Sodium
mmol/liter
89 ± 56
107 ± 61
Potassium
mmol/liter
110 ± 77
108 ± 68
Phosphorus
mg/dl
102 ± 58
109 ± 46
Calcium
mg/dl
3.3 ± 1.3
3.0 ± 1.3
Creatinine
mg/dl
147 ± 85
143 ± 87
Protein
mg/24hr
42.1 ± 29.1
Alkaline phosphatase
IU/24hr
16.2 ± 13.0d
Lactate dehydrogenase
IU/24hr
16.2 ± 2.0c
,
d
N-Acetyl-β-D-glucosaminidase
U/liter
6.9 ± 2.9c
,
d
Acid phosphatase
IU/24hr
1.02 ± 0.56d
Aspartate aminotransferase
IU/24hr
0.5 ± Q.2d
Alanine aminotransferase
IU/24hr
1.23 ± 1.21d
Creatinine clearance
ml/min/kg
3.7 ± 0.77c
,
d
Insulin clearance
ml/min/kg
3.89 ± 0.42c
,
d
PAH clearance
ml/min/kg
8.8 ± 1.0c
,
d
Glomerular filtration rate
ml/kg/hr
234 ± 16.4c
,
d
Urine flow rate
ml/kg/hr
3.5 ± 7.9c
,
d
PSP clearance
%
43.8 ± 11.1c,d
a
Summarized from Loeb and Quimby (1999).
b
Mean ± S.D.
c
Breed not specified.
d
Gender not specified.
Table IV
Normal Physiologic Data for Dogsa
Parameter
Awake
Anesthetized (if different)
Body temperature
99.5°–102.5°F (37.5°–39.2°C)
Heart rate (beats/min)
70–180
60–180
Respiration rate (breaths/min)
20–40
8–20
Capillary refill time (sec)
<1.5
Arterial pH
7.30–7.43
Arterial PC0
2 (mm Hg)
30–49
Arterial P0
2 (mm Hg)
91–97
90–500 (if >50% inspired 02)
Arterial HC03 (mEq/liter)
18–22
Arterial base excess (mEq/liter)
−3 to+3
a
Breed not specified. Modified from Birchard and Sherding (1994).
B.
Nutrition
Good nutrition and a sound, balanced diet are essential to the health, performance,
and well-being of the animal. The basic nutrient requirements for dogs have been compiled
by the NRC and represent the average amounts of nutrients that a group of animals
should consume over time to maintain growth and prevent deficiencies (National Research
Council, 1985). The reader is referred to these guidelines for useful reference points
for management of an animal's diet during various physiologic states (e.g., gestation,
lactation, maturational age).
Most commercially available balanced dog diets are “closed-formula” diets, in which
the labeled specific minimum requirements for protein and fat, and the maximum values
for ash and fiber, are met. These diets do not necessarily provide the identical composition
of ingredients from batch to batch. Ingredient composition varies, depending on the
cost relationships of the various ingredients as the manufacturer attempts to achieve
the label requirements at the lowest ingredient cost. An “open-formula” (or “fixed-formula”)
diet provides more precise dietary control. In these diets the ingredients are specified,
and the percentage of each ingredient is kept constant from batch to batch. “Semipurified”
diets provide for the strictest control of ingredients and are formulated from the
purified components: amino acids, lipids, carbohydrates, vitamins, and minerals.
The animal care provider should be aware of the manufacture date of the diet, which
should be clearly visible on the bag. As a general rule, diets are generally safe
for consumption up to 6 months following the manufacture date when stored at room
temperature. Refrigeration may prolong the shelf life, but the best strategy is to
use each lot based on the date of manufacture in order to prevent food from expiring
and to ensure that only fresh diets are fed. Specifications for feeding and watering
of dogs are provided in the regulations of the Animal Welfare Act.
Recommendations for feeding the appropriate amount of diet are determined by the dog's
metabolic requirements. The basal metabolic rate, or basal energy requirement (BER),
refers to the amount of energy expended following sleep, 12–18 hours after food consumption,
and during thermoneutral conditions (Kleiber, 1975; Lewis et al., 1987). The maintenance
energy requirement (MER) is the amount of energy used by a moderately active adult
animal in a thermoneutral environment, which in the dog is approximately twice the
BER (Lewis et al., 1987). For dogs weighing greater than 2 kg, the MER may be calculated
using this simplified linear equation: MER (metabolizable kcal/day) = 2(30 weightkg
+ 70) (National Research Council, 1985; Lewis et al., 1987). The quantity of a correctly
balanced diet to be fed to each dog can then be determined by dividing the MER by
the energy density of the diet.
Fat provides three major dietary functions, including absorption of fat-soluble vitamins
(A, D, E, and K), enhancement of palatability, and provision of essential (unsaturated)
fatty acids. Dietary fat is an excellent, highly digestible energy source, providing
2.25 times more energy on a per weight basis than either soluble carbohydrates or
proteins (Lewis et al., 1987). However, fats are not needed for this purpose when
adequate carbohydrate and protein are present. Consumption of fat in excess of an
animal's ability to metabolize it results in steatorrhea and has been related to the
development of acute pancreatitis, whereas lack of dietary fat may lead to a fatty
acid/energy deficiency. Fatty acid deficiency is associated with poor growth, poor
physical performance, reduced reproductive performance, and weight loss.
Dogs are considered to be “easy keepers,” because they do not have as many absolute
nutritional requirements as their domestic counterpart, the cat. However, they do
possess a unique requirement for certain polyunsaturated fatty acids, a deficiency
of which may predispose them to decreased growth rates and dermatologic abnormalities,
such as “hot spots.” Dogs require linoleic (Ω-6) acid, an essential fatty acid (National
Research Council, 1985), and more recently it has been demonstrated that the Ω-3 fatty
acids may play a role in maintaining healthy skin (Logas and Kunkle, 1994). Supplementation
with a balanced essential fatty acid product (e.g., Derm Caps) may alleviate allergy-related
dermatoses such as flea-bite dermatitis and pyoderma (Logas and Kunkle, 1994; Miller,
1989). Essential fatty acid deficiency can occur in dogs receiving low-fat dry dog
food that has been stored too long, particularly under warm, humid conditions (Lewis
et al., 1987).
There are 22 α-amino acids, 10 of which cannot be synthesized in sufficient quantity
to meet a dog's normal metabolic demands for growth and maintenance. Hence, as their
name implies, these essential amino acids are required by all dogs and must be provided
in the diet. The essential amino acids and the minimal requirements for growth are
listed elsewhere (Lewis et al., 1987).
Chronic excessive protein intake may be detrimental to the kidney by contributing
to accelerated renal aging and subsequent glomerulosclerosis (Lewis et al., 1987).
Conversely, inadequate protein intake results in retardation of growth and reduction
in production and/or performance. Protein deficiency, a potential consequence of decreased
food intake, results in decreased energy intake. As a compensatory mechanism for a
lack of fat or carbohydrate, body protein catabolism ensues in order to meet energy
demands, thus exacerbating the negative protein balance and contributing to the clinical
signs of edema/ascites, unkempt appearance, lethargy, and weight loss. Thus, caloric
needs must be met before protein needs (Lewis et al., 1987), an important concept
to bear in mind in the event of research experiments that may predispose to anorexia.
In general, providing a good quality commercial diet that supplies the required amount
of amino acids and caloric requirements of the animal, while avoiding excess protein,
will ensure nutritional stability and promote longevity.
Appropriate mineral balance in the diet is very important. The best approach in the
laboratory setting is to feed a commercial diet that has been formulated with the
proper amount and balance of minerals for normal growth. The recommended amount of
dietary minerals and the major causes and clinical signs of deficiencies are published
elsewhere (Lewis, 1987). Determining the specific mineral involved in an imbalance
can be a diagnostic challenge, because the clinical signs for several excesses/deficiencies
are similar and nonspecific. A definitive diagnosis is often made only after the diet
has undergone analysis of the mineral components. Once the imbalance has been identified,
the safest resolution to the problem is to discard the entire lot of misformulated
diet. Attempting to correct the imbalance through oral supplementation is likely to
be more harmful than beneficial, and it risks intensifying the problem by creating
additional mineral imbalances.
Vitamins function as enzymes that regulate a wide variety of physiologic processes.
They are divided into two groups based on their solubility. The fat-soluble vitamins
include A, D, E, and K, whereas the rest are water-soluble. A list of the vitamins,
their requirements, and clinical signs associated with deficiencies and toxicities
is published elsewhere (Lewis et al., 1987). Cases of dietary deficiency are rarely
encountered in the research setting, because laboratory dog chows are fortified with
vitamins. Additional vitamin supplementation may occasionally be required during prolonged
clinical illnesses, such as polyuria or diarrhea, which predispose to loss of water-soluble
vitamins (B complex and C) (Lewis et al., 1987). However, as with minerals, routine
supplementation of vitamins may induce inadvertent toxicity and exacerbation of an
imbalance.
C.
Reproduction
Management of a breeding colony requires broad knowledge of the dog's anatomy, reproductive
physiology, and behavioral needs during breeding, gestation, and parturition. Although
a comprehensive discussion of the biology of canine reproduction is beyond the scope
of this chapter, essential features of the broad topics noted above are presented.
This section is largely based on information assimilated from texts such as “Miller's
Anatomy of the Dog” (Evans and Christensen, 1993), “Veterinary Reproduction and Obstetrics”
(Arthur et al., 1989), and an issue of Veterinary Clinics of North America: Small
Animal Practice devoted to pediatrics of puppies and kittens (Hoskins, 1999).
1.
Anatomy and Reproductive Physiology of the Bitch
The ovaries of the bitch are attached to the dorsolateral walls of the abdominal cavity
caudal to the kidneys by the broad ligaments and are not palpable abdominally. The
uterus consists of the cervix, uterine body, and uterine horns. The cervix is an abdominal
organ, located approximately halfway between the ovaries and the vulva. When the bitch
is in proestrus and estrus, the cervix can be distinguished during abdominal palpation
as an enlarged, turgid, walnut-shaped structure. Catheterization of the cervix is
usually not possible in the normal bitch at any stage of the reproductive cycle, except
during or immediately following parturition. Thus, semen is deposited at the external
cervical os during natural or artificial insemination. The vagina is a long musculomembranous
canal that extends from the uterus to the vulva. When the vagina is examined, the
gloved finger or examination instrument should be introduced through the dorsal commissure
of the vulva so as to avoid the deep ventral clitoral fossa. Examination should proceed
at an angle of approximately 60° until the instrument or fingertip has passed over
the ischial arch, after which it can be directed further craniad toward the cervix.
The bitch has a monoestrous cycle, with clinical estrus occurring predominantly in
January or February and again in July or August (although it can occur at any time
of year). The estrous cycle consists of four stages: proestrus, estrus, diestrus,
and anestrus. The average duration of proestrus is 9 days. During this stage the vulva
is enlarged, turgid, and firm, and a sanguinous vaginal discharge is present. Endocrinologically,
proestrus is the follicular stage of the cycle, and estrogen levels peak at this time.
Estrus generally lasts 9 days, and the vulva is softer and smaller than in proestrus.
A vaginal discharge persists during estrus and may remain serosanguinous or become
straw-colored. The endocrine feature of estrus is the luteinizing hormone (LH) surge,
followed by ovulation within 24–72 hours. Diestrus begins approximately 9 days after
the onset of standing heat. The end of this stage is 60 days later, which would be
coincident with whelping if the bitch had become pregnant. Serum progesterone levels
peak during diestrus. The duration of anestrus is approximately 4 months. Anestrus
is the stage of reproductive quiescence, characterized by an absence of ovarian activity
and serum progesterone levels of less than 1 ng/ml.
2.
Anatomy and Reproductive Physiology of the Male Dog
Components of the canine spermatic cord include the ductus deferens, the testicular
artery and vein, the lymphatics and nerves, and the cremaster muscle. The cremaster
muscle and pampiniform plexus aid in thermoregulation of the testicles, which are
maintained at 2°–7°C lower than basal body temperature. Sweat glands in the scrotum
assist in lowering the scrotal temperature through evaporation. The penis is a continuation
of the muscular pelvic urethra and is attached to the ischiatic arch by two fibrous
crura. It is composed of fibrous tissue and three cavernous sinuses: corpus cavernosum,
corpus spongiosum penis, and corpus spongiosum glandis. The accessory sex glands of
the dog consist of only a well-encapsulated prostate gland that surrounds the pelvic
urethra, and ampullary glands at the termination of the vas deferens in the urethra.
The dog does not have seminal vesicles or bulbourethral glands.
The onset of puberty ranges from 5 to 12 months of age and is affected by breed, season,
and nutritional and disease status. Testicular growth is rapid at this time, and the
seminiferous tubules begin to differentiate. The Sertoli cells form the bloodtestis
barrier, the tubules become hollow, and spermatogenesis commences. This process is
initiated by the secretion of LH from the anterior pituitary, which stimulates the
production of testosterone by the interstitial, or Leydig's, cells. Secretion of follicle-stimulating
hormone (FSH) by the anterior pituitary stimulates the production of other key hormones
by the Sertoli cells, including inhibin, androgen binding protein, and estrogen. FSH
stimulates spermatogenesis in the presence of testosterone, while inhibin and estrogen
play a role in a feedback loop on the pituitary gland to decrease FSH production.
Spermatogenesis in the dog is completed in 45 days, with subsequent maturation of
sperm occurring in the epididymis for approximately 15 days. Thus, the entire process
from initiation of spermatogonial mitosis to delivery of mature sperm to the ejaculate
is 60 days.
A breeding soundness exam should be conducted to assess the probability of a male
dog's successful production of offspring. Factors affecting male fertility include
libido, ability to copulate, testicular size, and quality and number of sperm produced.
Problems with libido may occur in dogs due to early weaning, isolation, or inherited
abnormalities that suppress sexual behavior. Animals with poor hindlimb conformation
or with trauma to the back or hindlimbs may be unable to properly mount the female.
There is a positive correlation with the size of the testicles as measured by scrotal
circumference and the number of sperm produced. Finally, parameters used to assess
the quality of sperm include motility, morphology, volume, and concentration. An ejaculate
(5 ml) that contains approximately 500 million progressively motile sperm without
significant morphological abnormalities (such as a kinked tail) is a good indicator
of normal male fertility.
In general, erection, which involves muscular contractions and increased arterial
blood flow to the penis, is controlled by the parasympathetic nervous system, whereas
ejaculation is under sympathetic control. On mounting, the initial thrusting and ejaculation
of semen last about 1 minute. The bulbus glandis becomes enlarged, which lodges the
penis in the female reproductive tract. The male then dismounts and brings one hindleg
over the female, and the two continue to be joined “rear to rear,” a position classically
termed “the tie.” Ejaculation of the accessory gland fluid continues for 5–30 minutes.
The continued expulsion of prostatic fluid during the “tie” may serve to propel the
semen from the vagina through the cervix into the uterus.
Fertilization occurs in the oviduct and may occur as late as 8 days after coitus,
because of the long life span of sperm in the dog. However, once ovulated, oocytes
generally remain viable for only 12–24 hours. Therefore, the bitch should be bred
prior to ovulation to ensure the presence of sperm for fertilization of live oocytes.
3.
Detection of Estrus and Pregnancy
Cells of the vaginal epithelium mature to keratinized squamous epithelium under the
influence of estrogen. Because of the rise in estrogen throughout proestrus, with
peak levels occurring just prior to the onset of standing heat, the vaginal smear
can be used as an indicator of the bitch's readiness for breeding. The smear will
not confirm the presence of ovulation, nor is it of prognostic value in normal bitches
during anestrus.
The percentage of vaginal epithelial cell cornification is an index of estrogen secretion
by the ovarian follicles. As cornification of vaginal epithelial cells proceeds, the
cells become larger, with more angular borders. The nuclear–cytoplasmic ratio decreases
until the nuclei reach a point where they no longer take up stain (coincident with
the onset of estrus). The cells appear “anuclear” and are classified as “cornified”
or “anuclear squames.” Cornification occurs approximately 2 days prior to the estrogen
peak and 4 days prior to standing heat. The percentage of cornified cells (of the
total number of epithelial cells) decreases gradually to zero after the onset of diestrus.
The vaginal cytology smear of the bitch changes from predominantly cornified to noncornified
6 days after ovulation. The day of this change is the first day of diestrus. Other
epithelial cell types noted on vaginal cytology include superficial cells (large,
angular cells with small nuclei); intermediate cells (round or oval cells with abundant
cytoplasm and large, vesicular nuclei); and parabasal cells (small round or elongated
cells with large, wellstained nuclei, and a high nuclear–cytoplasmic ratio). Based
on vaginal cytology, the estrous cycle is classified as follows:
Proestrus, early: intermediate and superficial cells, red blood cells, and neutrophils.
Proestrus, late: superficial cells, anuclear squames, and red blood cells.
Estrus: more than 50% anuclear squames, superficial cells, with or without red blood
cells.
Diestrus: more than 50% intermediate cells, superficial cells and squames early, but
becoming completely noncornified with neutrophils present as diestrus proceeds.
Anestrus: small numbers of parabasal cells and intermediate cells, with or without
neutrophils.
Although vaginal cytology is a useful tool, it is not a substitute for observation
of behavioral estrus, which is the best criterion to use in breeding management. During
proestrus the male is attracted to the bitch and will investigate her hindquarters,
but she will not accept breeding. The behavioral hallmark of estrus is standing receptivity
toward the male. During this stage the bitch will exhibit “flagging,” or elevation
of her tail with muscular elevation of the vulva to facilitate penetration by the
male. In order to maximize the conception rate, and the number of pups whelped per
egg ovulated, it is recommended to breed the bitch on days 1, 3, and 5 of the standing
heat.
4.
Pregnancy
Fertilization is completed in the mid- to distal oviduct. Implantation is evident
by areas of local endometrial edema 17–18 days after breeding. There is no correlation
between the number of corpora lutea and the number of fetuses in the corresponding
uterine horn, suggesting transuterine migration of embryos.
The dog has endotheliochorial placentation. The endothelium of uterine vessels lies
adjacent to the fetal chorion, mesenchymal, and endothelial tissues, so that maternal
and fetal blood are separated by four layers. The canine placenta is also classified
as zonary and deciduate, indicating that the placental villi are arranged in a belt
and that maternal decidual cells are shed with fetal placentas at parturition. The
length of gestation is 59–63 days. Luteal progesterone is responsible for maintaining
pregnancy, and canine corpora lutea retain their structural development throughout
gestation. Serum progesterone rises from less than 1 ng/ml in late proestrus to a
peak of 30–60 ng/ml during gestation, then declines to 4–5 ng/ml just prior to parturition.
Progesterone is essential for endometrial gland growth, secretion of uterine milk,
attachment of the placentas, and inhibition of uterine motility.
Pregnancy detection can be performed by abdominal palpation of the uterus 28 days
after breeding. The embryos and chorioallantoic vesicles form a series of ovoid swellings
in the early gravid uterus. They are approximately 2 inches in length at 28–30 days,
the time at which pregnancy is most easily and accurately diagnosed. By day 35 the
uterus begins to enlarge diffusely, so that the vesicles (and, therefore, pregnancy)
are difficult to identify by palpation. Fetal skeletons become calcified and are radiographically
evident by day 42. Bitches in which a difficult whelping is anticipated should be
radiographed in late pregnancy to determine the litter size and to evaluate the size
of the fetal skulls in relation to the bony maternal birth canal. Real-time ultrasound
can be utilized for pregnancy detection of vesicles as early as 25–28 days.
5.
Parturition and the Neonate
An abrupt drop in body temperature to less than 100°F indicates impending parturition
within 18–24 hours. The process of parturition has been divided into three stages.
Stage 1 of labor lasts 6–12 hours and is characterized by uterine contractions and
cervical dilation. During this stage, the bitch may appear restless, nervous, and
anorexic. Other common clinical signs include hard panting and increased pulse and
respiration rates.
Fetal expulsion occurs during stage 2, which lasts approximately 3–6 hours. As the
fetus engages the cervix, the neuroendocrine system induces the release of oxytocin;
this is referred to as the Ferguson reflex. Oxytocin strengthens the uterine contractions
and may elicit voluntary abdominal contractions as well. The bitch is usually recumbent
during stage 2 but is able to inhibit this stage if labor if disturbed. The chorioallantois
ruptures either during passage of each neonate through the birth canal or by the bitch's
teeth at birth. Interestingly, posterior presentation is common in dogs but does not
predispose to dystocia. The time interval between delivery of each pup is irregular,
but the average time lapse is less than 1 hour between pups until parturition is complete.
Veterinary assistance is necessary if the bitch remains in stage 2 for more than 5
hours without delivering the first pup, or for more than 2 hours before delivering
subsequent pups.
The placentas are expelled during stage 3 of labor, immediately following delivery
of a pup, or up to 15 minutes thereafter. If two pups are delivered from alternate
uterine horns, then the birth of both puppies may precede expulsion of the respective
placentas. The bitch will lick the newborn vigorously to remove the membranes from
its head and to promote respiration. She will also sever the umbilical cord. The bitch
may ingest the placentas, although they confer no known nutritional benefit and may
induce a transient diarrhea.
Thermal support should be provided prior to parturition. Dogs housed on grated flooring
should be provided with mats, and those on solid floors would benefit from blankets
placed in a corner of the primary enclosure. Shavings are discouraged as they have
the potential to coat the umbilical cord, which may predispose to ascending infections.
Heat lamps may be placed 24 hours prior to parturition and remain until all neonates
demonstrate vigorous and successful suckling behavior. However, the use of heat lamps
necessitates strict supervision in order to prevent thermal burns. If possible, whelping
bitches should be housed in a quiet corridor in order to decrease periparturient stress,
especially in primiparous or young mothers. Thus, monitoring of parturition is important,
but human intervention should be minimal in order to prevent stress-induced cannibalism.
Weak or debilitated puppies may be cannibalized by the bitch before the research staff
recognizes the need for veterinary attention.
The postpartum use of oxytocin is required only in the event of uterine inertia, stillbirths,
or agalactia. In these cases, 5–20 units of oxytocin may be administered intramuscularly.
Uterine involution occurs during anestrus within 4–5 weeks of parturition. During
this time a greenish to red-brown vaginal discharge, or lochia, may be noted. Although
lochia is normal, the presence of an odiferous, purulent discharge, accompanied by
systemic signs of illness, indicates metritis or pyometra. Desquamation of the endometrium
begins by the sixth postpartum week, with complete repair by 3 months.
Newborn puppies are easily sexed by examination of the anogenital distance. In female
puppies the vulva is evident a short distance from the anus, whereas the prepuce of
male puppies is nearly adjacent to the umbilicus. Eyes are open at approximately 12
days, and ears are patent at approximately 12–20 days. Solid food can be introduced
between 4.5 and 6 weeks of age, and puppies can be weaned at 6–8 weeks.
6.
Artificial Insemination
Artificial insemination (AI) is indicated when the male is physically incapable of
mounting or penetrating the bitch, when there are vaginal abnormalities such as strictures,
or when the bitch refuses to stand for breeding. Semen for AI is collected using a
plastic centrifuge tube and rubber latex artificial vagina. The male is introduced
to the bitch's scent and manually stimulated. After collection of the first two fractions,
a sufficient amount of the third fraction, which consists predominantly of prostatic
fluid, is collected to bring the total semen volume to 4–6 ml. The semen is then drawn
into a sterile 10 or 12 ml syringe attached to a sterile disposable insemination pipette.
The bitch is inseminated either standing or with raised hindquarters. A gloved index
finger is inserted into the dorsal commissure of the vulva and directed craniodorsally
until it is over the ischial arch. The tip of the insemination pipette is introduced
and guided by the gloved finger toward the external cervical os. The semen is injected,
and 2–3 ml of air are then flushed through the syringe and pipette. The pipette is
withdrawn, and the gloved finger is used to feather the ceiling of the vagina until
contractions of the vaginal musculature are palpable. The bitch's hindquarters are
subsequently elevated to promote pooling of semen around the external cervical os.
As with natural breeding, AI should be performed on days 1, 3, and 5 of standing heat,
or on the days of maximal vaginal cornification. The bitch should be palpated for
pregnancy approximately 4 weeks after the first insemination.
7.
False Pregnancy
False pregnancy (pseudocyesis), a stage of mammary gland development and lactation
associated with nesting or mothering behavior, is common in the bitch. The condition
occurs after the decline in serum progesterone toward the end of diestrus. There is
no age or breed predisposition. Pseudopregnancy does not predispose the bitch to reproductive
disease or infertility. However, in the event of extreme discomfort due to mammary
gland enlargement, bitches may be treated with mibolerone (Cheque Drops) at an oral
dose of 16 μg/kg q24 hr for 3–5 days (Brown, 1984).
8.
Reproductive Life Span
Reproductive performance in the bitch is optimal prior to 4 years of age. Although
normal cycle lengths are reported to occur up to the ages of 5–7 years, the interestrous
interval tends to increase by 4 years of age. Cycling does not completely cease; however,
after 8 years of age, bitches demonstrate significant decreases in conception rate
and number of live pups whelped. By 8–9 years of age, pathologic conditions of the
uterus, such as cysts, hyperplasia, atrophy, and neoplasia are extremely common.
D.
Behavior
Beagles have been a popular animal model because of their docile nature. They are
easily handled and for the most part respond favorably to repetitive manipulations
such as body weight measurements, physical examination, electrocardiogram (ECG) recordings,
oral gavage, and venipuncture.
Dogs are sexually mature by 6–9 months of age, but they are not socially mature until
18–36 months of age. The socialization process should begin early during development,
when puppies are receptive to conspecific and human contact. For example, from 3–8
weeks of age, puppies are most capable of learning about how to interact with other
dogs. Between weeks 5 and 12, puppies are most capable of learning how to interact
with people. By 10–12 weeks of age dogs voluntarily wander and explore new environments.
Thus, early handling and mild stress (such as vaccination) appear to be extremely
beneficial components of a dog's social exposure.
The extent to which breed affects behavior has been the subject of popular speculation
but is difficult to prove. In general, breed-specific patterns do tend to emerge.
For example, it appears that beagle pups are very motivated by food reward (Overall,
1997). This is not surprising, because the breed was selected to work with its nose,
and this may be a useful attribute for laboratory investigations that are predicated
on food restriction.
Canid social systems use signals and displays that minimize the probability of outright
aggression. These behavior patterns are most likely elicited during distressful situations,
such as strange environments, being handled by strange people, or encountering new
animals. An excellent, illustrated discussion of normal canine behavior patterns can
be found in the third chapter of “Clinical Behavioral Medicine for Small Animals”
(Overall, 1997).
III.
DISEASES
By virtue of the dog's status as a companion animal, there are many veterinary publications
and reference texts on the diagnosis, medical management, pathology, and epidemiology
of the disorders that can affect this species. The authors of this chapter have chosen
to emphasize those diseases that are more frequently encountered in the research setting.
Especially noted in this chapter are infectious diseases associated with the use of
random-source dogs that have unknown vaccination history and have had intensive contact
with other similar animals at pounds and/or shelters, or conditions seen frequently
in the beagle, the most common breed used in biomedical research. For more thorough
and detailed discussion of these diseases, as well as those not discussed in this
chapter, the reader should consult standard veterinary textbooks, such as the “Current
Veterinary Therapy” series (J. D. Bonagura and R. W. Kirk, eds.), “Veterinary Internal
Medicine” (S. J. Ettinger and E. C. Feldman, eds.), and “Infectious Diseases of the
Dog and Cat” (C. E. Greene, ed.). Full citations of some chapters from these texts
are listed in the references (W. B. Saunders Co. of Philadelphia publishes all three
texts.)
A.
Infectious Diseases
1.
Bacterial, Mycoplasmal, and Rickettsial Diseases
a.
Canine Infectious Tracheobronchitis (Kennel Cough Complex)
Etiology.
Infectious tracheobronchitis (ITB) is a highly contagious illness of the canine respiratory
tract that usually manifests as an acute but self-limiting disease. Several organisms
have been incriminated as causative for this condition: Bordetella bronchiseptica;
canine parainfluenza virus (CPIV); canine adenovirus types 1 and 2 (CAV-1, CAV-2);
canine herpesvirus; canine reovirus types 1, 2, and 3; and mycoplasms and ureaplasms.
Clinical signs.
Clinical infectious tracheobronchitis can be subdivided into mild or severe forms.
The mild form is the more common presentation and is characterized by an acute onset
of a loud, dry, hacking cough. Increased formation of mucus sometimes results in a
productive cough, followed by gagging or retching motions. Cough is easily elicited
by tracheal palpation and may be more frequent with excitement or exercise. Otherwise
the dog is typically asymptomatic, with normal body temperature, attitude, and appetite.
Mild tracheobronchitis usually lasts 7–14 days, even if left untreated.
The severe form of tracheobronchitis generally results from mixed infections complicated
by poor general health, immunosuppression, or lack of vaccination. Secondary bronchopneumonia
can occur and may be the determinant of severity (Sherding, 1994). Animals are clinically
ill and may be febrile, anorexic, and depressed. Productive cough and mucopurulent
naso-ocular discharge are more common than in the mild form. These cases require more
aggressive treatment and may be fatal.
Epizootiology and transmission.
The natural reservoir for Bordetella bronchiseptica is considered to be the respiratory
tract of infected animals (Bemis, 1992). This bacterium is very easily spread by aerosol
and direct contact, and fomite transmission is also possible (Bemis, 1992). Transmission
is favored by confined housing of multiple animals. In experimental studies, B. bronchiseptica
transmission to susceptible individuals was 100% (Thompson et al., 1976; McCandlish
et al., 1978). The incubation period is 3–10 days. CPIV and CAV-2 are also spread
by aerosols. Of these two viruses, CAV-2 is the most persistent, lasting for up to
several months in the environment, whereas CPIV is fairly labile (Hoskins, 2000a).
Both viruses can be destroyed by quaternary ammonium disenfectants.
Pathogenesis.
The most common clinical isolates are CPIV and Bordetella bronchiseptica. However,
B. bronchiseptica may be a commensal organism, and it is often recovered from asymptomatic
animals. In cases of clinical infection, B. bronchiseptica attaches to the cilia on
the mucosal surface of the upper airway epithelium, causing suppurative tracheobronchitis
and bronchiolitis. Infections with CPIV or CAV-2 alone are usually subclinical; coinfections
with B. bronchiseptica or other microbes may result in clinical ITB (Keil and Fenwick,
1998; Wagener et al., 1994). The characteristic lesion from CPIV or CAV-2 infection
is necrotizing tracheobronchiolitis (Dungworth, 1985). Pathogenic infection of the
upper airways typically results in inflammation and ciliary dysfunction.
Diagnosis and differential diagnosis.
Diagnosis of infectious tracheobronchitis is often based on clinical signs. Isolation
of Bordetella bronchiseptica or mycoplasma by nasal swabs allows only a presumptive
diagnosis. Viral isolation or paired serology can be done but is often impractical
and expensive. If cough persists for more than 14 days, other disease conditions should
be considered. Canine distemper virus infection, pneumonia, heartworm disease, tracheal
collapse, and mycotic infections are differential diagnoses for dogs with similar
signs. Bronchial compression as a result of left atrial enlargement, hilar lymphadenopathy,
or neoplasia may also elicit a nonproductive cough (Johnson, 2000) and should be considered
as a differential for ITB.
Prevention.
Prevention is best achieved by avoiding exposure to infected animals, but this is
oftentimes not practical. Dogs should be vaccinated prior to, or at the time of, admission
to the animal research facility. Intranasal vaccine combinations for Bordetella bronchiseptica
and CPIV are preferred. Intranasal vaccines protect against both infection and disease,
can be given to dogs as young as 2 weeks of age, and can produce immunity within 4
days.
Control.
Sanitation and ventilation are critical for control. The animal care staff must practice
proper hygiene to prevent fomite transmission. Symptomatic animals should be isolated,
and animal-to-animal contact avoided. Kennels should be disinfected with agents such
as bleach, chlorhexidine (Nolvasan) or quaternary ammonium chloride (Roccal-D). Proper
ventilation and humidity are important in controlling spread of these infectious agents;
15–20 air changes per hour at 50% relative humidity are recommended (Sherding, 1994).
Treatment: No specific treatment is available for viral infections. Bordetella bronchiseptica
is typically sensitive to potentiated sulfas, chloramphenicol, quinolones, tetracyclines,
gentamicin, and kanamycin. Use of antibiotics is indicated when severe or persistent
clinical signs occur, and it should be continued for 14 days. Use of empirical antibiotic
treatment in mild cases may hasten the resolution of clinical signs. For severe or
unresponsive infection, treatment should be based on bacterial culture sensitivity
patterns; nebulized gentamicin may be helpful. Cough suppressants (e.g., dextromethorphan)
should be avoided if the cough is bringing up mucus (productive); however, their use
is indicated if coughing is causing discomfort or interfering with sleep. Bronchodilators
such as aminophylline, theophylline, or terbutaline can be helpful in reducing reflex
bronchoconstriction and minimizing discomfort.
Research complications.
Because infectious tracheobronchitis results in altered respiratory tract histology
and impaired mucociliary clearance, infected animals should not be used for pulmonary
studies. Animals with clinical disease would also be poor surgical candidates.
b.
Group C Streptococcus Infections
Etiology. β-Hemolytic Lancefield's group C streptococcus (Streptococcus zooepidemicus)
is a gram-positive non-spore-forming coccus and an etiologic agent for pneumonia and
septicemia in dogs.
Clinical signs.
Clinical signs vary based on the organ system affected. Pneumonic disease is typically
associated with coughing, weakness, fever, dyspnea, and hematemesis. Peracute death
without clinical signs has been reported in a previously healthy research dog (Bergdall
et al., 1996), and conjunctivitis can also be caused by this organism (Murphy et al.,
1978).
Epizootiology and transmission.
Lancefield's group C streptococci have been isolated as commensal flora in the upper
respiratory tract and the vagina of clinically normal dogs (Olson et al., 1973). Epizootics
have been reported in both racing greyhounds and research colonies (Sundberg et al.,
1981; Garnett et al., 1982). In these epizootics, and in the reported case of peracute
death (Bergdall et al., 1996), recent transportation (within 7 days) was associated
with the disease. As such, Lancefield's group C streptococcus may be an opportunistic
pathogen in dogs.
Pathologic findings.
In the peracute case reported (Bergdall et al., 1996), hemorrhage from the mouth and
nose and within the pleural cavity was the most striking lesion. Ecchymotic and petechial
hemorrhages were seen on other organ surfaces. The lungs were heavy and wet, and blood
oozed from cut surfaces. “Bull's-eye” lesions were observed on the pleural surface
of affected lung lobes, similar to ischemic lesions seen with fungal infections (Fig.
2
). Histologically, the lungs were characterized by areas of hemorrhage surrounding
foci of degenerative neutrophils, blood, and necrotic debris. Gram-positive cocci
were seen in both the lung and the tonsils.
Fig. 2
Bull's-eye lesions seen with peracute Lancefield's group C streptococcal pneumonia
in a random-source research dog. Lesions on the lung surface are characterized by
a ringed pale area and central ulceration that oozed blood.
Pathogenesis.
The pathogenesis for disease caused by Lancefield's group C streptococcus is unclear.
Strain variation with respect to virulence and host immune factors is probably significant.
Diagnosis and differential diagnosis.
Definitive diagnosis is made based on bacterial culture and identification. Any cause
of pneumonia and/or peracute death in dogs needs to be considered as a differential
diagnosis. Bacterial pneumonias or septicemias can be caused by other pathogenic Streptococcus
spp., Staphylococcus spp., Escherichia coli, Pasteurella multocida, Pseudomonas spp.,
Klebsiella pneumoniae, and Bordetella bronchiseptica. Nonbacterial causes include
rodenticide intoxication, coagulopathies, heartworm disease, pulmonary thromboembolism,
ruptured aneurysm, and left-sided congestive heart failure.
Prevention and control.
Too little is known about the pathogenesis of Lancefield's group C streptococcus to
make any recommendations about prevention and control.
Treatment.
Antibiotic therapy should be provided, based on culture and sensitivity. Intravenous
fluids are indicated for febrile or systemically ill patients. For dyspneic patients,
oxygen therapy and strict activity restriction are required.
Research complications.
Clearly, dogs with severe hemorrhagic pneumonia or septicemia are not appropriate
for any research study. The association between epizootics of this disease and transportation
shipment supports the philosophy of providing acclimation periods to animals upon
arrival at research facilities to evaluate health status and enable the animals to
normalize physiologically.
c.
Leptospirosis
Etiology.
Serovars of the spirochete Leptospira interrogans sensu lato cause canine leptospirosis.
Disease in dogs is primarily due to serovars canicola, icterohemorrhagiae, grippotyphosa,
pomona, and bratislava.
Clinical signs.
Leptospirosis may present as either an acute or a chronic problem. Clinical signs
are nonspecific and include lethargy, depression, abdominal discomfort, stiffness,
anorexia, and vomiting. Animals may be febrile and may be reluctant to move, because
of muscle or renal pain or meningitis. Icterus, congested mucous membranes, or signs
referable to disseminated intravascular coagulation (petechial/ecchymotic hemorrhages,
melena, epistaxis, or hematemesis) are also possible. Animals with peracute leptospirosis
are characterized by septicemia, shock, vascular collapse, and rapid death. Uveitis,
abortions, and stillbirths have also been associated with leptospirosis.
Epizootiology and transmission.
Vaccination and reduced exposure to reservoir hosts have markedly decreased the prevalence
of leptospirosis over the past 30 years. Wild animals, cattle, and rodents are reservoirs
for Leptospira. The epidemiology of the disease is not static, and recent changes
have been observed. Serovars pomona, grippotyphosa, and bratislava are becoming more
common causes of canine disease, with canicola and icterohemorrhagiae becoming less
common. This may be due to vaccination practices and increased movement of wildlife
reservoirs (raccoons, skunks, and opossums) into urban/suburban areas. Rats have been
implicated as important in the transmission of serovars canicola and icterohemorrhagiae
(Rentko et al., 1992; Brown et al., 1996; Kalin et al., 1999).
Transmission occurs primarily by environmental contact, and not directly from animal
to animal. Infected hosts shed leptospires in urine, thereby contaminating the environment;
naive animals are infected when the organisms contact mucous membranes or abraded
skin. Recovered animals may shed organisms in their urine for months to years. The
organisms are actually labile in the environment; moisture, moderate temperatures,
and alkaline soil favor survival and subsequent transmission. Close contact, bites,
ingestion of infected meat, and transplacental and venereal transmission are also
possible. Leptospirosis is a zoonotic disease.
Pathologic findings.
The kidneys consistently have gross and microscopic lesions. In the acute phase of
the infection, kidneys are swollen and have subcapsular and cortical ecchymotic hemorrhages.
Petechial or ecchymotic hemorrhages and swelling of the lungs and liver may also be
noted. Hepatic lesions during the acute phase consist of diffuse hemorrhage and focal
areas of necrosis (Searcy, 1995). In chronic stages of leptospirosis the kidneys become
small and fibrotic. Endothelial cell degeneration and focal to diffuse lymphocytic–plasmacytic
interstitial nephritis are the characteristic histopathological findings.
Pathogenesis.
Infection occurs after the leptospires penetrate a mucous membrane or abraded skin.
The organisms then invade the vascular space and multiply rapidly. Several days postinfection
the renal tubular epithelium (and, to a variable extent, the liver) is colonized.
The hematogenous phase lasts 4–14 days. Acute renal failure or progressive renal failure
leading to oliguria or anuria may occur. The most common clinical syndrome is chronic
or subclinical infections after recovery from the acute phase (Greene, 2000). The
nephritis may or may not be accompanied by hepatitis, uveitis, and meningitis. Icterus,
if it develops, is most common in the acute phase. The combination of azotemia and
icterus should alert the clinician to the possibility of leptospirosis. Disseminated
intravascular coagulation is often a secondary complication. The severity and course
of leptospirosis depend on the causative serovar and the age and immune status of
the patient.
Diagnosis and differential diagnosis.
Zinc toxicity in dogs most closely mimics the clinical syndrome of leptospirosis.
Other causes of acute and chronic renal failure, icterus, and acute hepatic failure
must also be considered. Paired serology is the most reliable means of definitive
diagnosis; however, seroconversion may not occur until after the first week of infection.
Prevention and control.
Vaccination for leptospirosis is standard veterinary practice. Bivalent inactivated
bacterins for serovars of L. interrogans canicola and serovars of L. interrogans icterohemorrhagiae
are commercially available. However, immunization does not prevent development of
the carrier state or protect against other serovars. For outdoor-housed dogs, an effective
program to prevent contact with wildlife reservoirs is important. Control requires
identification and either treatment or elimination of carrier animals.
Treatment.
Penicillins are the drugs of choice for treating leptospiremia, and prompt use reduces
fatal complications. Aggressive fluid therapy and supportive care may also be needed.
Elimination of renal colonization and the carrier state can be accomplished with dihydrostreptomycin
or doxycycline administration.
Research complications.
Dogs with clinical leptospirosis should not be used in research studies because of
the effects of the disease on renal and hepatic function.
d.
Campylobacteriosis
Etiology.
Campylobacteriosis in dogs is caused by Campylobacter jejuni, a thin, curved or spiral,
microaerophilic, thermophilic motile gram-negative rod.
Clinical signs.
Most adult animals infected with C. jejuni are asymptomatic carriers; clinical signs
are most commonly noted in dogs that are less than 6 months of age (Greene, 2000;
Burnens et al., 1992). In cases of clinical illness, small volumes of mucoid or watery
diarrhea, with or without frank blood, are most commonly noted. These signs are usually
mild, may be intermittent, and typically last 5–21 days. Tenesmus, inappetance, vomiting,
and a mild fever may accompany the diarrhea.
Epizootiology and transmission.
The role of C. jejuni as a primary pathogen has been questioned; it may require a
coen-teropathy to produce disease (Sherding and Johnson, 1994). Clinical signs of
disease most often occur in dogs less than 6 months of age, although any age may be
affected. Stress or immunosuppression may make animals more susceptible to clinical
disease. Pound and shelter populations have the highest rates of fecal excretion of
C. jejuni (Sherding and Johnson, 1994). Transmission is via the fecal–oral route,
mostly through fecally contaminated food or water. Unpasteurized milk, poultry, and
meat are other sources of infection. Campylobacter jejuni can be zoonotic; children
and immunocompromised individuals are at the greatest risk.
Pathologic findings.
The actual lesions observed depend upon the mechanism of the enteropathy (Van Kruiningen,
1995). Enterotoxin production results in dilated fluid-filled bowel loops, with little
or no histopathologic alteration. In cytotoxin-medi-ated disease, hyperemia and a
friable, hemorrhagic mucosal surface are noted. On histopathology the mucosal surface
is irregular and ulcerated, and a lymphocytic–plasmacytic ileitis or colitis may be
seen. When translocation occurs, the lamina propria becomes edematous and congested,
with focal accumulation of granulocytes in the crypts and lamina propria. Focal areas
of epithelial hyperplasia and decreased numbers of goblet cells are also noted. With
Warthin–Starry silver staining, C. jejuni may be seen between enterocytes but only
rarely inside them.
Pathogenesis.
Clinical disease may be produced by several different mechanisms after the Campylobacter
has populated the intestinal tract (Van Kruiningen, 1995). After colonization of the
enterocyte surface, C. jejuni can produce an enterotoxin that causes a secretory diarrhea.
Campylobacter jejuni can also cause an erosive enterocolitis by invasion of the ileal
and colonic epithelium along with production of a cytotoxic agent; this may be the
mechanism that causes hematochezia. In addition, C. jejuni can produce illness by
translocation, i.e., multiplication in the lamina propria and transportation to regional
lymph nodes by macrophages. This causes mesenteric lymphadenitis.
Diagnosis and differential diagnosis.
Fresh feces (per rectum) are best for ensuring an adequate diagnostic sample. Presumptive
diagnosis may be made by demonstration of highly motile curved or spiral organisms
with dark-field or phase-contrast microscopy. Gram-stained C. jejuni appear as gull-winged
rods. Definitive diagnosis requires isolation of the organism (Sherding and Johnson,
1994). Culture requires selective isolation media, and growth is favored by reduced
oxygen tension and a temperature of 42° C. Any disorder that can cause diarrhea in
dogs should be considered as a differential diagnosis, including canine parvovirus,
coronavirus, distemper virus, Giardia, and Salmonella infections; helminth infestations;
and hemorrhagic gastroenteritis.
Prevention and control.
Proper environmental sanitation, waste disposal, and food storage can prevent campylobacteriosis.
In enzootic situations, group housing should be avoided. Outbreaks are controlled
by isolation and treatment of affected individuals.
Treatment.
Neomycin, enrofloxacin, chloramphenicol, and doxycycline are all effective antibiotics.
Treatment should be for 10–14 days, and bacterial cultures should be repeated 1 and
4 weeks after treatment.
Research complications.
Dogs with clinical campylobacteriosis have altered intestinal histology and temporary
derangements to digestive and absorptive functions.
e.
Monocytic Ehrlichiosis
Etiology.
Monocytic ehrlichiosis is caused by Ehrlichia canis, a small, pleomorphic, gram-negative
bacteria in the family Rickettsiaceae. Monocytic ehrlichiosis has been historically
referred to as canine tropical pancytopenia, tracker dog disease, canine hemorrhagic
fever, or canine typhus.
Clinical signs.
Based on experimental infections in dogs, three phases to the disease have been described:
acute, subclinical, and chronic. Clinical signs observed vary with the phase of the
disease, and the acute and subclinical phases are often missed or misdiagnosed (C.
G. Couto, personal communication, 1993; Waddle and Littman, 1988; Woody and McDonald,
1985). A history of tick exposure may be noted prior to onset of signs.
In the acute phase, clinical signs range from mild to severe and may last 1–2 weeks.
They include inappetance, lethargy, fever, generalized lymphadenopathy, hepatosplenomegaly,
exercise intolerance or dyspnea, petechial or ecchymotic hemorrhages, and peripheral
edema. Central nervous system (CNS) signs may also be present such as hyperaesthesia,
myoclonus, and cranial nerve deficits. Clinical laboratory abnormalities noted during
the acute phase include thrombocytopenia, anemia, neutropenia or neutrophilia, and
bicytopenia or pancytopenia. Hyperplastic bone marrow, mild hyperglobulinemia, and
elevated hepatic enzymes may be noted during this phase (Kuehn and Gaunt, 1985).
Clinical signs are generally absent during the subclinical phase. Mild thrombocytopenia,
anemia, or leukopenia may be seen. The chronic phase develops 1–4 months after the
initial infection, and signs may be subclinical to severe. An extremely varied clinical
picture can emerge during this time and can mimic several other clinical syndromes.
The following constellation of clinical signs may be observed: chronic lethargy, weight
loss, inappetance or anorexia, fever, generalized lymphadenopathy, hepatosplenomegaly,
petechial or ecchymotic hemorrhages, epistaxis, hematuria, melena, pallor, anterior
or posterior uveitis, chorioretinitis, peripheral edema, ataxia, upper and lower motor
neuron deficits, altered mentation, cranial nerve deficits, and seizures.
Persistent thrombocytopenia is the most consistent laboratory abnormality noted for
all three stages. Many other hematologic abnormalites may be found, such as regenerative
or nonregenerative anemia (more frequently the latter), positive Coombs’ test, bicytopenia
or pancytopenia, and splenic plasmacytosis or lymphocytosis. On bone marrow evaluation,
plasmacytosis along with hypoplasia of erythroid, myeloid, and/or megakaryocyte lines
may be seen. Hyperglobulinemia as a result of polyclonal or occasionally monoclonal
gammopathy has been noted in 50–100% of E. canis seropositive or infected dogs (Kuehn
and Gaunt, 1985; Breitschwerdt et al., 1987; Shimon et al., 1996). Proteinuria and/or
hypoalbuminemia have also been seen.
Epizootiology and transmission.
Ehrlichia canis is an obligate intracellular parasite that infects mononuclear cells.
The definitive hosts are arthropods; domestic and wild canids are parasitized secondarily.
The primary vector and reservoir is the brown dog tick, Rhipicephalus sanguineus.
Ehrlichia canis is found worldwide and follows the distribution of the vector. Infection
in dogs is most prevalent in tropical and subtropical areas (Greene, 1991). In the
United States, cases are concentrated in the southeastern and southwestern states
but have been reported in almost every state (Breitschwerdt, 2000). Transmission is
primarily by tick bites, but it can also occur via blood transfusions from dogs infected
for as long as 5 years. Ticks become infected by feeding on an infected dog that is
in the first 10–15 days of an acute infection (Lewis et al., 1977), and ticks can
shed the organisms for up to 5 months. Within the tick population, E. canis is transmitted
transstadially (within developmental stages) but not transovarially (from female to
offspring) (Groves et al., 1977).
Pathogenesis.
In experimental infections, the incubation period prior to the onset of the acute
phase is 7–21 days. During the acute phase, which can last from 2–4 weeks, the bacteria
replicate within circulating and tissue monocytes, resulting in lymphoreticular hyperplasia
in affected tissues. Infected monocytes then spread hematogenously to other organs
in the body, in particular the lungs, kidney, and meninges. Infected cells adhere
to the vascular endothelium and induce vasculitis, which is the primary mechanism
whereby the organism causes disease. The thrombocytopenia during the acute phase is
due to both sequestration and destruction, and the development of anemia is a result
of red blood cell destruction and suppression of erythrocyte production.
The subclinical phase of the disease occurs 6–9 weeks after initial infection. During
this stage, dogs that can mount an effective immune response clear the infection.
Those that cannot mount such a response progress to the chronic stage. Infection does
not confer protective immunity in dogs that recover. German shepherds and Doberman
pinschers seem to be more severely affected than other breeds.
Pathologic findings.
Gross lesions are varied and change, depending on the phase of the disease. The most
common findings are petechial and ecchymotic hemorrhages and edema of dependent tissues
(Woody and Hoskins, 2000). The most common histologic abnormality noted is lymphocytic-plasmacytic
inflammation of numerous organs. Mononuclear phagocytic system hyperplasia, extramedullary
hematopoiesis, and splenic erythrophagocytosis may also be seen.
Diagnosis and differential diagnosis.
The most sensitive, specific, and commonly employed method for diagnosing E. canis
infections is the indirect fluorescent antibody (IFA) test. Antibodies can be detected
as early as 7 days postinfection, although some dogs may not seroconvert until 28
days postinfection (Buhles et al., 1974). Cross-reaction may occur between E. canis,
E. chaffeensis, and E. ewingii. Titers greater than 1:10 are considered positive and
indicative of infection and may persist for up to 1 year. Effective treatment typically
produces seronegative results in 6–9 months. In some cases, asymptomatic dogs may
remain seropositive for years after treatment or may be seropositive with a persistent
hematologic abnormality (Bartsch and Greene, 1996). The exact mechanism for this finding
has not been elucidated. Ehrlichia canis morulae can be demonstrated in circulating
monocytes of Giemsa-stained blood smears. However, this method is labor-intensive
and has low sensitivity, as morulae are present transiently and in low numbers. Using
buffy coat smears from capillary blood may increase the diagnostic yield. Polymerase
chain reaction (PCR) assays are also available to identify E. canis. Differential
diagnoses include immune-mediated hemolytic anemia/thrombocytopenia, multiple myeloma,
chronic lymphocytic leukemia, and lymphoma.
Prevention.
Preventing laboratory animals from contacting ticks is the primary means to avoid
monocytic ehrlichiosis in research dogs. Avoid exercising dogs in areas infested with
ticks. Use topical acaricides to prevent tick infestations. Keep kennel areas tick-free.
Dogs used as blood donors and dogs from unproven sources should be tested for E. canis.
Treatment.
Doxycycline is the drug of choice for treating monocytic ehrlichiosis. Oral doses
of either 2.5–5 mg/kg ql2 hr or 10 mg/kg q24 hr for 21 days are very effective at
eliminating the organism. Tetracycline, chloramphenicol, and enrofloxacin are also
effective antibiotics; however, chloramphenicol should not be used in animals with
cytopenias. In chronic cases, antibiotic treatment should be extended for an additional
1–2 weeks.
Research complications.
The most significant research complication is the thrombocytopenia that persists for
all stages of the disease. Additionally, there is probable alteration in immune function
and increased susceptibility to infectious agents. For these reasons, dogs positive
for antibodies to E. canis should not be used in research.
f.
Thrombocytic Ehrlichiosis
Etiology.
This disease, caused by Ehrlichia platys, was first described as cyclic thrombocytopenia
by Harvey et al. in 1978.
Clinical signs.
In most cases, infection with E. platys results in subclinical disease. A generalized
lymphadenopathy may be noted.
Epizootiology and transmission.
The vector for E. platys is assumed to be a tick; however, this mode of transmission
has not been established. Experimental studies by Simpson et al. (1991) failed to
demonstrate Rhipicephalus sanguineus as a vector for E. platys. Coinfection with E.
canis has been reported, which suggests a common vector for both organisms (French
and Harvey, 1983; Kordick et al., 1999). Dogs have been experimentally infected by
inoculation with infected blood or infected platelets from other dogs (Harvey et al.,
1978; Gaunt et al., 1990). The geographic distribution of thrombocytic ehrlichiosis
is assumed to follow that of other Ehrlichia organisms. The highest concentration
of cases seems to be in southeastern states, but isolated cases have been reported
as far north as Michigan and as far west as Oklahoma (Wilson, 1992; Mathew et al.,
1997). The prevalence of seropositive dogs can be high in some parts of the country.
A study by Bradfield et al. (1996) reported that 74% of the dogs entering a research
institute's quarantine facility from sources in eastern North Carolina were seropositive
for E. platys.
Hoskins et al. (1988) reported a 54.2% seropositive prevalence in healthy dogs from
kennels in Louisiana.
Pathologic findings.
Gross and histopathologic findings during experimental E. platys infection in dogs
have been described by Baker et al. (1987). Generalized lymphadenopathy was the only
gross lesion noted. Follicular hyperplasia and plasmacytosis were the predominate
findings in lymphoreticular tissues. All dogs also had extramedullary hematopoiesis,
erythrophagocytosis, and crescent-shaped hemorrhages in the spleen. Multifocal Kupffer's
cell hyperplasia was noted in the liver, and mild multifocal lymphocytic-plasmacytic
interstitial inflammation was seen in the kidneys.
Pathogenesis.
The pathogenesis of E. platys in dogs has primarily been determined through experimental
infection (Harvey et al., 1978). After inoculation the organism directly infects platelets.
Thrombocytopenia occurs by day 10–14 and fluctuates, along with parasitemia, at 10
to 14 day intervals. In some cases the rebound may be within the normal range for
thrombocyte counts. The nadir can be lower than 20,000 platelets/μ1. Concurrent with
low platelet counts is the development of megakaryocytc hyperplasia in the bone marrow.
Interestingly, despite extremely low platelet counts, spontaneous bleeding has not
been reported in cases of E. platys infection. The mechanism responsible for the cyclic
nature of the infection has not been elucidated.
Diagnosis and differential diagnosis.
Ehrlichia platys infection may be diagnosed on stained blood smears by visualization
of the organisms within platelets. However, this method is very unreliable due to
the cyclic nature of the parasitemia and the low numbers of infected thrombocytes.
Available IFA assays are much more sensitive and specific, and there is reportedly
no serologic cross-reaction with other Ehrlichia species. Dogs usually develop detectable
titers 2–3 weeks postinfection. PCR assays for E. platys have now been developed as
well (Chang and Pan, 1996; Mathew et al., 1997). Differential diagnoses for thrombocytic
ehrlichiosis include E. canis infection, immune-mediated thrombocytopenia, and disseminated
intravascular coagulation (DIC).
Prevention and treatment.
Prevention and treatment for E. platys is the same as described for E. canis, above.
Research complications.
Ehrlichia platys infection may increase the risk of bleeding during surgical or traumatic
procedures. Coinfection with E. platys may potentiate the pathogenicity of other infectious
agents, in particular E. canis (Breitschwerdt, 2000).
g.
Borreliosis (Lyme disease)
Etiology.
Lyme disease is caused by Borrelia burgdorferi sensu lato, a microaerophilic spirochete
that is primarily an extracellular pathogen.
Clinical signs.
Clinical signs may be highly variable; lameness due to polyarthritis has been reported
as the most common sign. The onset of lameness may be acute or chronic, shift from
limb to limb, and be accompanied by swelling and joint pain. Synovial fluid analysis
from affected joints is consistent with a diagnosis of suppurative arthritis. Other
clinical signs include fever, anorexia, lethargy, lymphadenopathy, and weight loss.
Over the course of the disease, signs may wax and wane over a period of weeks to months.
Dogs rarely develop erythema chronicum migrans (the characteristic rash seen in infected
people) and do not exhibit the severe arthritis and neurologic sequelae seen in human
beings (Greene, 1991; Manley, 1994). Hematologic and biochemical profiles are generally
unremarkable.
Epizootiology and transmission.
Lyme disease is thought to be the most common arthropod-borne disease of human beings
(and possibly of dogs) in the United States. It affects humans and dogs worldwide.
The geographic distribution of canine borreliosis is assumed to follow that of the
human disease and is related to the range of the arthropod vectors. Three major endemic
foci that have been identified in the United States account for 94% of reported human
cases (Appel and Jacobson, 1995). The distribution of these cases is as follows: Northeast/mid-Atlantic
focus, 85%; midwestern focus (Michigan, Wisconsin, Minnesota, Iowa, Illinois, and
Missouri), 10%; and California and Oregon, 4%. For the most part, dogs in the remainder
of the country are not at risk for contracting Lyme disease.
Borrelia burgdorferi is transmitted exclusively by Ixodes ticks. Other arthropod hosts
may carry the organism but have not as yet been implicated in the transmission of
disease. Ixodes scapularis, a three-host tick with a 2 to 3 year life cycle, is the
prototypical vector for North America. The spirochetes are spread by tick bites from
both nymphs and adults. Ticks become infected by feeding on an infected mammal and
by transstadial transmission (transovarial passage is rare). In endemic areas, 50–80%
of adult ticks may be infected (Appel and Jacobson, 1995). The primary reservoir for
the organism is the white-footed deer mouse, Peromyscus leucopus, which can carry
spirochetes for its life span without becoming ill. Evidence also indicates that the
eastern chipmunk, Tamias striatus, is an important reservoir (Slajchert et al., 1997),
and birds may also be a significant reservoir. Deer, however, serve only as hosts
for the tick vectors and not as a reservoir for the spirochete.
Pathogenesis.
The pathogenesis of Lyme disease is poorly understood, primarily because of a lack
of good animal models and the chronic nature of the disease. Infection can be induced
experimentally by the bite of a single infected tick. Clinical signs develop 60–90
days postinfection. Some evidence points to the host's inflammatory response to the
organism as etiologic for disease (Pershing et al., 1994; Greene, 1991). Seroconversion
in dogs occurs 4–6 weeks after infection with B. burgdorferi. Antibody titers may
remain extremely elevated for at least 18 months. IgM titers also remain elevated
for several months and are indicative of neither acute nor active infection (Appel
and Jacobson, 1995). Because antibiotic treatment may not eliminate the organism,
persistent infections in dogs (treated for 30 days with antibiotics) can be reactivated
by steroid treatment up to 420 days postinfection (Straubinger et al., 1998).
Diagnosis and differential diagnosis.
Appel and Jacobson (1995) recommend that three of the following four criteria be met
to establish a diagnosis of Lyme disease in dogs: (1) history of exposure to Ixodes
ticks in an endemic area, (2) characteristic clinical signs, (3) positive serology,
and (4) rapid resolution of clinical signs with antibiotic therapy. IFA or ELISA tests
for Borrelia antibodies are the assays of choice. It should be remembered, however,
that a positive titer in an endemic area indicates exposure and not necessarily disease
and that vaccinated dogs will also have a positive titer. Responses to vaccine versus
infection may be distinguished by Western blot. Culture or identification of the organism
provides a definitive diagnosis but is very difficult to perform. Differential diagnoses
include immune-mediated polyarthritis and septic arthritis from other etiologic agents.
Prevention and control.
Prevention and control are the same as for the other tick-borne diseases (see discussion
of monocytic ehrlichiosis, Section III,A,1,e above). A vaccine against B. burgdorferi
is available but should not be necessary in a research setting.
Treatment.
Doxycycline is the drug of choice for treating Lyme borelliosis. A typical dosing
regimen is 10 mg/kg ql2 hr for 3–4 weeks. Amoxicillin, tetracycline, and the quinolones
are also effective. Of significant note is that antibiotic treatment results in resolution
of clinical signs but may not result in elimination of the organism.
Research complications.
No complications to research have been reported. However, clinically infected dogs
are probably not appropriate models for orthopedic or rheumatological research.
h.
Helicobacteriosis
Etiology.
Helicobacters are gram-negative, microaerophilic, spiral bacteria that typically reside
in and infect the gastrointestinal tract. The following species are considered to
naturally infect dogs: Helicobacter felis, “H. heilmannii,” H. bizzozeronii, H. (Flexobacter)
rappini, H. canis, and H. bilis (Fox and Lee, 1997). “Helicobacter heilmannii” and
H. bizzozeroniiare thought be the same species, with the latter being the updated
nomenclature. This species, as well as H. rappini and H. canis, is considered to be
zoonotic (Fox and Lee, 1997).
Clinical signs.
Most infections are subclinical in the dog. Clinical infections may present with vomiting,
diarrhea, fever, and anorexia, pica, or polyphagia.
Epizootiology and transmission.
The epizootiology and transmission of Helicobacter spp. in the dog remains to be elucidated.
The prevalence of canine Helicobacter infections in colony or shelter situations has
been reported to range from 82% to almost 100% (Fox, 1995; Hermanns et al., 1995).
Both oral–oral and fecal–oral routes for transmission have been suggested.
Pathologic findings.
No gross lesions are noted; the primary lesion is that of histologic gastritis. This
is typically characterized by reduced mucus content of the surface epithelium; vacuolation,
swelling, karyolysis, and karyorrhexis of parietal cells; and multifocal infiltrates
of plasma cells and neutrophils into the subepithelium, primarily around blood vessels
and between the gastric pits (Hermanns et al., 1995). Focal areas of lymphocytic inflammation
and lymphoid follicles may also be seen.
Pathogenesis.
Some Helicobacter spp. colonize the gastric epithelium exclusively and other species
colonize lower parts of the gastrointestinal tract. Helicobacter felis and “H. heilmannii”
infections have been linked to gastric lesions in laboratory-raised beagles (Fox and
Lee, 1997). The mechanism by which these organisms cause disease may be related to
the host's inflammatory response to colonization and the Helicobacter's ability to
produce urease. Urease splits urea into ammonia and bicarbonate; ammonia is toxic
for the epithelial cells, and bicarbonate may help the organism survive the acidic
environment (Marshall et al., 1990; Shimoyama and Crabtree, 1998).
Diagnosis and differential diagnosis.
Any of the numerous causes of acute or chronic vomiting and diarrhea in the dog (including
canine distemper, viral or bacterial gastroenteritis, and ingested toxicants) should
be considered as differential diagnoses. Definitive diagnosis for dogs requires either
endoscopic or surgical biopsy. Confirmation of infection with Helicobacter spp. requires
demonstration of the organism in biopsy samples by histopathology, culture, or recognition
by PCR. A positive urease test on a biopsy sample may give a presumptive diagnosis,
but only for those species that produce urease. The use of Warthin-Starry silver stain
may increase the sensitivity for histopathologic diagnosis.
Prevention and control.
Until more is known about the epizootiology and transmission of Helicobacter spp.
in the dog, specific recommendations cannot be made about prevention and control in
this species.
Treatment.
Combination therapy has proven to be the most effective method for treating Helicobacter
spp. infections in dogs. Combination therapy of amoxicillin (10 mg/kg ql2 hr), metronidazole
(30 mg/kg q24 hr), and sucralfate (0.25–0.5 mg/kg q8 hr) for 21 days has been suggested
for dogs (Hall and Simpson, 2000). Replacing the sucralfate with famotidine (0.5 mg/kg
q24 hr), omeprazole (0.3 mg/kg q24 hr), or bismuth subsalicylate (0.2 ml/kg q4–6 hr)
may also be effective (Marks, 1997; Jenkins and Bassett, 1997; DeNovo and Magne, 1995).
The benefits of antimicrobial therapy in dogs still need to be established by controlled
therapeutic studies.
Research complications.
Helicobacter spp. infections could result in altered gastrointestinal responses to
drugs and toxic or carcinogenic compounds. Therefore, dogs used in gastric physiology
or oral pharmacology studies should be free from helicobacteriosis.
2.
Viral and Chlamydial Diseases
a.
Canine Parvovirus
Clinical signs.
Clinical signs of canine parvovirus usually appear 5 days after inoculation by the
fecal-oral route and are characterized by anorexia, fever, depression, and vomiting.
Profuse, intractable diarrhea ensues, which may become hemorrhagic. Approximately
85% of affected dogs develop severe leukopenia, with a total granulocyte/lymphocyte
count ranging from 500–2000 WBC/μl or less. Repeated hemograms may provide prognostic
value, because rebounds in leukocyte counts are indicative of impending recovery.
Terminally ill dogs may develop hypothermia, icterus, or disseminated intravascular
coagulation due to endotoxemia.
Epizootiology and transmission.
Parvovirus can infect dogs of any age, but puppies between 6 and 20 weeks of age appear
to be particularly susceptible. Puppies less than 6 weeks of age are generally protected
from infection by passive maternal antibody. Adult dogs probably incur mild or inapparent
infections that result in seroconversion.
Pathogenesis.
Canine parvovirus has an affinity for rapidly dividing cells of the intestine and
causes an acute, highly contagious enteritis with intestinal crypt necrosis and villus
atrophy. The virus also has tropism for the bone marrow and lymphoid tissues; thus
leukopenia and lymphoid depletion accompany the intestinal destruction.
Diagnosis and differential diagnosis.
Parvovirus can be detected in fecal samples with a commercially available ELISA from
CITE. At necropsy, diagnosis is based on gross and histopathologic evidence of necrosis
and dilatation of intestinal crypt cells with secondary villous collapse. Other lesions
include myeloid degeneration and widespread lymphoid depletion. Parvovirus can also
be demonstrated in frozen sections by fluorescent antibody techniques. Differential
diagnoses should include other viral enteritides, salmonellosis, and small intestinal
obstruction.
Prevention and control.
Prevention of transmission begins with isolation of affected animals and quarantine
for 1 week after full recovery. Disinfection of potentially infected kennel and diagnostic
areas with diluted bleach (1:30) or commercially prepared disinfectant (such as Kennesol,
available from AlphaTech, Lexington, Massachusetts) is essential for elimination of
the virus. Six-week-old puppies should be vaccinated every 2–4 weeks with a commercially
available modified live vaccine until 16–18 weeks of age. Young Rottweilers and Doberman
pinschers appear to be predisposed to parvoviral enteritis and should be vaccinated
every 3 weeks (5 times) from 6–18 weeks of age.
Treatment.
Treatment is largely supportive and is aimed primarily at restoring fluid and electrolyte
balance.
Research complications.
Infection with parvovirus obviously precludes the use of a particular dog in an experimental
protocol. Given the potential for significant discomfort of the affected animal, and
the cost of therapy, humane euthanasia is usually the option chosen in a research
setting.
b.
Canine Coronavirus
Canine coronavirus infection is usually inapparent or causes minimal illness. This
epitheliotropic virus preferentially invades the enterocytes of the villous tips,
resulting in destruction, atrophy, and fusion and subsequent diarrhea of varying severity.
Subclinical infections are most common, but abrupt gastrointestinal upset accompanied
by soft to watery, yellow-orange feces is possible. Definitive diagnosis by virus
isolation or paired sera is usually not made, because supportive therapy generally
results in rapid resolution of the diarrhea. Inactivated coronavirus is present in
commercially available combination vaccines, which are administered immunoprophylactically
at 6–8, 10–12, and 12–14 weeks of age and then annually thereafter. The role of these
vaccines in protection from coronaviral infection is unknown, because the virus typically
causes inapparent or mild illness (Hoskins, 1998).
c.
Canine Distemper
Etiology.
Canine distemper virus (CDV) belongs to the family Paramyxoviridae, within the genus
Morbillivirus, which includes human measles virus and rinderpest virus of ruminants.
Clinical signs.
Although there is only one serotype of CDV, there is a wide difference in strain virulence
and tissue tropism. Some strains produce mild clinical signs that are similar to tracheobronchitis,
whereas other strains cause generalized infections of the gastrointestinal tract,
integument, and central nervous system, resulting in enteritis, digital hyperkeratosis,
and encephalitis, respectively. Other factors contributing to the severity and progression
of clinical signs include environmental conditions, immune status, and age of the
host. A transient subclinical fever and leukopenia occur 4–7 days after exposure,
with a subsequent fever spike 7–14 days later, accompanied by conjunctivitis and rhinitis.
Other clinical signs associated with acute distemper include coughing, diarrhea, vomiting,
anorexia, dehydration, and weight loss. Secondary bacterial infections can cause progression
to mucopurulent oculonasal discharge and pneumonia. An immune-mediated pustular dermatitis
may develop on the abdomen; this is usually a favorable prognostic sign (Greene and
Appel, 1998), because dogs that develop skin lesions often recover.
Neurologic complications of distemper infection may occur weeks to months after recovery
from an acute infection. Dogs that develop late-onset disease are usually immunocompetent
hosts, suggesting that the virus may have escaped complete elimination by the immune
system, possibly because of protective effects by the blood–brain barrier. Classic
neurologic signs that may occur in acute or chronic CDV infection include ataxia,
incoordination, vocalization, “chewing gum” seizures, and myoclonus with or without
paresis of the affected limb. Canine distemper is the most common cause of seizures
in dogs less than 6 months of age. Dogs with extensive neurologic involvement often
have residual clinical deficits, including flexor spasm and olfactory dysfunction.
CDV has also been associated with two forms of chronic encephalitis in mature dogs:
multifocal encephalitis and “old dog encephalitis.”
Epizootiology and transmission.
The virus is highly prevalent and contagious to dogs and other carnivores, especially
at the age of 3–6 months, coincident with the waning of maternal antibody. Transmission
is primarily by aerosolization of infective droplets from body secretions of infected
animals.
Pathologic findings.
The predominant histopathologic lesion in neurologic forms of distemper is demyelination,
which may be accompanied by gliosis, necrosis, edema, and macrophage infiltration.
Acidophilic cytoplasmic inclusions can be found in epithelial cells of mucous membranes,
reticulum cells, leukocytes, glia, and neurons, while intranuclear inclusions are
often present in lining or glandular epithelium and ganglion cells.
Diagnosis and differential diagnosis.
Diagnosis of CDV is based on history of exposure and clinical signs. Young dogs who
have not received routine immunoprophylaxis (or similarly, mature dogs with a questionable
vaccination history) and present with rhinitis, mucopurulent oculonasal discharge,
plus or minus hyperkeratosis of the footpads and neurologic signs, are highly likely
to have CDV. Ophthalmologic examination may reveal chorioretinitis with acute disease
or retinal atrophy in chronic cases. Definitive diagnosis of acute infection can be
made by fluorescent antibody testing of intact epithelial cells from conjunctival
and mucous membranes. Attenuated strains of CDV, found in modified live vaccines,
are not disseminated from lymphoid tissue to epithelial cells and thus are not detected
by the fluorescent antibody. Serologic testing is usually not useful, because dogs
frequently fail to mount a measurable immunologic response. Because of the variety
of clinical signs, there are many differential diagnoses for canine distemper. An
important differential diagnosis for respiratory illness is infectious tracheobronchitis
(kennel cough). Bacterial, viral, and protozoal causes of gastroenteritis must be
considered for cases presenting with vomiting and diarrhea, and rabies, Pseudorabies,
bacterial meningitis, and poisonings are differential diagnoses for dogs with central
nervous system disorder.
Prevention and treatment.
A series of three immunizations from 6 to 14 weeks of age, followed by yearly boosters,
is a recommended preventative. Treatment is largely supportive, but because of the
profound immunologic effects and significant morbidity of CDV, humane euthanasia is
usually undertaken in the research setting.
d.
Canine Herpesvirus
Etiology.
Canine herpesvirus (CHV) infection causes a generalized hemorrhagic disease with a
high mortality rate in newborn puppies less than 2 weeks of age. In adult dogs, CHV
causes a persistent, latent infection of the reproductive tract with recrudescence
and shedding during periods of physiologic stress.
Clinical signs.
Clinically affected puppies do not suckle, cry persistently, become depressed and
weak, and fail to thrive. Petechial hemorrhages of the mucous membranes and erythema
of sparsely haired regions such as the caudal abdomen and inguinal area are evident.
Older puppies, aged 3–5 weeks, develop less severe clinical signs and are likely to
survive with neurologic sequelae such as ataxia and blindness resulting from reactivation
of latent infection. Infection in adult dogs may result in stillbirths, abortions,
and infertility. Lesions in adult bitches include raised vesicular foci in the vaginal
mucosa, accompanied by mild vaginitis. Adult males have preputial discharge due to
vesicular lesions at the base of the penis and on the preputial mucosa.
Epizootiology and transmission.
Transmission occurs during passage of puppies through the birth canal or venereally
in adult dogs. Puppies can also be horizontally infected by littermates. Entire primiparous
litters may be lost, with subsequent litters protected by colostral antibody.
Pathologic findings.
Pathologic findings include multifocal ecchymotic hemorrhages of the kidneys, liver,
lungs, and gastrointestinal tract. Basophilic intranuclear inclusions in necrotic
areas of parenchymal organs are characteristic findings.
Diagnosis and differential diagnosis.
Diagnosis of canine herpesvirus infection in adult dogs is based on a history of reproductive
infertility and the presence of genital vesicular lesions. Differential diagnoses
for stillbirths, abortions, and infertility include canine brucellosis, canine distemper
virus and parvovirus infections, and pyometra. The diagnosis in infected puppies is
usually made based on clinical history and characteristic lesions (multifocal systemic
hemorrhages) (Carmichael and Greene, 1998). Differential diagnoses for the disease
in neonates would include canine ehrlichiosis and causes of disseminated intravascular
coagulation, including bacterial endotoxemia.
Prevention and treatment.
A vaccine is not available, and there is no effective curative treatment. Supportive
therapy is unrewarding, and death usually ensues within 48 hours in infected neonates.
In general, adult bitches that have multiple abortions, stillbirths, or persistent
infertility should be culled from the breeding colony. Examination of these animals
may reveal raised vesicular lesions on the vaginal mucosa. Adult male dogs that have
vesicular lesions on the base of the penis and preputial mucosa should be similarly
culled.
Research complications.
Canine herpesvirus infection in adult dogs would obviously interfere with production
operations, and affected animals should be culled based on the criteria noted above
in the discussion of prevention and treatment. Because of the severity of clinical
illness in puppies, such animals should be humanely euthanatized.
e.
Rabies
Etiology.
Rabies virus is a member of the rhabdovirus family and is essentially contagious to
all species of warm-blooded animals.
Clinical signs.
Clinical progression of neurologic disease occurs in three stages. The first, or prodromal,
stage is characterized by a change in species-typical behavior. The loss of the instinctive
fear of humans by a wild animal is a classic sign of impending rabies. In the second,
or furious, stage animals are easily excited or hyperreactive to external stimuli
and will readily snap at inanimate objects. The third, or paralytic, stage is characterized
by incoordination and ascending ataxia of the hindlimbs due to viral-induced damage
of motor neurons. Death usually occurs within 2–7 days of the onset of clinical signs,
due to respiratory failure.
Epizootiology and transmission.
Wild animals such as raccoons, skunks, and bats are common reservoirs of infection
for domestic animals, which in turn are the principal source of infection for humans.
Transmission occurs primarily by contact of infected saliva from a rabid to a naive
animal (or human), usually via bite wounds.
Pathogenesis.
The incubation period for rabies is generally 3–8 weeks from the time of exposure
to the onset of clinical signs but can range from 1 week to 1 year. Bites of the head
and neck typically result in shorter incubation periods because of the proximity to
the brain. Following infection, the virus migrates centripetally via peripheral nerve
fibers to the central nervous system and eventually to neurons within the brain, resulting
in neurologic dysfunction. On reaching the brain, the virus migrates centrifugally
to the salivary glands, thus enabling shedding and subsequent transmission.
Diagnosis and differential diagnosis.
Diagnosis of rabies is based on clinical signs; differential diagnoses include pseudorabies,
canine distemper, bacterial meningitis, and toxicants that affect neurologic function.
Definitive diagnosis is based on fluorescent antibody demonstration of the virus in
Negri bodies of hippocampal cells.
Prevention and treatment.
Puppies should be vaccinated at 4–6 months of age, “boostered” in 1 year, then vaccinated
annually or triennially, depending on state and local laws and which vaccine product
is used. Treatment of rabies is not recommended, because of the risk of human exposure.
Research complications.
In a research setting, dogs are often not vaccinated for rabies, because of the low
incidence of exposure to wild-animal reservoirs. A healthy, purpose-bred dog that
bites a human in a research facility should be quarantined for 10 days and observed
for signs of rabies. This quarantine interval is based on the knowledge that dogs
do not shed rabies in the saliva for more than a few days before the onset of neurologic
disease. A random-source dog with an unknown vaccination history that bites a human
should be immediately euthanized. The brain should be examined for rabies virus to
determine if the dog was infected, and if the test is positive, postexposure immunization
should be initiated for the human patient. A rabies vaccine licensed for use in humans
is available, and immunoprophylaxis is recommended for animal care and research personnel
who may have high work-related risks of exposure.
3.
Parasitic Diseases
a.
Protozoa
i.
Giardiasis
Etiology.
Giardiasis is a small-intestinal disease of the dog caused by Giardia duodenalis (lamblia),
a binucleate flagellate protozoan.
Clinical signs.
Most Giardia infections are subclinical. When dogs are clinically affected, diarrhea
is the most prominent sign. The diarrhea is a result of intestinal malabsorption and
is often characterized as voluminous, light-colored, foul-smelling, and soft to watery.
Weight loss has also been associated with clinical infection. Clinical illness is
more often seen in young animals.
Epizootiology and transmission.
Giardia has a direct life cycle. Dogs (and people) typically become infected when
they consume water (or food) contaminated with Giardia cysts. The pH change from the
stomach (acid) to duodenum (neutral) causes excystation. Trophozoites migrate to the
distal duodenum and proximal jejunum and attach to the villus surface. Eventually
the trophozoites encyst and pass in the feces to perpetuate the life cycle.
Pathologic findings.
Giardiasis is rarely fatal. On histopathology of duodenal or jejunal specimens, Giardia
trophozoites can be seen attached to enterocytes. Mucosal inflammation and ulceration,
and villous atrophy, have been observed.
Pathogenesis.
The exact pathogenesis of Giardia-induced illness is unknown. It is thought that tissue
invasion, although occasionally observed, is unimportant for pathogenesis. It is suspected
that illness is caused by physical obstruction of enteric absorption, enterotoxicity,
competition for nutrients, excess mucus production, and/or secondary bacterial overgrowth.
Diagnosis and differential diagnosis.
Definitive diagnosis requires observation of the organism in fecal or intestinal samples.
Direct fecal smears are considered best for observing trophozoites, and zinc sulfate
flotation is preferred for detection of cysts. Commercial ELISA kits and direct immunofluorescent
tests are available to detect fecal Giardia antigens, but the diagnostic specificity
and/or sensitivity of these tests may not be sufficient to warrant substitution for
the less expensive direct fecal examination or zinc sulfate preparation (Barr, 1998).
Differential diagnoses for giardiasis include bacterial and protozoal enteritis, coccidiosis,
and whipworm infestation.
Prevention.
High-quality water sources will eliminate the possibility of infection developing
within an animal research facility. Use of dogs with a known husbandry and medical
background will minimize the chances of giardiasis developing in a research colony.
Control.
Once giardiasis has been diagnosed in a canine population, segregation of infected
animals will help to reduce further infection (provided other dogs were not preinfected
at the same source location as the signal case). Disinfection with quaternary ammonium
compounds, bleach, or steam is usually successful in eradication of Giardia cysts.
Treatment.
The most common treatment for giardiasis is metronidazole (Flagyl) at 25–30 mg/kg
per os twice per day for 5–10 days. Quinacrine hydrochloride (Atabrine) at 9 mg/kg
per os once per day for 6 days, furazolidone (Furoxone) at 4 mg/kg per os twice per
day for 7–10 days, and the anthelmintics albendazole and fenbendazole have been proposed
for use against metronidazole-resistant strains of Giardia. Al-bendazole is recommended
at 25 mg/kg per os ql2 hr for 2 days, and fenbendazole at 50 mg/kg per os q24 hr for
3 days. Fenbendazole was thought to be safer for both puppies and pregnant females
(nonteratogenic) (Barr, 1998).
Research complications.
Typical asymptomatic infections probably have no consequence on research protocols,
with the exception of intestinal physiology or immunology studies. Clinical diarrhea
would clearly need to be treated before a dog could be used as a research subject.
ii.
Coccidiosis
Etiology.
Intestinal coccidia that have been associated with enteropathy in dogs include Cystoisospora
canis, C. ohioensis, C. burrowsi, and C. neorivolta.
Clinical signs.
Dogs are typically asymptomatic when infected with intestinal coccidia, and oocysts
are an incidental finding on fecal flotation or direct smear. Dogs that are clinically
infected usually develop diarrhea, which can vary from soft to watery and may contain
blood or mucus. Vomiting, dehydration, lethargy, and weight loss can also be seen.
Epizootiology and transmission.
Cystoisospora oocysts are typically spread by fecal–oral transmission, usually by
ingestion of fecal-contaminated food or other objects in the environment. An indirect
form of transmission is also possible, whereby the dog consumes a rodent or other
animal that is serving as a transport host. Once inside the small intestine, the cyst
releases sporozoites that infect enteric epithelium. Several generations of asexual
reproduction can occur in the enterocyte before sexual reproduction produces gamonts.
The gamonts fuse to become a zygote, which encysts, ruptures the enterocyte, and passes
in the feces. Once in the environment the cyst sporulates and is now an infective
stage for ingestion by another host.
Pathologic findings.
Dogs with coccidiosis may have hyperemia or fluid retention at affected intestinal
segments. The mucosa may appear normal, raised, or ulcerated. Histologically, there
may be necrosis of enterocytes, hyperemia, and submucosal inflammation. The oocysts
are usually readily apparent within the epithelial cells (Van Kruiningen, 1995).
Pathogenesis.
Intestinal coccidia are opportunistic organisms; they do not typically cause illness
unless other predisposing factors are present. Such factors include immunodeficiency,
malnutrition, and/or concurrent disease. Overcrowding and unsanitary conditions can
also promote clinical coccidiosis by providing a high population of infective oocysts
to stressed animals.
Diagnosis and differential diagnosis.
Diagnosis is somewhat difficult, as coccidian oocysts (of both Cystoisospora and non-Cystoisospora
spp.) can be seen on fecal examinations of clinically healthy dogs, as well as animals
with diarrhea. Other causes for diarrhea (e.g., parvovirus, roundworms, Giardia spp.,
Campylobacter jejuni, and inflammatory bowel disease) should be excluded before a
coccidial etiology is implicated.
Prevention.
Clinical coccidiosis can be readily prevented by adhering to proper sanitation guidelines,
reducing any overcrowding, and providing as stress-free an environment as possible.
Treatment.
Treatment for the presence of coccidial oocysts may often not be necessary, because
Cystoisospora infections are typically self-limiting and clinically insignificant.
Treatment may, however, help to limit the number of oocysts shed in a kennel housing
situation and may be necessary in cases of protracted clinical illness. Possible choices
for treatment include daily administration of sulfadimethoxine (25–30 mg/lb per os
for 10 days), trimethoprim sulfa (15 mg/lb per os for 10 days), or quinacrine (5 mg/lb
per os for 5 days). Amprolium, which is not labeled for dogs, can also be used as
a coccidiostat. It can be given in gelatin capsules for 7–12 days at a daily dose
of 100 mg for small-breed pups and 200 mg for larger breeds.
Research complications.
As with any enteric disease, the presence of clinical coccidiosis can cause aberrations
in gastrointestinal physiological parameters. Dogs used in intestinal pharmacokinetic
studies should be confirmed to be free of Cystoisospora infections.
b.
Nematodes
i.
Ascarids
Etiology.
The most common ascarid of dogs is Toxocara canis. Toxascaris leonina can also infect
both dogs and cats.
Clinical signs.
Ascarid infestations are most commonly subclinical. However, large worm burdens can
cause diarrhea, vomiting, dehydration, and abdominal discomfort with vocalization.
Puppies may have a classical “potbellied” appearance and dull hair coat. Heavy infestations
can cause intussusception and/or intestinal obstruction, in which case the young dogs
may be found dead. Visceral larval migrans caused by Toxocara canis can cause pneumonia.
Epizootiology and transmission.
Toxocara canis typically infects puppies. In fact, a unique characteristic of T. canis
is its ability to infect prenatal puppies by transplacental migration, and neonatal
puppies by transmammary migration. Ingestion of infective eggs that have been shed
in the feces is another common route of transmission, and infection by ingestion of
a transport or intermediate host is also possible.
Pathologic findings.
Puppies that die from ascarid infestations typically have large worm populations in
the lumen of the small intestine. Such populations can cause intestinal obstruction
and may also result in intussusception or intestinal perforation. Puppies that experience
lung migrations of large larval worm populations can have severe pulmonary parenchymal
damage and develop fatal pneumonia.
Pathogenesis.
The infective stage of T. canis is the third-stage larva (L3). Infections initiated
by ingestion of infective eggs have three possibilities for larval migration: liver–lung
migration (which leads to intestinal infection), somatic tissue migration, and intestinal
wall migration. Older dogs that become infected typically have an age-related resistance
to liver-lung migration and instead experience the other two migratory patterns. These
larval migrations are often asymptomatic, and progression of the L3 larvae is arrested
in the tissues. It is these larvae that become reactivated in a pregnant bitch, thus
establishing the transplacental and transmammary routes of transmission. If the source
of infection is transplacental, puppies may be born with L3 larvae in their lungs,
because larval migration is already in progress (Sherding, 1989).
Diagnosis and differential diagnosis.
The characteristic large (70–85 μm in diameter) and relatively round ascarid eggs
can be readily diagnosed by standard fecal flotation methods.
Prevention and control.
Monthly administration of milbemycin or ivermectin plus pyrantel pamoate (Heartgard
Plus) is recommended for prevention and control of canine ascarid infestation (Hall
and Simpson, 2000).
Treatment.
Most anthelmintics are effective for treatment of ascariasis. Pyrantel pamoate (Nemex)
and fenbendazole (Panacur) are commonly used. Treatment should be started early in
puppies (2, 4, 6, and 8 weeks) because of the possibility of prenatal or neonatal
infection. Pyrantel pamoate, dosed at 5 mg/kg per os, is safe for puppies and is also
effective in treatment of hookworms (see Section III,A,3,b,ii). In breeding colonies
in which ascarid infestation is a known problem, treatment of the pregnant and nursing
bitch may be advantageous. Extended fenbendazole therapy (50 mg/kg per os twice per
day for 14 days or once per day from day 40 of gestation through day 14 of lactation)
has been shown to be experimentally safe and effective in decreasing ascarid burdens
in puppies.
Research complications.
Puppies with large worm burdens make poor research subjects and should be treated
aggressively before placement on an experimental study.
ii.
Hookworms
Etiology.
The most common and most pathogenic hookworm of dogs is Ancylostoma caninum. Other,
less pathogenic canine hookworms found in North America are A. braziliense, which
can be found in the American tropics and southern United States, and Uncinaria stenocephala,
which is distributed in the northern United States and Canada.
Clinical signs.
Only A. caninum infestation typically results in clinical illness, because of the
amount of blood that it consumes. Puppies with A. caninum infestations are typically
pale and weak (from anemia), with bloody diarrhea or melena. Other clinical signs
include lethargy, anorexia, dehydration, vomiting, and poor weight gain.
Epizootiology and transmission.
Infective larvae (L3) are typically ingested by puppies and develop directly in the
intestinal tract. Ingestion can be from the bitch's milk (transmammary migration occurs
with A. caninum), from food or objects contaminated with infective larvae, or from
ingestion of a paratenic host. Transplacental migration does occur with A. caninum,
but to a much lesser extent than is seen with Toxocara canis. Larvae can also penetrate
intact skin, migrate to the lung via somatic or circulatory routes, and be coughed
and swallowed to reach the intestine. The prepaient period is 3 weeks.
Pathologic findings.
Infected puppies often have severe anemia and eosinophilia. The anemia can be from
acute blood loss or can also be an iron-deficiency anemia caused by chronic blood
loss coupled with limited iron reserves. On gross necropsy, the small-intestinal tract
contains worms admixed with intestinal contents containing fresh or digested blood
(Fig. 3a
). Ulcerative enteritis caused by hookworm attachment is evident on histopathologic
examination, and worms with mouthparts embedded in the mucosa can be identified in
some sections (Fig. 3b).
Fig. 3
(a) Gross necropsy presentation of hookworm infestation in a research dog. Intestinal
lumen contains fresh and digested blood and adult worms. (b) Photomicrograph of hookworm
infestation in a research dog. Longitudinal and cross sections of worms can be seen
within the intestinal lumen, surrounded by erythrocytes, cellular debris, and fecal
material. Magnification: ×10.
Pathogenesis.
The severe pathogenicity of A. caninum is a direct result of its voracious consumption
of blood and body fluids. Each adult hookworm can consume 0.01–0.2 ml of blood; thus
an extensive infection could deplete a puppy of 20 ml of blood per day, which is approximately
15% of the blood volume of a 2.0 kg animal. In contrast, A. braziliense and U. stenocephala
consume 0.001 and 0.0003 ml per worm, respectively.
Diagnosis and differential diagnosis.
Diagnosis of ancylostomiasis is made by identification of eggs or larvae from fecal
samples by either flotation or direct smear. Parvovirus should be considered for puppies
with bloody diarrhea, and autoimmune hemolytic anemia should be considered in the
diagnosis of a young dog with anemia.
Prevention and control.
Purchase of purpose-bred animals will limit the exposure to hookworm larvae, and effective
sanitation programs will easily eradicate the infective larvae. Unlike ascarid eggs,
hookworm eggs are readily killed by drying, sunlight, or cold; however, they do survive
readily in warm, moist environments. Monthly administration of milbemycin or ivermectin
plus pyrantel pamoate (Heartgard Plus) is recommended for prevention and control of
canine ascarid infestation (Hall and Simpson, 2000).
Treatment.
Pyrantel pamoate (Nemex) is the anthelmintic of choice because it is safest in young
ill animals and is also effective against ascarids and other enteric helminths. Because
of the possibility of transplacental or milk-borne infection, puppies should be treated
every 2 weeks from weeks 2–8. A follow-up treatment at 11 weeks is recommended to
kill any larvae that have migrated and matured since the initial therapy. Severely
ill puppies may require supportive fluid therapy and possibly whole blood transfusions
and iron supplementation.
Research complications.
Anemic puppies with large worm burdens make poor research subjects and should be treated
aggressively before placement on an experimental study.
iii.
Strongyloides
Etiology.
Strongyloides stercoralis is a small strongyle that can cause hemorrhagic enteritis
in puppies. It is found in warm, humid climates such as the southeastern United States.
Epizootiology and transmission.
Strongyloides stercoralis infects dogs and other animals by third-stage larval penetration
of the skin or mucous membranes. Larvae migrate via the circulatory system to the
lung and then are coughed and swallowed to initiate the intestinal parasitism. The
eggs of S. stercoralis hatch within the gut lumen, and so it is the first-stage larvae
that pass in the feces and need to be identified by diagnostic examination. Once passed,
the larvae can either develop into the infectious third-stage larvae or mature into
free-living, nonparasitic adults.
Diagnosis and differential diagnosis.
The Baermann procedure is usually performed on fresh feces in order to detect the
motile first-stage larva (280–310 μm × 30–80 μm). The larvae must be distinguished
from larva of Filaroides hirthi and hatched Ancylostoma caninum.
Treatment.
The usual treatment for S. stercoralis is fenbendazole (Panacur) at 50 mg/kg per day
for 5 days.
iv.
Whipworms
Etiology.
Trichuris vulpis, the canine whipworm, can cause acute or chronic large-intestinal
diarrhea. The adult whipworm typically resides in the cecum or ascending colon.
Clinical signs.
Most whipworm infections are subclinical. In symptomatic cases, the typical clinical
sign is diarrhea with blood and/or mucus. Abdominal pain, anorexia, and weight loss
are also seen. Dogs may have eosinophilia, anemia, and/or hypoproteinemia on clinical
hematology. Severe dehydration with electrolyte imbalance has occurred occasionally
as an acute crisis episode.
Epizootiology and transmission.
Trichuris vulpis has a direct life cycle. Adult worms residing in the canine large
intestine intermittently release eggs that pass in the feces. The eggs are very hardy
and can persist for years. In optimal conditions, the eggs develop into an infective
embryo within 10 days. After ingestion by a dog, the larvae hatch in the small intestine,
burrow into the small-intestinal mucosa, and then reemerge several days later to travel
and burrow into the cecal and colonic mucosa. The prepatent period is typically 2–3
months long.
Pathologic findings.
Dogs do not typically die from whipworm infestations. Lesions seen as incidental findings
feature adult worms embedded into the colonic and cecal mucosae, causing local granulomatous
inflammatory reactions and mucosal hyperplasia.
Pathogenesis.
The penetration of the adult worm into the enteric mucosa, and the associated inflammation,
can lead to the clinical development of diarrhea. Factors that influence the possible
development of clinical symptoms are the number and location of adult whipworms; the
severity of inflammation, anemia, or hypoproteinemia in the host; and the overall
condition of the host.
Diagnosis and differential diagnosis.
Whipworm infestation is diagnosed by the presence of characteristic trichurid eggs
on fecal flotation. These eggs are barrel-shaped, with thick walls and bipolar plugs.
Because of the intermittent release of eggs by the adult female worms, negative fecal
flotation does not exclude the possibility of clinical whipworm infection. Adult worms
can be seen on colonoscopy (Jergens and Willard, 2000). Differential diagnoses for
whipworm infestation include giardiasis, coccidiosis, and bacterial enteritis.
Prevention and control.
Trichuris eggs are resistant to disinfection, making control difficult. Dessication
or incineration is the only completely effective means to eradicate whipworm eggs
from the environment.
Treatment.
Fenbendazole, oxibendazole, and milbemycin have all been recommended for treatment
of whipworms. Treatment for whipworm infestation should be at monthly intervals for
3 months (Jergens and Willard, 2000). Treatment is also suggested in cases wherein
whipworm infestation is suspected but not confirmed by multiple fecal flotation. Rapid
response to treatment would be indicative of a correct diagnosis; lack of response
should prompt further diagnostic efforts.
Research complications.
Whipworm infestation has not been documented to interfere with research protocols,
although one would anticpate that aberrations in local enteric immune function and
absorptive functions of the large intestine could result from trichuriasis.
v.
Dirofilariasis (heartworm infestation)
Etiology.
Heartworm disease of dogs is caused by the filarial worm, Dirofilaria immitis. Adult
heartworms reside in the pulmonary artery; severe infestations can result in the presence
of worms in the right ventricle and atrium. Microfilariae, the immature worms produced
by the adults, circulate in the bloodstream until a mosquito (intermediate host) ingests
them.
Clinical signs.
Most heartworm infestations are asymptomatic. The most common clinical signs observed
are coughing and dyspnea. Clinical signs of exercise intolerance and right-sided heart
failure can be seen in severe infestations.
Epizootiology and transmission.
Successful heartworm transmission requires the presence of mosquitoes. For this reason,
random-source dogs or dogs housed in outdoor kennels are much more likely to have
heartworm infestations than indoor, purpose-bred dogs. Mosquitoes become infested
with heartworm microfilariae when they take a blood meal from the dog. The microfilaria
progress through several larval stages within the mosquito, eventually terminating
at the third stage. This stage is then returned to the canine bloodstream during feeding.
This stage matures within the dog's circulatory system, and the adults reside in the
pulmonary artery. Male and female heartworms will then sexually reproduce to create
more microfilariae and propagate the parasitic life cycle. In the United States, transmission
of heartworm by mosquitoes occurs over a 6-month or shorter period, except for the
southeastern and Gulf Coast states. Here, climatic conditions enable longer survival
of the mosquitoes (possibly year-round), thus resulting in the highest prevalence
of heartworm infestation (Knight, 2000).
Pathologic findings.
On necropsy, the small, slender worms can be seen in the pulmonary artery, right ventricle,
and/or right atrium (Fig. 4a
). There may be no histologic abnormalities associated with a minor worm burden, although
typically the arterial endothelium in these areas is hyperplastic (Fig. 4b). Endothelial
cell hyperplasia, vascular smooth muscle hyperplasia, inflammation, and thrombosis
of the pulmonary arteries and arterioles characterize more significant infestations.
Severe infestations can lead to right-sided heart failure and its pathologic sequelae
of ascites, pleural effusion, hepatomegaly, and right heart and pulmonary artery enlargement.
Verminous pulmonary embolism can result from treatment of dogs with anthelmintics
when a worm burden is present. Immune responses to circulating microfilariae can cause
pathologic lesions, most commonly glomerulonephritis.
Fig. 4
(a) Canine lung with adult heartworms occluding the pulmonary artery. (b) Intimal
proliferation in the pulmonary artery of a dog, caused by dirofilariasis. Magnification:
×25.
Pathogenesis.
The physical presence of the worms in the pulmonary artery is partially responsible
for clinical signs observed in severe cases. However, the host immunologic response
to this infestation, coupled with secretion by the heartworms of physiomodulative
factors, contributes significantly to the complications seen with this disease. Endothelial
cell proliferation, damage, and sloughing stimulates periarteritis and proliferation
of the vascular media of pulmonary arteries and arterioles. These changes lead to
thrombosis of these vessels and the arterial truncation that can be seen radiographically
in severe infestations. The heartworms also release circulating factors that affect
vascular tone and can promote bronchoconstriction (Dillon, 2000). These factors are
discussed in more detail below, under “Research complications.”
Diagnosis and differential diagnosis.
For dogs used in biomedical research, diagnosis of asymptomatic heartworm disease
is important, especially if the dogs are used in cardiovascular, pulmonary, or long-term
studies. A diagnosis of dirofilariasis is typically made by detection of adult heartworm
antigens in a blood sample. Use of adult heartworm antigen tests has virtually eliminated
the historical status of “occult” heartworm disease, which was caused by infestation
of adult worms without corresponding microfilarial circulation. Commercial test kits
that assay for the presence of adult heartworm antigens, and designed for use by veterinary
practitioners, are readily available. False-negative results can occur during the
prepatent period after initial infection (first 6–7 months), and when the adult worm
burden is light or predominantly male. Infections consisting of more than three mature
female worms are usually detected by antigenic serology (Knight, 2000). A significant
feature of these tests for circulating antigen is that they have a very high specificity
(low rate of false-positive results). If a dog were negative on initial testing because
of prepatency or small worm burden, it will more than likely be detected on a follow-up
test 7 months later.
Examination for circulating microfilariae could be used to confirm an antigenic diagnosis
of dirofilariasis or to establish that microfilarial production had occurred. Microfilarial
detection can be done by microscopic examination of the buffy coat of a microhematocrit
tube or by concentration techniques, such as the modified Knott test and filter tests.
Tests that examine for microfilariae have the inherent problem of false positives
caused by microfilariae of Dipetalonema reconditum, a nonpathogenic filarial worm.
Other serologic diagnostic tests that were more common historically, and that may
still be useful, include detection of antibodies to either adult heartworm antigens
or microfilarial antigens.
These same techniques can be used to diagnose clinical heartworm disease. Additional
diagnostic tests that can augment a diagnosis of clinical heartworm disease include
thoracic radiography (pulmonary artery and right-heart enlargement), electrocardiography
(right-heart enlargement), and hematology (eosinophilia). Differential diagnoses for
symptomatic heartworm disease (coughing, dyspnea, and exercise intolerance) include
canine distemper, canine infectious tracheobronchitis
(complicated), streptococcal or other bacterial pneumonia, nocardiosis, and congestive
heart failure.
Prevention and control.
For dogs used in biomedical research, prevention is primarily via insect control and
housing of the dogs in a controlled, indoor environment. Purpose-bred dogs reared
in such an environment are usually free from dirofilariasis. However, any dog (random-source
or purpose-bred) exposed to mosquitoes could become inoculated with infective larvae
and, if untreated, could develop adult heartworm disease. There are many commercial
anthelmintic preparations used to prevent heartworm infestation by killing the larval
stages in the canine bloodstream before they become adult worms (e.g., ivermectin,
milbemycin, and diethylcarbamazine). These could be used in a research setting in
which heartworm-negative dogs are housed outdoors and thus could potentially be infected
through mosquito bites. If a research facility is conditioning random-source dogs
for long-term use, the presence of circulating adult heartworm antigen should disqualify
an animal from the conditioning program.
Treatment.
Treatment for eradication of heartworms (adults, juveniles, and microfilaria) is a
long process that can pose a significant risk to the patient with regard to both drug
side effects (Hoskins, 1989) and immunologic reactions to dead worms lodged in the
pulmonary vasculature. For this reason, medical treatment of heartworm disease is
not usually attempted in research dogs. In a rare instance when such treatment was
in the best interest of a long-term canine experiment, thiacetarsamide (Caparsolate)
and ivermectin (Ivomec) were used to eradicate adults and microfilariae, respectively
(authors’ personal experience). Alternative choices include melarsomine (Immiticide)
as an adulticide and milbemycin (Interceptor), levamisole (Leva-sol), or fenthion
(Spotton) as microfilaricidal agents. Dosing regimens for these agents are detailed
in Dillon (2000).
Research complications.
The physiomodulative properties of heartworm infection have been studied. Such studies
have looked at factors released by adult heartworms, as well as changes in the function
of host tissues in response to the worm presence. Probably the most consistent finding
is that endothelial cell–dependent relaxation of pulmonary arterial smooth muscle
is depressed in heartworm-infected dogs as compared with control dogs, indicative
of alterations in local endothelial cell behavior (Maksimowich et al., 1997; Matsukura
et al., 1997; Mupanomunda et al., 1997). The extension of this effect on peripheral
arteries (in vivo and in vitro) has been supported in some studies (Kaiser et al.,
1992) but refuted in others (Tithof et al., 1994). It is thought that the endothelium
is perturbed by a factor released from the adult Dirofilaria, possibly a cyclooxygenase
product such as prostaglandin D2 (Kaiser et al., 1990, Kaiser et al., 1992). These
products have also been demonstrated to cause constriction in in vitro rat tracheal
ring preparations (Collins et al., 1994), suggesting that bronchoconstriction could
be an aspect of the pathogenesis of the infestation. Platelet reactivity was also
been found to be enhanced in dogs naturally infected with Dirofilaria, when compared
with uninfected controls (Boudreaux and Dillon, 1991). Based on these data, dogs that
are positive for adult heartworm antigen should be considered inappropriate for use
as research subjects and, if used, should be restricted to nonsurvival preparations
that do not require physiological measurements.
c.
Cestodes
Etiology.
Several species of cestodes (tapeworms) parasitize the small intestine of dogs. The
most common is Dipylidium caninum. Other species include Taenia pisiformis and, more
rarely, Echinococcus granulosus, Multiceps spp., Mesocestoides spp., and Spirometra
spp.
Clinical signs.
Most cestode infestations are subclinical. Severe infestations with Dipylidium can
be associated with diarrhea, weight loss, and poor growth.
Epizootiology and transmission.
The cestode life cycle requires an intermediate host. For Dipylidium caninum, the
intermediate hosts are fleas and lice. Thus this species of tapeworm can be readily
transmitted by ingestion of arthropods that are canine parasites in and of themselves.
Taenia pisiformis requires small ruminants, rabbits, or rodents for intermediate hosts,
so spread is less likely, especially in a research setting. Echinococcus granulosus
uses not only sheep as an intermediate host but also human beings, and thus the zoonotic
potential of this cestode must be considered.
Pathologic findings.
Adult cestodes in the small intestine are usually an incidental finding at necropsy.
Diagnosis and differential diagnosis.
Definitive diagnosis is usually made by the identification of egg capsules or proglottids
(tapeworm segments) on the surface of the feces or around the anus. Dipylidium egg
packets are large (100 × 150 μm) and contain 1–63 eggs per packet (Hall and Simpson,
2000).
Prevention and control.
The most significant means to limit cestode infestation is to control the population
of fleas and/or lice infesting the colony. See the sections on these ectoparasites
for effective means to treat infested dogs and kennels.
Treatment.
Praziquantel at 5–12.5 mg/kg orally or subcutaneously is the standard treatment for
cestodiasis, especially Taenia or Echinococcus species. Fenbendazole, mebendazole,
or oxfendazole may also be effective against Dipylidium caninum (Hall and Simpson,
2000).
Research complications.
Unless the infestation is severe, canine tapeworms probably have minimal impact on
experimental studies. However, dietary and nutritional studies would require dogs
to be free of all intestinal parasites.
d.
Trematodes
i.
Pulmonary trematodiasis
Etiology.
Canine lung fluke infestation is caused by Paragonimus kellicotti in the Western Hemisphere
and P. westermani in Asia.
Clinical signs.
Most lung fluke infestations are inapparent, but coughing can develop in cases that
prompt a strong inflammatory response. Pneumothorax has been a sequela of cyst rupture,
in which case dyspnea with reduced lung sounds would be the typical presentation.
Epizootiology and transmission.
The lung fluke life cycle requires two intermediate hosts: a snail and then a crayfish.
Dogs become infested after eating crayfish, which essentially limits this disease
to random-source dogs. On ingestion, the immature flukes (metacercariae) migrate to
the lungs and encyst in the pulmonary parenchyma. Eggs produced by adult flukes are
passed into the bronchioles, coughed up, swallowed, and passed in the feces to complete
the life cycle.
Pathologic findings.
Grossly, the trematode cysts containing adult flukes can be seen in the lung parenchyma.
Areas of eosinophilic inflammation surround the cysts, and eosinophilic granulomas
can also be seen encircling released eggs. Pleural hemorrhages may also be caused
by the migrating metacercariae (Lopez, 1995).
Pathogenesis.
Clinical illness is usually a result of a severe eosinophilic inflammatory response,
pneumothorax caused by cyst rupture, or secondary bacterial pneumonia.
Diagnosis and differential diagnosis.
Definitive diagnosis of Paragonimus infestation requires identification of the characteristic
ovoid eggs (80–115 μm long) with a single operculum in either the feces or a transtracheal
wash. Identification from fecal samples requires sedimentation techniques. Other causes
of coughing in dogs (e.g., infectious tracheobronchitis, dirofilariasis, congestive
heart failure) need to be considered. Radiographically, the appearance of (multi)focal
densities within the air-filled lung field needs to be differentiated from pulmonary
neoplasia (primary or metastatic) or systemic fungal pneumonias.
Prevention.
Use of purpose-bred dogs virtually eliminates the chance of pulmonary trematodiasis
in a research animal.
Treatment.
Praziquantel (at 25 mg/kg q8 hr × 3 days) or fenbendazole (25–50 mg/kg q12 hr × 10–14
days) are recommended for treatment of canine Paragonimus infestation (Hawkins, 2000).
Effectiveness is monitored by fecal sedimentation tests for eggs and resolution of
radiographic lesions (which may never resolve entirely). Early diagnosis of pulmonary
trematodiasis should warrant discontinuation of a dog from a long-term study because
of the possibility of more serious clinical sequelae, such as pneumothorax.
Research complications.
Experimental studies involving the immune system, especially eosinophilic or local
pulmonary responses, would be significantly affected by even minor infestations. Clinical
illness would complicate almost any research project and makes dogs poor anesthetic
risks. Radiographic lesions may confound diagnostic evaluation for pulmonary metastasis
of tumors.
e.
Mites
i.
Demodicosis
Etiology.
Canine demodicosis is caused by Demodex canis, a commensal mite that lives in the
hair follicles. It is considered to be normal fauna of dog skin, but certain conditions
(i.e., immunosuppression) cause development of clinical illness.
Clinical signs.
Demodex canis infestation is typically asymptomatic. Clinical demodicosis presents
with variable and nonspecific clinical signs, such as alopecia, erythema, pruritus,
crusts, and hyperpigmentation. It can occur anywhere on the body but is often seen
on the feet and the face and around the ears (DeManuelle, 2000a). Secondary bacterial
pyoderma is a common complication.
Epizootiology and transmission.
Demodex canis mites pass to nursing pups from the dam. They live their entire lives
on one dog and are not considered contagious to other dogs or humans. Certain breeds
are predisposed to the generalized form of Demodex dermatitis (see “Pathogenesis,”
below). Beagles are among the predisposed breeds, as are German shepherds, Doberman
pinschers, Old English sheepdogs, collies, boxers, and shorthair brachycephalic breeds
(Muller et al., 1983).
Pathologic findings.
Histologically, Demodex infections are characterized by perifolliculitis and folliculitis
with mites and keratin debris visible in the hair follicles. Cases with generalized
demodicosis (see “Pathogenesis,” below) may have a minimal cellular response with
no eosinophils, indicative of severe immunosuppression (Scott et al., 1995).
Pathogenesis.
When clinical demodicosis develops, it is classified into “localized” or “generalized”
(e.g., more than one foot affected, or five or more small areas, or one large body
area). Localized demodicosis is typically seen in juvenile dogs (< 18 months) and
usually resolves without treatment as natural immunological control develops. Generalized
demodicosis can develop in juvenile or adult populations. Juvenile-onset generalized
demodicosis occurs in dogs with a genetic predisposition, thought to be an inherited
T-lymphocyte dysfunction. Adult-onset generalized demodicosis is usually indicative
of an underlying endocrine (hyperadrenocorticism, diabetes mellitus, hypothyroidism)
or neoplastic disorder or can develop as a result of immunosuppressive therapy (such
as corticosteroid administration).
Diagnosis and differential diagnosis.
Demodex is readily identified from deep skin scrapings of lesioned areas (Campbell,
2000; Noli, 2000). Demodex canis has a characteristic “cigar shape,” with short, stubby
legs on a body 100–400 μm long. Differential diagnoses for local demodicosis include
dermatophytosis, allergic contact dermatitis, and seborrheic dermatitis. The primary
differential diagnosis for generalized demodicosis is primary bacterial pyoderma;
remember, however, that bacterial pyoderma is a common secondary complication of the
generalized form of this parasitism.
Prevention and control.
Dogs with generalized demodicosis should not be maintained in a breeding colony.
Treatment: Ivermectin (Ivomec) at 200–600 μg/kg and oral milbemycin (Interceptor)
at 1–2 mg/kg/day have been found to be effective treatments. These parasiticides are
probably the most practical to use in a research setting, although they are not labeled
for treatment of Demodex canis. Amitraz (Mitaban) dips (250 ppm every 14 days) can
be used for more problematic cases. Treatment duration can be extensive and must be
accompanied by repeated skin scrapings.
Research complications.
Dogs with generalized demodicosis should not be used in research studies, because
this disease is indicative of another underlying disorder (endocrine or immunological).
Dogs that receive immunosuppressive agents or paradigms could develop generalized
demodicosis as an unexpected consequence of the experimentation.
ii.
Sarcoptic mange
Etiology.
Canine sarcoptic mange is caused by Sarcoptes scabiei var. canis.
Clinical signs.
The most common clinical sign is an intense pruritus, usually beginning at sparsely
furred areas such as the ear pinnae, elbows, and ventral thorax and abdomen. Lesions
are characterized by alopecia and yellowish dry crusts with a macular papular eruption.
These lesions may be exacerbated by excoriation due to the pruritic nature of the
condition.
Epizootiology and transmission.
Sarcoptes mites live their entire lives in the stratum corneum of the host animal;
however, they can survive for 1–3 weeks away from the host, and it is this ability
that enables them to spread from dog to dog. Sarcoptes scabiei var. canis can also
infect cats and humans.
Pathologic findings.
Histologic examination can be unrewarding because mites are rarely seen on tissue
sections, and the associated dermatitis is nondiagnostic: perivascular and interstitial
dermatitis with hyperkeratosis, with or without eosinophilic infiltration. Suggestive
histopathologic lesions are epidermal “nibbles,” small foci of edema, exocytosis,
degeneration, and necrosis (Scott et al., 1995).
Pathogenesis.
Lesions and illness are a result of the female mites burrowing through the epidermal
layers to deposit eggs, and the larvae migrating back to the surface. The typical
locations of mange lesions are a result of the mite's preference for relatively hairless
areas.
Diagnosis and differential diagnosis.
Sarcoptic mange can be difficult to diagnose because multiple skin scrapings can yield
negative results with this parasitic disorder. Hopefully, adult mites, mite eggs,
or mite feces can be observed on superficial skin scrapings. Even if scrapings are
negative, however, a therapeutic trial should be initiated if the clinical signs and
history suggest a Sarcoptes etiology. Demonstration of anti-mite IgE in either the
serum or via an intradermal antigen test can be used as a diagnostic aid (Campbell,
2000). An important differential diagnosis is flea allergy dermatitis; in contrast,
mange is non-seasonal and contagious.
Prevention and control.
Use of purpose-bred dogs limits the possibility of having research animals with sarcoptic
mange. For random-source dogs, an ectoparasite control program should be in place
to limit possible infestations. Many institutions use ivermectin as a means to control
both endoparasites and ectoparasites.
Treatment.
Unless treatment would interfere with research objectives, all dogs with sarcoptic
mange (no matter how minor the lesions) and their kennel mates should be treated because
of the contagious nature of the disease and its zoonotic potential. In research colonies,
the usual means of treatment is either ivermectin (Ivomec) at 200–400 μg/kg ql4 days
or milbemycin (Interceptor) at 3 oral doses of 2 mg/kg q7 days (Scott et al., 1995).
Neither of these agents is approved for treatment of sarcoptic mange, but they are
considered to be effective. Acaricidal dips (e.g., lime sulfur, organophosphates,
amitraz) can also be used.
Research complications.
The local skin inflammation and systemic immune response to sarcoptic mange probably
make infected dogs poor subjects for dermatologic and immunologic studies.
f.
Lice and Ticks
i.
Lice
Etiology.
Dogs can be infested by one species of sucking louse (Linognathus setosus) and two
species of biting lice (Trichodectes canis and Heterodoxus spiniger).
Clinical signs.
Mild cases of pediculosis may be asymptomatic or may cause pruritic areas of dry skin.
More severe infestations can cause significant pruritus and produce alopecia, papules,
and crusts. These lesions lead to excoriation and secondary bacterial dermatitis.
Severe Linognathus infestations could cause anemia, because this species feeds on
blood.
Epizootiology and transmission.
Louse infestations are uncommon in both pet animal practice and the research setting.
They would most likely be seen in random-source dogs that were obtained from a pound
or shelter. Transmission is usually by direct contact, for lice spend their entire
lives on the host species. Lice are host-specific and not zoonotic.
Pathogenesis.
The biting lice usually cause more local irritation than the sucking louse and therefore
are more apt to induce clinical dermatologic signs. Trichodectes canis can serve as
vector for the canine tapeworm Dipylidium caninum. The most severe complication of
infestations by the sucking louse is the potential anemia.
Diagnosis and differential diagnosis.
Pediculosis is diagnosed by direct observation of the lice or nits (eggs) on the dog's
skin. Cellophane tape can be used to pick up surface debris from skin lesions, which
may include nits or immobilized lice (Muller et al., 1983). Differential diagnoses
include dermal acariasis, flea allergy dermatitis, and seborrhea.
Prevention.
Use of high-quality conditioned dogs for research should prevent pediculosis from
ever being seen within a research facility. Random-source dogs should be shampooed
or treated prophylactically with topical insecticide before being permitted to enter
the research colony.
Treatment.
Most commercially available insecticide shampoos and dips readily treat louse infestations.
Treatment should be repeated in 10–14 days, because any nits that were not killed
would have hatched by that time (Muller et al., 1983).
Research complications.
There is probably minimal interference with research, unless severe Linognathus infestations
cause anemia.
ii.
Ticks
Etiology.
Ticks are obligate arachnid parasites that require vertebrate blood as their sole
food source. Except for the brown dog tick (Rhipicephalus sanguineus), ticks have
a wide host range and are not especially host-specific; so any number of tick genera
and species can be found on dogs. Genera that more commonly infest dogs in the United
States include species of Rhipicephalus, Dermacentor, and Ixodes. The primary significance
of tick infestation is the tick's ability to be a vector for many other infectious
diseases, including Rocky Mountain spotted fever (caused by Rickettsia rickettsii),
Lyme disease (Borrelia burgdorferi), and the canine forms of ehrlichiosis (Ehrlichia
canis and E. platys), babesiosis (Babesia canis), haemobartonellosis (Haemobartonella
canis), and hepatozoonosis (Hepatozoon canis).
Clinical signs.
As an entity unto itself, tick infestation causes minimal clinical signs. Most infestations
are subclinical, although some dogs may lick and bite at the site, aggravating the
local lesion. Some dogs can develop a hypersensitivity reaction after several tick
bites; these dogs develop a more granulomatous response at the location of the bite
(Merchant and Taboada, 1991). Some species of ticks (primarily Dermacentor andersoni
and D. variabilis) produce a salivary neurotoxin that can cause an ascending flaccid
paralysis (Malik and Farrow, 1991). The paralysis develops within 5–9 days of tick
attachment and can result from a single tick. This paralysis is fatal once the respiratory
musculature is affected.
Epizootiology and transmission.
In dogs used for biomedical research, tick infestation may occasionally be seen in
random-source dogs, because these dogs are more likely to have been in tick habitats
than purpose-bred dogs. Ticks commonly reside in wooded areas until they contact a
suitable host for a blood meal. The brown dog tick may reside within kennels (attics,
bedding, wall insulation) (Garris, 1991).
Pathologic findings.
Under most circumstances, tick infestation will be an incidental finding on necropsy
(unless tick paralysis was the cause of death).
Pathogenesis.
Tick-bite paralysis is caused by the presence of a salivary neurotoxin released by
female ticks of certain genera (e.g., Dermacentor) while consuming a blood meal (Malik
and Farrow, 1991). Interestingly, dogs seem to be most affected by this condition,
whereas cats appear to be resistant. The primary dysfunction appears to be at the
neuromuscular junction, as stimulation of the motor nerves fails to elicit a response,
but direct stimulation of the muscle tissue results in contractions. Tick bites can
also transmit pathogen microorganisms to the dog, because ticks serve as vectors for
several infectious diseases, including Lyme borreliosis, ehrlichiosis, babesiosis,
and Rocky Mountain spotted fever.
Diagnosis and differential diagnosis.
For uncomplicated tick bites and tick-bite paralysis, definitive diagnosis is made
by identification of the offending arachnid (and improvement of paralysis after removal).
Differential diagnoses for tick-bite paralysis include botulism, snakebite, polyradiculoneuritis,
and idiopathic polyneuropathy (Malik and Farrow, 1991).
Prevention.
Purpose-bred dogs should be free from all ectoparasites, but ticks can occasionally
be seen on random-source animals. Research dogs should not be exercised in outdoor
areas infested with ticks, and kennels must be cleaned properly and regularly so as
to remain free of ticks and other parasites.
Treatment.
Removal of the offending tick is the primary treatment for both local inflammation
as well as tick-bite paralysis. Dogs with tick-bite paralysis usually show improvement
within 24 hr, with complete recovery within 72 hr (Malik and Farrow, 1991) To remove
an attached tick from a dog, forceps should be used to grasp the tick as close to
the dog's skin as possible. The tick should not be grabbed by the body, as this may
cause the parasite to either rupture or inject its body contents into the dog. The
tick should be pulled away from the dog with steady pressure. Many of the diseases
transmitted by ticks are zoonotic so precautions, such as wearing gloves, should be
taken. Use of topical acaricide/insecticides on newly arrived random-source dogs should
help to limit infestations.
Research complications.
Simple uncomplicated tick bites probably have minimal impact on research variables.
The significant concern for tick infestation is the possible development of tick-bite
paralysis or of any one of a number of systemic diseases spread by ticks (see Sections
III,A,l,e–g).
g.
Other
i.
Flea infestation
Etiology.
Fleas are laterally flattened wingless insects that feed on animal blood. The most
common flea to infest dogs is Ctenocephalides felis, the cat flea. Other fleas that
can affect dogs are Ctenocephalides canis, Pulex irritans, and Echidno-phaga gallinacea.
The fleas are speciated by the shape of their head and by the presence or absence
of ctenidae (spiny combs on or behind the head) (Campbell, 2000).
Clinical signs.
Flea infestations usually cause foci of alopecia and pruritus. Dogs that are hypersensitive
to antigenic proteins in flea saliva develop the more severe “flea allergy dermatitis,”
which features papules and crusting. Acute moist dermatitis (“hot spots”) can also
be seen in these cases, and secondary pyoderma or seborrhea can develop. Lesions from
flea allergy dermatitis generally appear in the dorsal lumbosacral region, as well
as the flanks, thighs, and abdomen (Muller et al., 1983). The lesions are typically
worse in the summer and autumn months and are progressively more severe as the dog
ages.
Epizootiology and transmission.
Fleas are readily transmitted between animals and even between host species. They
move readily between the host and the environment, making transmission easy and control
difficult. Because fleas require host blood for food, they can survive off of a host
for only 1–2 months (Muller et al., 1983).
Pathologic findings.
Biopsy samples are usually nondiagnostic in cases of flea allergy dermatitis. Lesions
are typically characterized by perivascular eosinophilic inflammation and may feature
pustules and folliculitis if secondary pyoderma develops (Muller et al., 1983).
Pathogenesis.
Fleas are parasites that require animal blood for their meals. When they bite host
animals, they inject some saliva into the host's skin. If the host develops an allergic
response to the flea saliva, it will develop the more pruritic flea allergy dermatitis.
Fleas can also transmit or serve as vectors for other pathogens (e.g., Dipylidium
tapeworms).
Diagnosis and differential diagnosis.
Flea infestations and flea allergy dermatitis are definitively diagnosed by observing
the fleas on the host's skin. Given that this may be difficult because of the mobility
of the flea and the majority of the time it spends off of the host, diagnosis is often
based on clinical signs, history, and lesion distribution. Sometimes the presence
of flea excrement (“flea dirt”) on the dog's skin can support a presumptive diagnosis
(DeManuelle, 2000b). Circulating eosinophilia is seen in some dogs with flea allergy
dermatitis. Differential diagnoses include mite and louse infestations, bacterial
folliculitis, and allergic or atopic conditions that present with skin lesions in
dogs (e.g., food, drug, or contact hypersensitivity).
Prevention.
Most dogs obtained from high-quality purpose-bred facilities should be free from flea
infestations. Dogs received from pounds, shelters, or licensed dealers would be more
likely to be affected by fleas (or any ectoparasitism). Thorough knowledge of prevention,
control, and treatment measures at these facilities should be obtained, and dogs from
sources where proper prevention and/or therapy are not practiced should be evaluated
and/or empirically treated upon arrival at the facility.
Control.
Thorough cleaning of the dog's housing environment should remove the risk of perpetuating
or transmitting flea infestation in the colony.
Treatment.
Treatment for fleas needs to address treatment of both the dog and the environment.
Many insecticide formulations such as shampoos, sprays, dips, powders, and oral systemics
can be used for initial treatment of the individual dog. The active ingredients include
Pyrethrins, pyrethroids, carbamates, and organophosphates. Flea control in the kennel
may need to include outdoor areas in warm climates. Typically combinations of adult
insecticides and juvenile growth regulators are used for environmental treatment.
Directed sprays are the most effective means of treating housing areas, because flea
“bombs” or foggers do not penetrate adequately into tight areas where fleas might
hide (DeManuelle, 2000b).
In addition to insecticide therapy, dogs with flea allergy dermatitis may also require
anti-inflammatory medication to relieve clinical signs. Oral prednisone or prednisolone
at 0.5 mg/kg q12 hr for 5–7 days has been proposed as a starting therapy (Muller et
al., 1983). The use of hyposensitization with flea-bite antigens is controversial
and not practical for the research setting.
Research complications.
Mild flea infestation probably has minimal impact on most research protocols, and
treatment measures may in fact be more detrimental to the experimental objective than
the actual ectoparasitism. In a research setting, the residual effects of insecticides
may preclude their use in experimental animals. Such treatments should be used judiciously
to ensure that experimental results are not more seriously affected by the therapy
rather than the infestation. Dogs with flea-allergy dermatitis are more severely affected
by the flea infestation and should be treated appropriately; however, systemic corticosteroids
may also interfere with experimental objectives, especially in studies involving functions
of the immune system. The ability of fleas to transmit other parasitic diseases must
also be considered.
4.
Fungal Diseases
a.
Superficial Dermatophytoses
Etiology.
Dermatophytoses (“ringworm”) are fungal skin infections, which in dogs in the United
States are usually caused by either Microsporum canis, M. gypseum, or Trichophyton
mentagrophytes (Muller et al., 1983).
Clinical signs.
Uncomplicated superficial dermatophytoses are characterized by circumscribed circular
areas of alopecia, usually with minimal to no inflammation. These skin lesions are
usually seen around the face, neck, and forelimbs but can be found anywhere on the
body. Secondary bacterial infections can develop; these lesions are called kerions
and are self-limiting, for the fungus cannot survive in inflamed skin (Muller et al.,
1983).
Epizootiology and transmission.
The fungi that cause skin infections are very contagious and readily transmissible
between dogs and other species (including human beings), but they can also be obtained
from the soil.
Pathologic findings.
On close inspection of skin samples, broken hair shafts (and not complete hair loss)
would be seen with uncomplicated dermatophytosis. Histologically, fungal elements
can be seen within the stratum corneum or in and around the hair and hair follicles
(Muller et al., 1983). Stains that facilitate visualization of fungal elements include
periodic acid–Schiff (PAS) or Gomori methenamine–silver. The pattern of inflammation
in the affected foci is very variable and can feature folliculitis, perivascular dermatitis,
hyperkeratosis, and/or vesicular dermatitis.
Pathogenesis.
The dermatophytes typically infect the hair shaft itself, the hair follicle, and possibly
the skin around the affected hair. The hair follicle is not destroyed (unless by secondary
bacterial infection), but the hair itself becomes brittle and breaks. This causes
short stubbly hair to be seen within the lesion. As the lesion progresses, the hairs
in the center recover from the infection, thus leading to the classic “ringworm” appearance
of the alopecic areas. It is postulated that the inflammatory process produces an
environment that is unfavorable for dermatophyte survival, whereas the periphery of
the lesion still enables continued fungal growth (Muller et al., 1983).
Diagnosis and differential diagnosis.
Diagnosis of dermal fungal infection is typically made by scraping the affected area
to obtain hair and superficial epidermal cells. These scrapings are then digested
with potassium hydroxide to facilitate observation of fungal elements. Fungal elements
can also be seen on skin biopsy samples. For speciation of a fungus, skin scrapings
can also be inoculated onto agars that promote fungal growth, such as Sabouraud's
medium or dermatophyte test medium (DTM). Incubation should be at 30°C with 30% humidity
for 14–30 days. Lesions caused by M. canis may fluoresce when inspected using a Wood's
(253.7 nm ultraviolet) light. Unfortunately, some strains of M. canis do not fluoresce,
and neither does M. gypseum or T. mentagrophytes. Differential diagnoses for dermatomycosis
include seborrhea, localized demodecosis, folliculitis, histiocytoma, and acral lick
dermatitis (Muller et al., 1983).
Prevention.
Purpose-bred dogs are typically free of infectious dermatophytes, but ringworm may
be diagnosed on random-source animals.
Control.
In cases of dermatophytosis, isolation of the affected animal(s) is prudent, because
the fungi are easily spread to other dogs, as well as to people. Treatment, if acceptable,
should be started immediately.
Treatment.
Topical antifungal therapy is most commonly used. Shampoos, rinses, and creams containing
miconazole, ketoconazole, enilconazole, or Chlorhexidine are commercially available
to treat ringworm (Stannard et al., 2000). Severe cases may require systemic therapy
with griseofulvin, ketoconazole, itraconazole, or fluconazole. However, these systemic
antifungal agents may have considerable side effects (such as vomiting and teratogenicity
with griseofulvin). Many of the newer agents are also expensive and not labeled for
use in dogs.
Research complications.
The infection itself probably has no impact on most research applications for dogs.
Unfortunately, the zoonotic implications of dermatophytoses force the issue of aggressive
treatment, and many antifungal agents may not be compatible with biomedical research
studies.
b.
Systemic Fungal Infections
Systemic fungal infections disseminate to multiple organ systems from a single mode
of entry (usually through the respiratory tract). Dogs are susceptible to several
fungi that characteristically cause systemic mycosis, including Blastomyces dermatitidis,
Histoplasma capsulatum, Coccidioides immitis, and Cryptococcus neoformans var. neoformans.
These diseases are not typically seen in the research setting, because of the low
overall incidence and noncontagious nature of these disorders and because of the use
of purpose-bred animals. These conditions could, however, present in the rare random-source
dog that was subclinical at its point of origin, especially if the animal becomes
immunosuppressed (either naturally or by virtue of experimental manipulation). Typical
clinical signs include weight loss, fever, lymphadenopathy, and cough and dyspnea
(if the lungs are affected). The reader is advised to read veterinary medical text
chapters (e.g., Taboada, 2000) for more complete information on these disorders and
their possible treatments.
B.
Metabolic and Nutritional Diseases
1.
Genetics
a.
Hypothyroidism
Although the incidence of hypothyroidism in the canine population is not high (Kemppainen
and Clark, 1994), deficiency in thyroid hormone can significantly affect basal metabolism
and immune function. Because these factors are important in many biomedical research
studies, it is imperative that laboratory animal veterinarians be able to recognize,
diagnose, and treat this problem.
Etiology.
The majority of cases of canine hypothyroidism are due to lymphocytic thyroiditis,
an autoimmune disorder, or idiopathic atrophy of the thyroid gland. Both of these
causes result in a gradual loss of functional thyroid tissue (Kemppainen and Clark,
1994). Lymphocytic thyroiditis is the major cause of hypothyroidism in laboratory
beagles and appears to be familial in that breed (Tucker, 1962; Beierwaltes and Nishiyama,
1968; Manning 1979). Rarely, congenital defects or nonfunctional tumors may cause
hypothyroidism (Peterson and Ferguson, 1989; Kemppainen and Clark, 1994).
Clinical signs.
Because it affects metabolism in general, hypothyroidism can produce a large number
of clinical signs referable to many organ systems. An individual dog with hypothyroidism
may have one or any combination of clinical signs. Hypothyroidism reduces the dog's
metabolic rate, which then produces such signs as obesity, lethargy, cold intolerance,
and constipation. Additionally, hypothyroidism can produce several dermatologic abnormalities,
including alopecia, hyperpigmentation, seborrhea, and pyoderma (Peterson and Ferguson,
1989; Panciera, 1994). Several clinicopathologic abnormalities have also been reported
in a large percentage of hypothyroid dogs. These aberrations include increased serum
cholesterol and triglycerides due to a decrease in lipolysis and decreased numbers
of low-density lipopolysaccharide receptors (Peterson and Ferguson, 1989; Panciera,
1994). Normocytic normochromic nonregenerative anemia and increased serum alkaline
phosphatase and creatine kinase have also been reported in a significant number of
hypothyroid dogs (Peterson and Ferguson, 1989; Panciera 1994). Neurologic signs of
hypothyroidism, which include lameness, foot dragging, and paresis, may be caused
by several mechanisms such as segmental nerve demyelination or nerve entrapment secondary
to myxedema (Peterson and Ferguson, 1989). Mental impairment and dullness have also
been reported in hypothyroid dogs, secondary to atherosclerosis and cerebral myxedema
(Peterson and Ferguson, 1989). Hypothyroidism has been implicated in other neurological
abnormalities such as Horner's syndrome, facial nerve paralysis, megaesophagus, and
laryngeal paralysis; however, these conditions do not always resolve with treatment
(Bischel et al., 1988; Panciera, 1994), and so the relationship between hypothyroidism
and these problems has not been completely defined (Panciera, 1994). Myopathies associated
with hypothyroidism are caused by metabolic dysfunction and atrophy of type II muscle
fibers and can present with signs similar to neurological disease (Peterson and Ferguson,
1989). Hypothyroidism can also cause bradycardia as a result of decreased myocardial
conductivity. Abnormalities that may be detected by ECG include a decrease in P and
R wave amplitude (Peterson and Ferguson, 1989) and inverted T waves (Panciera, 1994).
These electrocardiographic abnormalities are caused by lowered activity of ATPases
and calcium channel function. Several reports have suggested that hypothyroidism is
associated with von Willebrand's disease and bleeding abnormalities. However, the
relationship is probably one of shared breed predilection and not a true correlation.
It has been demonstrated that dogs with hypothyroidism are not deficient in von Willebrand's
factor when compared with other dogs. In addition, the replacement of thyroid hormone
in dogs did not increase the levels of vWF:Ag in naturally occurring (Panciera and
Johnson, 1994) or experimentally induced (Panciera and Johnson, 1996) hypothyroidism.
Epizootiology.
The prevalence of hypothyroidism in the general canine population has been reported
to be less than 1% (Panciera, 1994). The disorder occurs most often in large-breed
dogs but has been reported in several other breeds as well as mongrels. Doberman pinschers
and golden retrievers appear to have a higher incidence of hypothyroidism when compared
with other breeds (Panciera, 1994; Peterson and Ferguson, 1989; Scarlett, 1994). There
have been several reports about hypothyroidism in laboratory colonies of beagles (Manning,
1979; Tucker, 1962; Beierwaltes and Nishiyama, 1968). In general, the problem is usually
recognized in middle-aged animals, and some reports state that there is a higher incidence
of hypothyroidism in spayed female dogs (Panciera, 1994; Peterson and Ferguson, 1989).
Diagnosis and differential diagnosis.
Because of the large number of clinical manifestations in dogs, the recognition of
hypothyroidism is not always straightforward. Likewise, the diagnosis of hypothyroidism
can be difficult because of the lack of definitive diagnostic tests available for
the dog. The tests currently available and in popular use will be discussed further.
However, a complete understanding of the diagnosis of hypothyroidism requires a familiarity
with thyroid hormone metabolism and function that is beyond the scope of this writing.
For additional information, the reader is referred to one of several manuscripts available
(Peterson and Ferguson, 1989; Ferguson, 1994).
Currently, the ability to diagnose hypothyroidism relies heavily on the measurement
of serum total T4 (thyroxine) and free T4 (Peterson and Ferguson, 1989; Ferguson,
1994). T4 serves primarily as a precursor for T3 in the body and is heavily protein-bound.
Free T4 represents the unbound fraction that is available to the tissues (Peterson
and Ferguson, 1989). Using the measurement of serum total T4 and free T4, hypothyroidism
can usually be ruled out if the values are within the normal range or higher. If both
hormone concentrations are low, it is highly likely that the patient has hypothyroidism,
and a therapeutic trial is in order (Peterson and Ferguson, 1989). However, it must
be noted that nonthyroidal illnesses and some drugs (e.g., glucocorticoids, anticonvulsants,
phenylbutazone, salicylates) can falsely lower these values (Peterson and Ferguson,
1989; Ferguson, 1994). Therefore, low values do not always indicate that hypothyroidism
is present, and animals should not be treated solely on the basis of serum hormone
levels if clinical signs are absent. If the clinical signs are equivocal or if only
total T4 or free T4 is decreased, further diagnostic testing is warranted (Peterson
and Ferguson, 1989). Although T3 is the most biologically active form of thyroid hormone
in the body, the measurement of serum T3 levels is an unreliable indicator of hypothyroidism
(Peterson and Ferguson, 1989; Ferguson, 1994). Like T4, serum T3 can be falsely lowered
by many nonthyroidal illnesses and many drugs (see above). In addition, T3 may be
preferentially released, and conversion of T4 to T3 may be enhanced in the hypothyroid
dog (Peterson and Ferguson, 1989; Ferguson, 1994). T3 was within normal limits in
15% of the hypothyroid dogs in one study (Panciera, 1994). Autoantibodies can be responsible
for false elevations in the concentrations of T3 and T4 found in these respective
assays. It has been recommended that free T4, measured by equilibrium dialysis, be
assayed in dogs that are suspected of hypothyroidism and have autoantibodies with
normal or high T3 and T4. Autoantibodies have been found in less than 1% of the samples
submitted to one laboratory (Kemppainen and Behrend, 2000).
Other means of diagnosing hypothyroidism have been described. In humans, endogenous
TSH (thyroid-stimulating hormone) levels provide reliable information on thyroid status,
and an assay for endogenous TSH is now available in dogs. However, TSH levels can
be normal in some dogs with hypothyroidism, and high TSH levels have been noted in
normal dogs. Therefore, it is recommended that TSH levels be considered along with
other information (clinical signs, T4) prior to diagnosis and treatment (Kemppainen
and Behrend, 2000). TSH stimulation testing using exogenous bovine TSH provides a
good and reliable method for establishing a diagnosis. Unfortunately, the availability
and expense of TSH limit the use of this diagnostic tool (Peterson and Ferguson, 1989;
Ferguson, 1994). Another drawback of TSH testing is that the test must be postponed
for 4 weeks if thyroid supplementation has been given (Peterson and Ferguson, 1989).
When TSH is available for testing, there are several recommendations for dosage, routes
of administration, and sampling times. One recommendation is 0.045 U of TSH per pound
of body weight (up to a maximum of 5 U) to be administered IV. For this protocol,
blood samples are taken prior to administration of TSH and 6 hours after. A normal
response to the administration of TSH should create an increase of T4 levels at least
2 μg/dl above the baseline levels or an absolute level that exceeds 3 μg/dl (Peterson
and Ferguson, 1989; Wheeler et al., 1985).
Treatment.
The treatment of choice for hypothyroidism in the dog is L-thyroxine (sodium levothyroxine).
A recommended dosing regimen is 0.02 mg/kg once a day or 0.05 mg/m2 (body surface
area)/day for very small or very large dogs. If drugs that decrease thyroxine levels
are being administered concurrently, it may be necessary to divide the thyroxine dose
for twice daily administration. After the supplementation has begun, the thyroid hormone
level should be rechecked in 6–8 weeks, and blood samples should be drawn 4–8 hours
after the morning pill. A clinical response is usually seen in 6–8 weeks and would
include weight loss, hair regrowth, and resolution of other signs (Panciera, 1994).
ECG abnormalities also return to normal (Peterson and Ferguson, 1989). For dogs with
neurologic signs, the prognosis is guarded, because the signs do not always resolve
with supplementation (Panciera, 1994).
2.
Management-Related Issues
a.
Obesity
Weight gain and eventual obesity are also frequent findings in dogs in the research
environment. Because obesity can adversely affect several body systems as well as
general metabolism, the laboratory animal veterinarian must be aware of the development
of obesity and the potential effect that it can have on research.
Etiology.
Obesity is defined as a body weight 20–25% over the ideal. In general, obesity occurs
when the intake of calories exceeds the expenditure of energy. Excessive caloric intake
results from overeating or eating an unbalanced diet. Overeating is a common cause
of obesity in pet dogs and may be triggered by boredom, nervousness, or conditioning
(MacEwen, 1992). In addition, pet animals are often subjected to unbalanced diets
supplemented with high-fat treats. In the laboratory animal setting, overeating is
less likely than in a household, because access to food is more restricted and diets
are usually a commercially prepared balanced ration. However, obesity can still be
a problem if specific guidelines for energy requirements are not followed. In addition,
the necessary caging of dogs in the research environment and thus the limitation to
exercise reduces energy expenditure and predisposes dogs to weight gain. It is also
important to realize that other factors may predispose dogs to obesity, even when
guidelines for caloric intake and energy expenditure are followed (Butterwick and
Hawthorne,. 1998). As in humans, genetics plays an important role in the development
of obesity in dogs. It has been established that certain breeds are more predisposed
toward obesity. In a study of dogs visiting veterinary clinics in the United Kingdom,
Labrador retrievers were most likely to be obese. Other breeds affected included Cairn
terriers, dachshunds, basset hounds, golden retrievers, and cocker spaniels. The beagle
was also listed as a breed predisposed to obesity in the household environment (Edney
and Smith, 1986).
In addition to genetics, several metabolic or hormonal changes are associated with
obesity. It has been well established that neutering promotes weight gain. In one
study, spayed female dogs were twice as likely to be obese when compared with intact
females (MacEwen, 1992). The authors proposed that the absence of estrogen promotes
an increase in food consumption. A similar trend toward obesity was found in castrated
male dogs (Edney and Smith, 1986). In addition, hypothyroidism and hyperadrenocorticism
may present with obesity as one of the clinical signs (MacEwen, 1992).
Epizootiology.
Obesity affects up to 40% of pet dogs (MacEwen, 1992).
Diagnosis and differential diagnosis.
The diagnosis of obesity is somewhat subjective and relies on an estimate of ideal
body weight. The ideal body condition for dogs is considered to be achieved when the
ribs are barely visible but easily palpated beneath the skin surface. When the ribs
are not easily palpated and/or the dog's normal function is impaired by its weight,
the animal is considered obese. There are few objective, quantifiable methods for
establishing this diagnosis. Ultrasound has been evaluated for measurement of subcutaneous
fat in dogs, and measurements taken from the lumbar area can be used to reliably predict
total body fat (Wilkinson and McEwan, 1991). After a diagnosis of obesity has been
made, additional diagnostic tests should be performed to determine if there is an
underlying cause for the problem. A complete physical exam should be performed to
look for signs of concurrent disease and to establish if obesity has adversely affected
the individual. Serum thyroid hormones should be evaluated (see Section III,B,1,a),
and serum chemistry may reveal an increased alkaline phosphatase associated with hyperadrenocorticism.
Treatment.
Restricting food intake readily treats obesity, and this is easily done in the research
setting. It has been suggested that a good weight loss program involves restriction
of intake to 60% of the calculated energy requirement to maintain ideal body weight.
It has been shown that restriction of calories down to 50% produces no adverse health
effects. However, T3 levels will decrease in direct proportion with caloric intake.
Ideally, weight loss will occur at a rate of 1–2% of body weight per week (Laflamme
et al., 1997). With more severe calorie restriction and more rapid weight loss, the
individual is more likely to rebound and gain weight after restrictions are relaxed.
There has been a great deal of attention in humans as to the correct diet to be fed
to encourage weight loss. Likewise, the type of diet fed to dogs has been examined.
As mentioned above, the restriction of calories is most important, and feeding less
of an existing diet can do this. Alternatively, several diet dog foods are available,
and there is some evidence that these diets are superior to simple volume restriction
(MacEwen, 1992). There has been much concern about the addition of fiber to the diet
in both humans and animals as a method for reducing caloric intake while maintaining
the volume fed. Studies in dogs have examined the addition of both soluble and insoluble
fibers to calorie-restricted diets. These studies have shown that the addition of
fiber does not have an effect on satiety in dogs and therefore does not have a beneficial
effect in weight loss protocols (Butterwick and Thome, 1994; Butterwick and Thorne,
1997).
Research complications.
It is important to control weight gain in research animals, because of the association
of obesity and several metabolic changes. Although an association between obesity
and reproductive, dermatologic, and neoplastic problems has been reported (MacEwen,
1992), this relationship is not consistently apparent (Edney and Smith, 1986). Obesity
in dogs over 10 years of age appears to be related to an increase in cardiovascular
problems (Edney and Smith, 1986), and obesity has been linked to hypertension. Joint
problems including osteoarthritis and hip dysplasia have also been related to obesity
(MacEwen, 1992; Kealy et al., 1997). In addition, diabetes mellitus has been linked
to obesity, and obesity induces hyperinsulinism in several experimental models (MacEwen,
1992).
C.
Traumatic Disorders
1.
Generalized Traumatic Wound Care
In the laboratory setting, the majority of traumatic wounds will be small in size.
In facilities with good husbandry practices and a diligent staff, traumatic wounds
will generally be observed quickly and attended to promptly. Under these conditions,
proper initial treatment will lead to uncomplicated wound healing. Complications such
as infection and delayed healing arise when wounds are not noticed immediately or
when the basic principles of wound management are not followed.
To aid in the description of wounds and in decision making about wound therapy, several
classification systems have been developed for traumatic injuries. At one time, decisions
about wound therapy were largely based upon the length of time since wounding, or
the concept of a “golden period.” It is now recognized that several factors must be
considered prior to initiating wound care, including (but not limited to) the type
and size of the wound, the degree of wound contamination, and the capability of the
host's defense systems (Swaim, 1980; Waldron and Trevor, 1993). One of the most widely
used classification systems is based upon wound contamination and categorizes wounds
as either clean, clean–contaminated, contaminated, or dirty (see Table V
).
Table V
Classification and Treatment of Traumatic Woundsa
Classification
Description
Examples
Treatment options
Clean
Aseptic wound
Surgical incision
Aseptic, immediate closure
Clean–contaminated
Recent wound with minimal, easily removed contamination
Simple laceration, broken toenail
Wound lavage. debridement, ±immediate closure
Contaminated
Several hours since wounding; grossly contaminated
Bite wounds, old lacerations, fecal contamination
Wound lavage, debridement ±drain placement, ± delayed closure
Dirty
Purulent exudate and infection already present
Infected bite wound, anal sac abscess, postsurgical infection
Wound lavage, debridement, drain placement, delayed or no closure
a
Modified from Waldron and Trevor (1993).
The vast majority of the wounds seen in the laboratory setting will fall into the
clean and clean–contaminated categories. These wounds may be treated with the basic
wound care described below and primary closure of the wound. Contaminated and dirty
wounds, which are seen infrequently in the laboratory setting, require more aggressive
therapy. Dirty wounds can occur as postsurgical infections or complications of initial
wound therapy. When one is in doubt as to the classification of a wound, the worst
category should be presumed in order to provide optimal therapy and reduce the chance
for complications.
The initial treatment of a wound is the same regardless of the wound's classification.
When first recognized, the wound should be covered with a sterile dressing until definitive
treatment is rendered. Bleeding should be controlled with direct pressure; tourniquets
are discouraged because of the complications that may arise with inappropriate placement
(Swaim, 1980). It is best to avoid using topical disinfectants in the wound until
further wound treatment (culture, debridement, lavage) has been performed (Swaim,
1980).
When the treatment of a wound begins, anesthesia or analgesia may be necessary, and
the choice of anesthetic regimen will depend on the size and location of the wound
as well as the preference of the clinician. If the wound is contaminated or dirty,
bacterial cultures, both aerobic and anaerobic, should be performed at this time.
Then a water-soluble lubricant gel may be applied directly to the wound. A wide margin
of hair should then be clipped from around the wound, using a #40 blade. After the
clipping, a surgical scrub is performed around the edges of the wound. Povidone-iodine
alternating with alcohol or chlorhexidine gluconate scrub alternating with water is
most often recommended for surgical preparation of the skin surface (Osuna et al.,
1990a,Osuna et al., 1990b). Simple abrasions that involve only a partial thickness
of the skin do not generally require further treatment. Full-thickness wounds require
further attention, including irrigation with large quantities of a solution delivered
under pressure. Two solutions, 0.05% chlorhexidine diacetate in water (Lozier et al.,
1992) and 1% povidone-iodine in saline, are most often recommended for wound lavage
(Waldron and Trevor, 1993). The chlorhexidine solution may offer the advantage of
greater bactericidal activity but does not significantly alter wound healing when
compared with povidone-iodine (Sanchez et al., 1988). Actually, the type of solution
chosen may not be as important to wound care as the volume and pressure at which the
solution is delivered. It has been suggested that 8 psi is required to obtain adequate
tissue irrigation, and this may be achieved by using a 35 ml syringe and an 18- or
19-gauge needle (Waldron and Trevor, 1993).
For wounds that are contaminated or dirty, debridement is an important part of initial
therapy. Debridement usually proceeds from superficial to deeper layers. Skin that
is obviously necrotic should be removed. Although it is often recommended to remove
skin back to the point at which it bleeds, this may not be feasible with large wounds
on the limbs. In addition, other factors such as edema or hypovolemia may reduce bleeding
in otherwise viable skin (Waldron and Trevor, 1993). If one is unsure about tissue
viability in areas that are devoid of extra skin, the tissue may be left (Swaim, 1980;
Waldron and Trevor, 1993), and nonviable areas will demarcate within 2–3 days (Waldron
and Trevor, 1993). Necrotic fat should be resected liberally, because it does not
have a large blood supply and will provide an environment for infection. Often, resection
of subcutaneous fat is necessary to remove debris and hair that could not be removed
during wound irrigation. Damaged muscle should also be liberally resected (Swaim,
1980). The wound should be irrigated several times during debridement and again after
completion.
After initial wound treatment, the options concerning wound closure must be weighed.
The principles of basic surgery are discussed in several good texts, and readers are
encouraged to pursue additional information. Primary wound closure is defined as closure
of the wound at the time of initial wound therapy and is the treatment of choice for
clean and clean-contaminated wounds. Closure is performed in two or more layers, carefully
apposing tissues and obliterating dead space. If dead space will remain in the wound,
a drain should be placed. Subcutaneous closure should be performed with absorbable
suture such as polydioxanone (PDS), polyglactin 910 (Vicryl), or polyglycolic acid
(Dexon). It is best to use interrupted sutures and avoid leaving excess suture material
in the wound. It may be necessary to choose tension-relieving suture patterns, such
as horizontal mattress. Skin closure is generally performed with nylon (3-0 or 4-0).
In situations where gross contamination cannot be completely removed, closure of the
wound should be delayed or avoided. After debridement and irrigation, the wound should
be bandaged. Initially, the wound can be covered by gauze sponges soaked in saline
or chlorhexidine to create a wet-to-dry bandage. When the sponges are later pulled
from the wound, dried exudates will also be removed. When the wound appears clean,
the layer in contact with the wound may be changed to a nonadherent dressing such
as vaseline-impregnated gauze (Swaim, 1980). The contact layer is covered by cotton
padding, and the entire bandage is covered by a supportive and protective layer. The
bandages should be changed once or twice daily, depending upon the amount of discharge
coming from the wound. Wound closure within 3–5 days of wounding (prior to the formation
of granulation tissue) is considered delayed primary closure. When the wound is closed
after 5 days, this is considered secondary closure (Waldron and Trevor, 1993). Second-intention
healing involves allowing the wound to heal without surgical intervention. This type
of healing is often used on limbs when there is an insufficient amount of skin to
allow complete closure (Swaim, 1980). It is important to note that second-intention
healing will take longer than surgical repair of a wound, and in the case of large
wounds it will be more expensive because of the cost of bandaging materials.
Several factors must be weighed concerning the use of antibiotics in traumatic wounds,
including the classification and site of the wound, host defenses, and concurrent
research use of the animal. When wounds are clean or clean-contaminated, antibiotics
are seldom necessary unless the individual is at high risk for infection. When wounds
have been severely contaminated or are dirty, antibiotics are indicated, and the type
of antibiotic will ultimately depend on culture and sensitivity results. Until such
results are available, the choice of antibiotic is based on the most likely organism
to be encountered. In skin wounds, Staphylococcus spp. are generally of concern, whereas
Pasteurella multocida should be considered in bite wounds. Cephalosporins, amoxicillin-clavulanate,
and trimethoprim sulfas are often recommended for initial antibiotic therapy (Waldron
and Trevor, 1993).
2.
Pressure Sores (Decubital Ulcers)
Etiology.
Pressure sores (decubital ulcers) can be a problem in long-term studies that require
extended periods of recumbency. Decubital ulcers usually develop due to continuous
pressure from a hard surface contacting a bony prominence such as the elbow, the tuber
ischii, tarsus, or carpus. The compression of the soft tissues between the hard surfaces
results in vascular occlusion, ischemia, and ultimately tissue death (Swaim and Angarano,
1990). Several factors that increase pressure at the site and/or affect the integrity
of the skin will predispose an individual to develop pressure sores. These factors
include poor hygiene, self-trauma, low-protein diet, preexisting tissue damage, muscle
wasting, inadequate bedding, and ill-fitting casts or bandages (Swaim and Angarano,
1990).
Clinical signs.
At first, the skin at the developing site will appear red and irritated. Over time,
constant trauma can result in full-thickness skin wounds and can progress to necrosis
of underlying structures such as bone. The severity of the sores may be graded from
I to IV, according to the depth of the wound and the tissues involved, from superficial
skin irritation to bone necrosis.
Epizootiology.
The problem usually occurs in large-breed dogs, but any type of dog can be affected.
Prevention and control.
Minimizing or eliminating those factors that can predispose to decubital ulcers is
important to both the prevention and the control of this condition. If the dogs are
going to experience long periods of recumbency, adequate bedding or padding must be
provided. Skin hygiene is of the utmost importance when trying to prevent or treat
pressure sores. The skin should be kept clean and dry at all times. If urine scalding
is a problem, the affected area should be clipped, bathed, and dried thoroughly at
least once or twice daily. Finally, an appropriate diet to maintain good flesh and
adequate healing is also important (Swaim and Angarano, 1990).
Treatment.
The treatment of pressure sores must involve care of the wound and attention to the
factors causing the wound. The extent of initial wound management will largely depend
on the depth of the wound. For simple abrasions and small wounds involving the skin
only, simple wound cleansing and open-wound management provide adequate treatment.
When wounds involve deeper tissues, including fat, fascia, or bone, more aggressive
therapy must be performed. The affected area should be radiographed to assess bone
involvement, and the wound should be cultured. All of the damaged tissue should be
debrided, and wound management guidelines should be followed (see Section III,C,1).
When a healthy granulation bed has formed over the entire wound, a delayed closure
over a drain may be performed (Swaim and Angarano, 1990). With extensive lesions,
reconstruction with skin flaps may be necessary.
Bandaging should be performed on all full-thickness wounds; however, it is important
to remember that ill-fitting or inadequately padded bandages or casts may worsen the
problem. The area over the wound itself should not be heavily padded, because this
will increase the pressure over the wound. The wounded area should be lightly covered
and then a doughnut, created from rolled gauze or towel, should be fitted around the
wound. This will displace the forces acting on the wound over a larger area and over
healthier tissue. Then the doughnut is incorporated into the bandage. If a cast has
been applied to the area for treatment or for research purposes, a hole can be cut
over the wound to reduce pressure in that area and allow treatment of the wound (Swaim
and Angarano, 1990). Bandages should be removed at least once or twice a day to allow
wound care.
After wound care has been initiated the causative factors for the pressure sore must
be addressed (see “Prevention and control,” above). Recumbent animals should be moved
frequently to prevent continuous compression on the wound. If the dog tends to favor
a position that aggravates the problem, splinting the body part to reduce contact
with hard surfaces may be necessary.
3.
Acral Lick Granuloma
Etiology.
Acral lick granuloma is a psychodermatosis, a skin lesion caused by self-trauma. In
a few cases, self-trauma begins because of identifiable neurologic or orthopedic causes
(Tarvin and Prata, 1988). However, the majority of the cases begin because of repetitive
licking by dogs that are confined and lack external stimuli (Swaim and Angarano, 1990).
It has been theorized that the self-trauma promotes the release of endogenous endorphins,
which act as a reward for the abnormal behavior (Dodman et al., 1988). The laboratory
setting is an environment that could promote this abnormal behavior and lead to acral
lick granuloma.
Epizootiology.
The lesions associated with acral lick granuloma are seen most often in large-breed
dogs, but any type of dog can be affected (Walton, 1986).
Clinical signs.
At first, lesions appear as irritated, hairless areas usually found on the distal
extremities (Swaim and Angarano, 1990). The predilection for the limbs may be due
to accessibility or possibly may be caused by a lower threshold for pruritus in these
areas. As the lesions progress, the skin becomes ulcerated, and the wound has a hyperpigmented
edge. The wounds may partially heal and then be aggravated again when licking resumes.
Diagnosis and differential diagnosis.
Aerai lick granulomas must be differentiated from several other conditions, including
bacterial or fungal infection, foreign bodies, and pressure sores. In addition, mast-cell
tumors and other forms of neoplasia can mimic the appearance of acral lick granuloma.
Many of these problems can be ruled out by the history of the animal. When in doubt,
a biopsy should be taken. An uncomplicated acral lick granuloma would feature hyperplasia,
ulceration, and fibrosis without evidence of infection or neoplasia (Walton, 1986).
Prevention and control.
Behavior modification and relief of boredom are important aspects of preventing (and
treating) acral lick granuloma. The environment of a dog with this problem can be
enriched with exercise and the introduction of toys. In addition, the relief of boredom
or anxiety can be attempted through the use of drugs such as phenobarbital, megestrol
acetate, and progestins. These drugs may produce side effects, however (Swaim and
Angarano, 1990), and may interfere with experimental results.
Treatment.
Several treatments have been reported for acral lick granuloma, and none of them have
been proven to be successful in all cases. One of the most important aspects of treatment
is to break the cycle of self-trauma. Mechanical restraint with an Elizabethan collar
is one of the easiest methods to accomplish this goal. Several direct treatments have
been examined, including intralesional and topical steroids, perilesional cobra venom,
acupuncture, radiation, and surgery (Swaim and Angarano, 1990; Walton, 1986). Opioid
antagonists have been used in an attempt to treat acral lick granuloma by blocking
endogenous opioids. In one study, either naltrexone (1 mg/kg SQ) or nalmefene (1–4
mg/kg SQ) successfully reduced the excessive licking behavior in 7 of 11 dogs; however,
lesions returned after the drug was discontinued (Dodman et al., 1988). The use of
a mixture of flunixin meglumine, steroid, and dimethyl sulfoxide (3 ml of Banamine
[Schering] mixed with 8 ml of Synotic [Diamond Laboratories]) applied topically twice
daily has also been shown to be effective (Walton, 1986). The prognosis for aeral
lick granuloma should be considered guarded, because the lesions often recur or new
lesions develop when treatment is discontinued.
4.
Elbow Hygroma
Etiology.
Hygromas are fluid-filled sacs that develop as a result of repeated trauma over a
bony prominence. The area over the olecranon is most frequently affected, but hygromas
have been reported in association with the tuber calcis, greater trochanter, and stifle
(Newton et al., 1974).
Epizootiology.
Elbow hygromas are most frequently reported in large and giant breeds of dogs around
6–18 months of age (Johnston, 1975; Bellah, 1993). Elbow hygromas are seen infrequently
in the laboratory animal setting because the commonly affected breeds are seldom used
in research. However, the housing environment for research dogs predisposes them to
hygromas, because these animals spend a large amount of time on hard surfaces such
as cage bottoms or cement runs. For this reason, laboratory animal veterinary and
husbandry staff should be familiar with this condition.
Clinical signs.
A dog with an elbow hygroma presents with a unilateral or bilateral, painless, fluctuant
swelling over the point of the elbow. The animals are not usually lame. Over a long
period of time, elbow hygromas may become inflamed and ulcerated. If the hygroma is
secondarily infected, the animal may exhibit pain and fever (Johnston, 1975; Bellah,
1993).
Pathology.
The fluid-filled cavity in the hygroma is lined by granulation and fibrous tissue.
Hygromas lack an epithelial lining and therefore are not true cysts. The fluid within
the cavity is yellow or red and is a serous transudate. This fluid is less viscous
than joint fluid, and elbow hygromas do not communicate with the joint (Johnston,
1975).
Treatment.
The treatment of elbow hygromas should be conservative whenever possible, and surgical
options should be reserved for complicated or refractory cases. Conservative management
of the elbow hygroma is aimed at relieving pressure at the point of the elbow by providing
a padded cage surface and/or bandaging the elbow in a manner similar to that used
to treat pressure sores (see Section III,C,2). More aggressive therapy, including
needle drainage and the injection of corticosteroid into the hygroma, has been described
but is not recommended, because infection is a serious complication of this treatment
(Johnston, 1975). Likewise, simple surgical excision of elbow hygromas can be associated
with complications such as wound dehiscence and ulceration (Johnston, 1975). A technique
that has been used successfully involves placement of multiple Penrose drains. The
drains are kept in place for 2–3 weeks, and the limb remains bandaged for 4 weeks
with this technique (Bellah, 1993). Another technique has been described that involves
the removal of a crescent-shaped piece of the skin and capsule. The remaining dead
space is closed with mattress sutures over stents, and then the wound is closed in
a routine fashion. The stents are removed in 5–7 days, and the wound is bandaged until
suture removal in 10–14 days (Newton et al., 1974). Regardless of the method used
to treat an elbow hygroma, recurrence of the problem is likely unless the predisposing
factors are identified and relieved.
5.
Corneal Ulcers
Etiology.
In the research environment, corneal ulcers are most often associated with either
direct trauma, contact with irritating chemicals, or exposure to the drying effects
of air during long periods of anesthesia. Chronic or recurrent corneal ulcers may
also be associated with infection or hereditary causes in some breeds of dogs; however,
these cases would be rare in the laboratory setting.
Clinical signs.
The signs of corneal ulceration are blepharospasm, epiphora, and photophobia. The
eye may appear irritated and inflamed. In minor cases, the cornea may not appear abnormal;
however, in cases of deeper ulceration, the cornea may appear roughened or may have
an obvious defect. In addition, the periocular tissues may be swollen and inflamed
because of self-inflicted trauma from rubbing at the eye.
Diagnosis.
A tentative diagnosis of corneal ulcer or abrasion may be based on the clinical signs.
A definitive diagnosis of corneal ulcers may be made by the green appearance of the
cornea when stained with fluorescein dye. When a corneal ulcer has been diagnosed,
the eye should be inspected for underlying causes such as foreign bodies or abnormal
eyelids or cilia.
Treatment.
The treatment of corneal ulcers will depend on the depth and size of the affected
area. Deep ulcers may require debridement and primary repair. In such cases, a third
eyelid or conjunctival flap may be applied to the eye until experienced help can be
obtained. Superficial abrasions are generally treated with topical application of
antibiotics. A triple antibiotic ointment that does not contain steroids given 3 times
a day for 2–3 days usually provides adequate treatment. Ointments are preferred over
drops, because use of the former requires less frequent. Simple corneal ulcers are
restained with fluorescein after 3 days and should show complete healing at that time.
If the ulcer is not healed, this may indicate that the ulcer has an undermined edge
impeding proper healing. Topical anesthetic should be applied to the eye, and a cotton-tipped
applicator can be rolled over the surface of the ulcer toward its edge. This will
remove the unattached edge of the cornea and healing should progress normally after
debridement. In all cases, an Elizabethan collar or other restraint may be necessary
to prevent additional trauma to the eye.
D.
Iatrogenic Diseases
1.
Indwelling Intravascular Catheter Infections
Indwelling intravascular catheters, including intracaths and vascular access ports,
often play a vital role in research protocols. The catheters are most often placed
in a central vein or artery where they may be used for repeated blood sampling, administration
of anesthetics and experimental compounds, or measurement of hemodynamic parameters.
Although catheters vary in composition, number of ports, and port placement, the basic
principles of their implantation and maintenance are similar. It is important that
the laboratory animal veterinarian be familiar with these principles and the potential
complications of catheter use. When appropriately maintained, indwelling catheters
may remain functional for months without serious complication.
The actual incidence of complications associated with indwelling vascular catheters
in dogs is unknown. This is due largely to the fact that many of the problems may
be incidental findings or related to a particular research protocol. One study (Hysell
and Abrams, 1967) examined the lesions found at necropsy in animals with chronic indwelling
catheters (exact vascular locations not specified). The lesions found were categorized
as traumatic cardiac lesions, visceral infarcts, and fatal hemorrhages. The traumatic
cardiac lesions consisted primarily of masses of fibrin and inflammatory cells on
the heart valves. The visceral infarcts were noted in the spleen, kidney (Fig. 5
), and brain and resulted from fibrin embolization from either the valvular lesions
or the catheter tip. Fatal hemorrhages were most often found in animals with experimentally
induced hypertension. These animals developed clinical signs of sepsis and later ruptured
a major vessel associated with mycotic infection and aneurysm.
Fig. 5
Chronic renal infarct in the kidney of a dog that had an indwelling aortic catheter.
Etiology.
The leading complication associated with the use of indwelling vascular catheters
is infection, either systemic or local at the point of entry through the skin. Septicemia
may develop from bacterial colonization of either the tract around the catheter or
the catheter lumen.
Clinical signs.
The signs and treatment of systemic infection are covered in Section III,D,3. Problems
with the skin defect associated with the catheter port vary from mild skin irritation
to obvious infection. The signs may include redness and swelling of the skin around
the external port, discharge from the skin wound, or even abscess formation.
Prevention.
Because indwelling catheters play an important role in many research protocols, it
is highly desirable to prevent catheter complications that may result in loss of the
device. The catheter should be made of nonthrombogenic material. In addition, it is
recommended that catheters be as simple as possible. A catheter with extra ports or
multiple lumens requires additional management and supplies more routes for infection.
The use of vascular access ports that lie entirely under the skin eliminates many
problems with infection. It has also been found that a long extension of tubing connected
to the port may actually reduce the potential for infection of the catheter (Ringler
and Peter, 1984). The initial placement of an indwelling catheter must be done under
aseptic conditions by individuals who are familiar with the procedure. The placement
of the catheter should be verified by radiography. Catheters that are used for delivery
of drugs or blood sampling should be positioned in the vena cava and not in the right
atrium, thereby minimizing trauma to the tricuspid valve. After catheter placement,
the animals should be observed daily for signs of either local or systemic infection.
The catheter entry site should be disinfected, coated with antibiotic ointment, and
rebandaged every other day. Once a month, the catheter line may be disinfected with
chlorine dioxide, as described below (see “Treatment”). Throughout the life of the
catheter, injections into and withdrawals from the catheter should be done in a sterile
manner, and the number of breaks in the line should be kept to a minimum.
Treatment.
The treatment of catheter infections almost invariably involves removal of the catheter,
as demonstrated in both dogs and monkeys (Ringler and Peter, 1984; DaRif and Rush,
1983). Superficial wound irritation or infection may be treated locally with antibiotic
ointment, sterile dressing changes and efforts to minimize catheter movement; however,
more extensive problems require aggressive therapy. Systemic antibiotic therapy should
be initiated for a 10-day period. The choice of drug will ultimately be based on previous
experience and culture results. Aerobic and anaerobic cultures of blood and locally
infected sites should be performed (Ringler and Peter, 1984). Localized abscesses
or sinus tracts may be managed by establishing drainage and flushing with chlorhexidine.
Again, the catheter should be removed. If retention of a catheter is important, the
catheter lumen may be disinfected by filling with chlorine dioxide solution. It has
been shown that there are no adverse effects from the use of chlorine dioxide in catheters
(Dennis et al., 1989). The solution is removed after 15 min and replaced with heparinized
saline. All of the extension lines and fluids used in the catheter should be discarded.
The blood cultures should be repeated 3 days after the antibiotic therapy has ceased.
If bacteria are still cultured, the catheter must be removed.
2.
Intestinal Access Ports
Intestinal access ports have been used to study the pharmacokinetics of drugs at various
levels in the intestinal tract. These catheters are usually vascular access ports
with several modifications to allow secure placement in bowel (Meunier et al., 1993).
When placed and managed correctly, these ports may remain in place for months without
complications.
The most frequently reported complication associated with these catheters is infection
around the port site (Meunier et al., 1993, Kwei et al., 1995). These infections lead
to removal of the catheters despite treatment with local lavage and systemic antibiotics.
There have also been reports of catheters dislodging from the intestinal tract and
resulting in peritonitis. This complication has largely been eliminated with the improved
security afforded by a synthetic cuff added to the end of the catheter (Meunier et
al., 1993). The chapter authors have also seen migration of the catheter end within
the lumen of the intestine (caused by peristaltic motion to egest the catheter), extensive
intra-abdominal adhesions, and intestinal torsion (Figs. 6a,b
) as complications of intestinal access ports.
Fig. 6
(a) Jejunal volvulus in a research dog caused by the presence of surgically implanted
intestinal ports. The point of insertion of the jejunal catheter (on the right in
the photograph) is coincident with the distal margin of the ischemic bowel. The catheter
on the left in the photograph enters the duodenum. (b) Another view of the dog in
(a), highlighting the swollen and ischemic segment of jejunum affected by the torsion.
The procedures for placement and maintenance of the catheters are similar to those
outlined previously for indwelling vascular catheters. It is important that the catheters
be firmly secured to the intestine to prevent migration or dislodgment. An omental
patch placed over the site of entry may help form a firm adhesion. In addition, it
is important to place the proper length of catheter within the peritoneal cavity;
excess catheter length can promote adhesion formation, whereas insufficient catheter
length to account for visceral organ motion can result in detachment. The placement
and patency of the catheters can be verified periodically by contrast radiography
using iodinated contrast material or by fecal occult blood testing after a small amount
of blood has been injected through the catheter (Meunier et al., 1993).
3.
Sepsis
Etiology.
Sepsis is defined as the systemic response to infection. Most often, sepsis is a result
of infection with gram-negative bacteria; however, sepsis may also be associated with
gram-positive bacteria and fungi. In laboratory animals, sepsis is seen as a complication
of surgical procedures or associated with chronic implants. Sepsis may also be seen
as a complication of infectious diseases such as parvovirus.
Clinical signs.
The signs of sepsis can vary, depending on the source of the infection and the stage
of the disease. Early in the course of sepsis, dogs will present with signs of a hyperdynamic
response, including an increased heart rate, increased respiratory rate, red mucous
membranes, and a normal to increased capillary refill time. Systemic blood pressure
and cardiac output will be increased or within the normal range. The animals will
often be febrile. Later in the course of the syndrome, the animals will show the classic
signs of septic shock, including decreased temperature, pale mucous membranes, and
a prolonged capillary refill time. Cardiac output and blood pressure are decreased
as shock progresses. Peripheral edema and mental confusion have also been reported
(Hauptman and Chaudry, 1993).
Pathogenesis.
The pathophysiology of sepsis is complex and is mediated by immune responses involving
mediators such as cytokines, eicosinoids, complement, superoxide radicals, and nitric
oxide. The body responds to overwhelming infection with an attempt to optimize metabolic
processes and maximize oxygen delivery to tissues. However, if inflammation is left
unchecked, the system may be unable to compensate, and the result is cardiovascular
collapse.
Diagnosis.
In general, a presumptive diagnosis of sepsis is made based on the occurrence of several
in a group of signs, including altered body temperature, increased respiratory and/or
heart rate, increased or decreased white blood cell count, increased number of immature
neutrophils, decreased platelet count, decreased blood pressure, hypoxemia, and altered
cardiac output. However, extreme inflammation without infection (e.g., pancreatitis,
trauma) may create similar signs. One study examined the diagnosis of sepsis in canine
patients at a veterinary hospital based on easily obtainable physical and laboratory
findings. That study found that septic individuals had higher temperatures, WBC counts,
and percentage of bands than non-septic individuals, whereas platelet counts were
lower in the septic dogs. There were no differences in respiratory rate or glucose
levels between the groups. Using these criteria, the results had a high sensitivity
and a tendency to overdiagnose sepsis (Hauptman et al., 1997). Ultimately, the presence
of a septic focus simplifies diagnosis greatly; however, the focus may not be obvious.
If the signs of sepsis are evident but the focus is not, several systems should be
evaluated for infection, including urinary tract, reproductive tract, abdominal cavity,
respiratory tract, teeth, and heart valves (Kirby, 1995).
Treatment.
The treatment of sepsis has three aims. The first aim is to support the cardiovascular
system. All septic animals should be treated with fluids to replace deficits and to
maximize cardiac output. Crystalloids are most frequently used to maintain vascular
volume, primarily because of their low cost. Colloids offer the advantage of maintaining
volume without fluid overload and may have other positive effects on the cardiovascular
system. Acid-base and electrolyte imbalances should also be addressed.
After the animal has stabilized, the treatment of sepsis should be aimed at removing
the septic focus. Obvious sources of infection should be drained or surgically removed.
If an implant is associated with the source of infection, the implant should be removed.
Antibiotic therapy should also be instituted. The choice of antibiotic will ultimately
depend upon the results of culture; however, the initial choice of antibiotics is
based on previous experience, source of infection, and Gram stains. The organisms
associated with sepsis are often gram-negative bacteria of gastrointestinal origin
or are previously encountered nosocomial infections. Ideally, the antibiotic chosen
for initial therapy should be a broad-spectrum, bactericidal drug that can be administered
intravenously. Second- or third-generation cephalosporins provide good coverage, as
does combination therapy with enrofloxacin plus metronidazole or penicillin.
Finally, the treatment of sepsis is aimed at blocking the mediators of the systemic
response. Several studies have examined the effects of steroids, nonsteroidal anti-inflammatory
drugs, and antibodies directed against endotoxin, cytokines, or other mediators of
the inflammatory response; however, none of these treatments have proven greatly effective
in clinical trials. Consequently, there is no “magic bullet” for the treatment of
sepsis at this time. Successful therapy remains dependent on aggressive supportive
care coupled with identification and elimination of the inciting infection.
4.
Aspiration Lung Injury
Etiology.
In research animals, aspiration into the lungs may occur accidentally during the oral
administration of various substances or by the misplacement of gastric tubes. Aspiration
of gastric contents may also occur as a complication of anesthesia. In pet animals,
aspiration is often seen as a result of metabolic and anatomical abnormalities; however,
such occurrence would be rare in the research setting.
Clinical signs.
The signs of aspiration lung injury may include cough, increased respiratory rate,
pronounced respiratory effort, and fever. When respiration is severely affected, the
oxygen saturation of blood will be decreased. The diagnosis of this problem is based
on a history consistent with aspiration and the physical findings. Classically, radiographs
of the thorax demonstrate a bronchoalveolar pattern in the cranioventral lung fields.
However, these lesions may not appear for several hours after the incident of aspiration.
In addition, the location of the lesions may be variable, depending on the orientation
of the animal at the time of aspiration.
Pathogenesis.
Aspiration of gastric contents or other compounds can create lung injury of variable
severity, depending upon the pH, osmolality, and volume of the substance. The compounds
aspirated can produce direct injury to lung tissue, but more importantly, the aspiration
provokes an inflammatory response probably mediated by cytokines. The result is a
rapid influx of neutrophils into the lung parenchyma and alveolar spaces. The inflammation
leads to increased vascular permeability with leakage of fluid into the alveolar spaces
and can eventually lead to alveolar collapse. If the condition is severe, it may result
in adult respiratory distress syndrome and respiratory failure. It should be noted
that infection is not present in the early stages of this condition but may complicate
the problem after 24–48 hr.
Treatment.
The treatment of aspiration lung injury is largely supportive and depends upon the
severity of the inflammation and the clinical signs. In cases in which a small amount
of a relatively innocuous substance (e.g., barium) has been aspirated, treatment may
not be necessary. When severe inflammation is present, systemic fluid therapy should
be instituted. Support of the cardiovascular system should be performed judiciously;
fluid overload could lead to an increase in pulmonary edema. The use of colloids is
controversial because of the increase in vascular permeability that occurs in the
lungs. Oxygen therapy is also controversial, because it may increase lung injury if
administered at high concentrations for long periods of time (Nader-Djahal et al.,
1997). Several studies have addressed the use of anti-inflammatory agents to reduce
lung injury associated with aspiration; however, none are used clinically in human
or veterinary medicine at this time.
In humans, antibiotics are reserved for use in cases with confirmed infection, in
order to prevent the development of antibiotic–resistant pneumonia. It has been suggested
that dogs should be treated with antibiotics immediately when the aspirated material
is either not acidic or has potentially been contaminated by oral bacteria associated
with severe dental disease. Amoxicillin-clavulanate has been recommended as a first
line of defense, reserving enrofloxacin for resistant cases (Hawkins, 2000). The presence
of pneumonia should be verified by tracheal wash and cultures.
5.
Burn Wounds
Etiology.
In laboratory animals, accidental burns usually result from thermal injury (heating
pads, water bottles) or harsh chemicals (strong alkalis, acids, disinfectants). The
insult to the skin results in desiccation of the tissue and coagulation of proteins.
In addition, the severely injured area is surrounded by a zone of vascular stasis,
which promotes additional tissue damage. Even small burns can result in significant
inflammation that could affect the outcome of some research investigations and cause
considerable discomfort to the animal. The proper and immediate treatment of burn
wounds can reduce the effects of the injury on both the individual and the research.
Clinical signs.
The clinical signs vary with the type and degree of burn injury. Initially, the injury
may not be noticed. The first signs may be oozing from the skin and matting of the
overlying hair. Within a couple of days, progressive hair and skin loss may be observed
(Johnston, 1993). The wounds may vary in severity from very superficial (involving
only the epidermis) to those in which the epidermis and dermis are completely destroyed.
Superficial wounds appear as red, inflamed skin similar to sunburn in humans. The
pain associated with these injuries usually subsides in 2–3 days, and the wound reepithelializes
without complications in 3–5 days. Deeper burns develop a thick covering, or eschar,
composed of the coagulated proteins and desiccated tissue fluid. The wound heals by
granulation under the eschar, which eventually sloughs or is removed to allow further
healing by contraction and reepithelialization. Within 2–3 days of injury, the burn
wound will be colonized by gram-positive bacteria that rapidly cover the entire wound.
Several days later, gram-negative organisms can appear in the burn wound (Johnston,
1993). At this point, signs of wound infection and sepsis may occur (see Section III,D,3).
Treatment.
Appropriate and timely treatment of a burn wound will reduce the extent of the injury.
Thermal injuries should be immediately cooled to reduce edema and pain (Demling and
Lalonde, 1989). Chemical burns should be thoroughly lavaged for 60 min after wounding.
The damaged tissues may be unable to mount appropriate responses to changes in temperature;
therefore, the lavage should be performed with warm water to prevent hypothermia.
After the initial treatment, all burn wounds should be gently cleansed 2–3 times a
day (Demling and Lalonde, 1989). Burns involving the epidermis and part of the dermis
can be extremely painful, and analgesia should be addressed throughout the treatment
period.
Systemic antibiotics are unable to penetrate eschar and are not adequately distributed
through the abnormal blood supply of burned tissues. Therefore, topical wound dressings
are recommended in the early stages of treatment. A thin film of a water-soluble broad-spectrum
antibiotic ointment should be applied to the wound surface after each cleaning. Silver
sulfadiazine has a broad spectrum, penetrates eschar well, and is often the preparation
of choice for burn wound therapy. Povidone-iodine ointment will also penetrate thin
eschar and provides a broad spectrum. Mafenide has a good spectrum that covers gram-negative
organisms well and is often used to treat infected wounds, although it is associated
with pain upon application (Demling and Lalonde, 1989). When signs of wound or systemic
infection are present, systemic antibiotics should be employed, and their ultimate
selection should be based on culture and sensitivity results.
After the topical antibiotic has been applied, a nonadherent dressing should be placed
on the wound. Burn wounds covered in such a manner tend to epithelialize more rapidly
and are less painful than uncovered wounds. When the eschar over a burn wound has
formed and become fully defined, a small or moderately sized wound may be completely
resected.
Prevention.
Obviously, prevention of burn wounds is preferable to a long course of treatment.
Care should be taken to prevent direct exposure to harsh chemicals. Tables, floors,
and other surfaces should be rinsed thoroughly after chemical use, prior to allowing
any animal contact. Electric heating pads should be avoided, and only heated water
blankets or circulating warm-air devices should be used to provide warmth to the animals.
In rare instances, heated water blankets have also caused burns; therefore these devices
should be carefully monitored. As a precaution, a thin towel may be placed between
the animal and the water blanket.
6.
Perivascular Extravasation of Drugs
Etiology.
Research and/or anesthetic protocols may require the intravenous injection of various
solutions. When these substances have a pH or osmolarity significantly different from
that of the surrounding tissues, the accidental perivascular extravasation of the
solutions may result in tissue damage. Several drugs have been shown to cause problems
when injected perivascularly, including pentobarbital, thiamylal, thiopental, thiacetarsemide,
vincristine, vinblastine, and doxorubicin (Swaim and Angarano, 1990; Waldron and Trevor,
1993).
Clinical signs.
The immediate signs of perivascular injection are swelling at the injection site and
withdrawal of the limb or other signs of discomfort. Later, the area may appear red,
swollen, and painful as inflammation progresses. Often there will be eventual necrosis
of the skin around the injection site. In cases of doxorubicin extravasation, signs
may develop up to a week after the injection, and the affected area may progressively
enlarge over a 1 to 4 month period. This is because the drug is released over time
from the dying cells (Swaim and Angarano, 1990).
Prevention.
Because the degree of injury and extensive treatment associated with perivascular
extravasation of a drug can be detrimental to research protocols and can cause severe
discomfort to the dog, prevention of these injuries is preferred. Prior to the use
of any substance, the investigator should be aware of its chemical composition and
the potential for problems. If a potentially caustic compound is to be used in a fractious
subject, sedation of the dog is warranted if this will not interfere with the research
protocol. Whenever possible, insertion of an indwelling catheter is extremely important.
Access to a central vessel such as the cranial or caudal vena cava is preferred over
the use of peripheral vessels. When peripheral catheters are used, the injection should
be followed by a vigorous amount of flushing with saline or other physiological solution
and removal of the catheter. Additional injections are best given through newly placed
catheters in previously unused vessels. The repeated use of an indwelling peripheral
catheter should be approached cautiously and done only out of necessity. Prior to
use, the catheter should be checked repeatedly for patency by withdrawal of blood
and injection of saline. Any swelling at the catheter site or discomfort by the subject
indicates that the catheter should not be used.
Treatment.
The treatment of perivascular injections will depend on the amount and type of substance
injected. In most cases, dilution of the drug with subcutaneous injections of saline
is recommended. In addition, steroids may be infiltrated locally to reduce inflammation.
Topical application of dimethyl sulfoxide (DMSO) may also be helpful in reducing the
immediate inflammation and avoiding the development of chronic lesions (Swaim and
Angarano, 1990). The addition of lidocaine to subcutaneous injections of saline has
been used in cases of thiacetarsemide injection (Hoskins, 1989), and local infiltration
of hyaluronidase accompanied by warm compresses has been suggested for use in cases
of vinblastine injection (Waldron and Trevor, 1993). Despite these treatments, necrosis
of skin may be observed and would require serial debridement of tissues with secondary
wound closure or skin grafting. In cases of doxorubicin extravasation, early excision
of affected tissues is advocated to prevent the progressive sloughing caused by sustained
release of the drug from dying tissues (Swaim and Angarano, 1990). In all cases, the
condition can be painful, and analgesia should be addressed.
7.
Hepatic Encephalopathy
Etiology.
Hepatic encephalopathy is the result of the derangements in metabolism associated
with abnormal liver function. This condition may be seen in young dogs with congenital
portosystemic shunting of blood flow. However, in the research setting, encephalopathy
occurs more often in canine models of hepatic disease that lead to liver failure.
A well-developed knowledge of the pathophysiology of liver disease is necessary for
the initial treatment and long-term management of hepatic encephalopathy.
Pathogenesis.
When the liver function is severely impaired because of either portosystemic shunting
of blood flow or loss of metabolically active hepatic tissue, the result is an accumulation
of ammonia, toxic amines, aromatic amino acids, and short-chain fatty acids (Hardy,
1989; Center, 1998). These compounds have several toxic effects that result in a decrease
in cerebral energy metabolism and a decrease in excitatory neurotransmitter synthesis.
Concurrently, there is an increase in the concentration of false neurotransmitters
and the inhibitory substance γ-aminobutyric acid (GABA).
Clinical signs.
The signs of hepatic encephalopathy include lethargy, depression, muscle tremors,
and convulsions.
Diagnosis and differential diagnosis.
A presumptive diagnosis of hepatic encephalopathy may be based on the appearance of
clinical signs following experimental manipulation of the liver. Additional diagnostic
tests to verify the loss of liver function can be performed to confirm the diagnosis.
Serum glucose and protein levels may be low if hepatic function is severely impaired.
A low serum urea nitrogen level suggests that the normal hepatic metabolism of ammonia
into urea has been impaired. Elevated levels of serum bile acids and blood ammonia
also verify the loss of liver function (Hardy, 1989). Measurement of serum hepatic
leakage enzymes are nondiagnostic, because they can be low, high, or normal.
Treatment.
Because of the severity of hepatic encephalopathy, treatment may be initiated based
on a presumptive diagnosis. During initial treatment, supportive care with fluids
and electrolytes should be instituted, based on the results of serum chemistry and
blood gas analysis. The majority of animals with hepatic dysfunction will be hypokalemic,
alkalotic, and hypernatremic; therefore, either 0.45% sodium chloride or 0.45% sodium
chloride with 2.5% dextrose, supplemented with potassium chloride, is recommended
(Hardy, 1989). The type of drug to be used for seizure control is controversial. The
short half-life of diazepam makes it an attractive choice compared with barbiturates,
which have prolonged metabolism when hepatic function is impaired (Maddison, 1995).
However, endogenous benzodiazepines mediate some of the CNS signs seen with hepatic
encephalopathy. Therefore, the use of diazepam has been discouraged in favor of phenobarbital
(Johnson, 2000). The drug selected for seizure control should be titrated carefully,
given the altered liver metabolism.
Most importantly, the treatment of dogs with hepatic encephalopathy must be aimed
at reducing the levels of toxic metabolites in the bloodstream. Because protein metabolism
is a major source of ammonia, all oral food intake should cease until the signs of
hepatic encephalopathy have abated. Because gastrointestinal bleeding may occur in
individuals with liver failure and this is also a source of protein, the use of H2
blockers such as cimetidine or ranitidine is suggested (Swalec, 1993). In addition,
lactulose retention enemas should be performed (10–15 ml/lb of a 30% solution in water,
retained for 20–30 min) (Hardy, 1989). Lactulose is an indigestible semisynthetic
sugar that is metabolized in the gut to lactic and other acids. The decrease in colonic
pH reduces ammonia levels in the bloodstream by converting intestinal ammonia into
less diffusible ammonium ions. Lactulose will also cause an osmotic diarrhea. Antibiotics
such as neomycin (10 mg/lb, 3–4 times/day) or metronidazole (9 mg/lb, 3 times/day)
should also be used to reduce the intestinal load of urease-producing bacteria responsible
for splitting urea into ammonia (Hardy, 1989).
When the signs of hepatic encephalopathy have resolved, the dog may be fed a low-protein
diet. Diets suitable for dogs with renal insufficiency are recommended initially.
This type of diet is not suitable for long-term use, however, because it appears that
individuals with some types of hepatic disease actually have increased protein requirements.
These requirements may be met by slowly increasing protein in the diet as long as
signs of hepatic encephalopathy do not recur. To maintain the appropriate balance
of aromatic and branched-chain amino acids, the diet should be based on vegetable
and dairy protein instead of meat or fish protein (Center, 1998). In addition, the
antibiotics suggested above should be continued to reduce the effects of increasing
dietary protein levels.
E.
Neoplastic Diseases
1.
Introduction
The prevalence of cancer in the general canine population has increased over the years
(Dorn, 1976). This can be attributed to the longer life spans resulting from improvements
in nutrition, disease control, and therapeutic medicine. Because of these changes,
cancer has become a major cause of death in dogs (Bronson, 1982).
In a lifetime cancer mortality study of intact beagles of both sexes, Albert et al.
(1994) found death rates similar to the death rate of the at-large dog population
(Bronson, 1982). Approximately 22% of the male beagles died of cancer. The majority
of the tumors were lymphomas (32%) and sarcomas (29%), including hemangiosarcomas
of the skin and fibrosarcomas. Of the female beagles dying of cancer (26% of the population
studied), three-quarters had either mammary cancer (40%), lymphomas (18%), or sarcomas
(15%). Of the sarcomas in females, one-third were mast cell tumors. In addition to
these tumors that cause mortality, the beagle is also at risk for thyroid neoplasia
(Hayes and Fraumeni, 1975; Benjamin et al., 1996).
Because of the popularity of the beagle as a laboratory animal, discussion of specific
neoplasms will focus on the tumors for which this breed is at risk, as well as tumors
that are common in the general canine population.
2.
Biopsy Techniques
Fine-needle aspirates are generally the first diagnostic option for palpable masses,
because they can easily be performed in awake, cooperative patients. This technique
allows for rapid differentiation of benign and neoplastic processes. In cases where
cytologic results from fine-needle aspirates are not definitive, more invasive techniques
must be used.
Needle-punch or core biopsies can also be performed in awake patients but typically
require local anesthesia. An instrument such as a Tru-Cut needle (Travenol Laboratories,
Inc., Deerfield, Illinois) is used to obtain a 1 mm × 1 to 1.5 cm biopsy of a solid
mass. A definitive diagnosis may be limited by the size of the sample acquired using
this technique.
Incisional and excisional biopsies are utilized when less invasive techniques fail
to yield diagnostic results. Excisional biopsies are the treatment of choice when
surgery is necessary, because the entire mass is removed. Surgical margins should
extend at least 1 cm around the tumor, and 3 cm if mast cell tumors are suspected
(Morrison et al., 1993). Incisional biopsies are performed when large soft-tissue
tumors are encountered and/or when complete excision would be surgically difficult
or life-threatening. When performing an incisional biopsy, always select tissue from
the margin of the lesion and include normal tissue in the submission.
3.
Lymphomas
Etiology.
Lymphomas are a diverse group of neoplasms that originate from lymphoreticular cells.
Whereas retroviral etiologies have been demonstrated in a number of species (e.g.,
cat, mouse, chicken), conclusive evidence of a viral etiology has not been established
in the dog. In humans, data implicate the herbicide 2,4-dichlorophenoxyacetic acid
(2,4-D) as a cause of non-Hodgkin's lymphoma, but studies in dogs with similar conclusions
have come under scrutiny (MacEwen and Young, 1991).
Clinical signs.
Multicentric and alimentary lymphomas account for most cases of canine lymphoma. In
multicentric lymphoma, animals usually present with enlarged lymph nodes and nonspecific
signs such as anorexia, weight loss, polyuria, polydypsia, and lethargy. When the
liver and spleen are involved, generalized organomegaly may be felt on abdominal palpation.
Alimentary lymphoma is associated with vomiting and diarrhea, in addition to previous
clinical signs.
Less commonly, dogs develop mediastinal, cutaneous, and extranodal lymphomas. Dogs
with mediastinal lymphoma often present with respiratory signs secondary to pleural
effusion. Hypercalcemia is most frequently associated with this form of lymphoma and
may result in weakness. Cutaneous lymphoma varies in presentation from solitary to
generalized and may mimic any of a number of other skin disorders. The tumors may
occur as nodules, plaques, ulcers, or dermatitis. Approximately half of the cases
are pruritic. A number of extranodal forms of lymphoma have been reported, including
tumors affecting the eyes, central nervous system, kidneys, or nasal cavity. Clinical
presentation varies, depending on the site of involvement.
Epizootiology.
The incidence of lymphoma is highest in dogs 5–11 years old, accounting for 80% of
cases. Although the neoplasm generally affects dogs older than 1 year, cases in puppies
as young as 4 months have been reported (Dorn et al., 1967).
Pathologic findings.
Enlarged neoplastic lymph nodes vary in diameter from 1 to 9 cm and are moderately
firm. Some may have areas of central necrosis and are soft to partially liquefied.
The demarcation between cortex and medulla is generally lost, and on cut section,
the surface is homogenous. The spleen may have multiple small nodular masses or diffuse
involvement with generalized enlargement. The enlarged liver may have disseminated
pale foci or multiple large, pale nodules. In the gastrointestinal tract, both nodular
and diffuse growths are observed. These masses may invade through the stomach and
intestinal walls.
Histologically, the most common lymphomas are classified as intermediate to high grade
and of large-cell (histiocytic) origin. The neoplastic lymphocytes typically obliterate
the normal architecture of the lymph nodes and may involve the capsule and perinodal
areas.
Pathogenesis.
All lymphomas regardless of location should be considered malignant. A system for
staging lymphoma has been established by the World Health Organization. The average
survival time for dogs without treatment is 4–6 weeks. Survival of animals undergoing
chemotherapy is dependent on the treatment regimen as well as the form and stage of
lymphoma (MacEwen and Young, 1991).
Hypercalcemia is a paraneoplastic syndrome frequently associated with lymphoma. The
pathogenesis of this phenomenon is not fully understood but may be a result of a parathormone-like
substance produced by the neoplastic lymphocytes.
Diagnosis and differential diagnosis.
Differential diagnoses for multicentric lymphoma include systemic mycosis; salmon-poisoning
and other rickettsial infections; lymph node hyperplasia from viral, bacterial, and/or
immunologic causes; and dermatopathic lymphadenopathy. Alimentary lymphoma must be
distinguished from other gastrointestinal tumors, foreign bodies, and lymphocytic-plasmacytic
enteritis. In order to make a definitive diagnosis, whole lymph node biopsies and
full-thickness intestinal sections are frequently needed.
Treatment.
Therapy for lymphoma typically consists of one or a combination of several chemotherapeutic
agents. The treatment regimen is based on the staging of the disease, the presence
of paraneoplastic syndromes, and the overall condition of the patient. MacEwen and
Young (1991) provide a thorough discussion of therapeutic options for the treatment
of lymphomas in the dog.
Research complications.
Given the grave prognosis for lymphoma with or without treatment, euthanasia should
be considered for research animals with significant clinical illness.
4.
Tumors of the Skin and Soft Tissue
a.
Fibrosarcomas
Etiology.
The fibrosarcoma group of tumors encompasses not only malignant tumors of fibroblasts
but also a number of indistinguishable tumors, all of which are capable of collagen
production (Pulley and Stannard, 1990). Frequently classified in this group are undifferentiated
leiomyosarcomas, liposarcomas, malignant melanomas, and malignant schwannomas.
Clinical signs.
Although these neoplasms can arise throughout the body, they are most commonly found
in the skin, subcutaneous tissues, and oral cavity. Fibrosarcomas are extremely variable
in size and can grow to be quite large. In general, they are irregular and nodular,
poorly demarcated, and nonencapsulated, and they frequently invade deeper tissues.
Epizootiology.
Most fibrosarcomas develop in adult and aged animals but can affect dogs as young
as 6 months or less.
Pathologic findings.
Histologically, fibrosarcomas appear as interwoven bundles of densely packed spindle-shaped
fibroblasts. The cells show large numbers of mitotic figures, while undifferentiated
tumors may exhibit multinucleated giant cells and cells with bizarre shapes.
Pathogenesis.
Fibrosarcomas exhibit rapid, invasive growth, recurring frequently after excision.
Metastasis occurs in only one-fourth of cases, usually by the bloodstream to the lungs.
Less frequently, spread to local lymph nodes is observed.
Diagnosis and differential diagnosis.
Differential diagnoses for fibrosarcomas vary with the location of the tumor. Histopathologic
exam should be used to distinguish these tumors from round cell tumors (mast cell
tumors, histiocytomas, transmissible venereal tumors), papillomas, and other neoplasms.
Treatment.
Treatment of any soft-tissue sarcoma would begin with wide surgical excision. If the
tissue margins indicate incomplete resection, radiotherapy could be used. For any
high-grade tumors, adjuvant chemotherapy would be recommended (see MacEwen and Withrow,
1991a, for a complete discussion).
Research complications.
Because fibrosarcomas are locally invasive and often recur, dogs with these neoplasms
should not be considered good subjects for long-term studies.
b.
Lipomas
Etiology.
Neoplasms of lipocytes and lipoblasts are well-differentiated tumors referred to as
lipomas.
Clinical signs.
These growths can be found as single or multiple round, ovoid, or discoid masses in
the subcutaneous tissues of the lateral and ventral thorax, abdomen, and upper limbs.
Generally they are well circumscribed, encapsulated, and soft on palpation. Further,
the skin is freely movable over the tumor.
Epizootiology.
Lipomas occur principally in aged animals (average 8 years), and the incidence increases
with age (Pulley and Stannard, 1990). The tumors are most commonly seen in overweight
female dogs, but no breed predisposition is observed.
Pathologic findings.
Histologically, lipomas are indistinguishable from normal adipose tissue except when
a fibrous capsule is present.
Pathogenesis: Lipomas are typically slow-growing and do not recur after complete surgical
excision.
Diagnosis and differential diagnosis.
Lipomas are not frequently confused with other tumors but can sometimes be difficult
to distinguish from normal adipose tissue. Generally, the distinction can be made
from the clinical history.
Treatment.
Treatment for lipomas is not usually necessary unless the mass is causing problems
with normal ambulation. In such cases, surgical excision is usually curative.
Research complications.
Lipomas usually do not complicate research studies unless they are interfering with
other systemic functions or ambulation.
c.
Histiocytomas
Etiology.
Histiocytomas are benign skin growths that arise from the monocyte–macrophage cells
in the skin. Some debate exists as to whether this growth is actually a neoplasm or
a focal inflammatory lesion (Pulley and Stannard, 1990).
Clinical signs.
The most frequent sites for histiocytomas are the head (especially the pinna) and
the skin of the distal forelegs and feet. The masses are usually domelike or buttonlike
(often referred to as “button tumors”) and usually measure 1–2 cm in diameter.
Epizootiology.
Histiocytomas are the most common tumors of young dogs, mostly occurring in dogs less
than 2 years of age.
Pathologic findings.
Histologically, these tumors contain round to ovoid cells with pale cytoplasm and
large nuclei. The cells infiltrate the dermis and subcutis, displacing collagen fibers
and skin adnexa. Despite being benign lesions, histiocytomas characteristically have
a high mitotic index.
Pathogenesis.
This tumor typically exhibits rapid growth (1–4 weeks) but does not spread. Most histiocytomas
will spontaneously regress in less than 3 months.
Diagnosis and differential diagnosis.
Histiocytomas must be distinguished from potentially metastatic mast cell tumors.
This is accomplished by staining with toluidine blue, which would stain the cytoplasmic
granules of mast cells red or purple.
Treatment.
Although most histiocytomas will spontaneously resolve, conservative surgery or cryosurgery
will provide an expeditious resolution.
Research complications.
Histiocytomas should not interfere with most studies.
d.
Mast Cell Tumors
Etiology.
Neoplastic proliferations of mast cells are the most commonly observed skin tumor
of the dog (Bostock, 1986). Mast cells are normally found in the connective tissue
beneath serous surfaces and mucous membranes, and within the skin.
Clinical signs.
Well-differentiated mast cell tumors are typically solitary, well-circumscribed, slow-growing,
1 to 10 cm nodules in the skin. Alopecia may be observed, but ulceration is not usual.
Poorly differentiated tumors grow rapidly, may ulcerate, and may cause irritation,
inflammation, and edema. Mast cell tumors can be found on any portion of the dog's
skin but frequently affect the hindquarters, especially the thigh and inguinal and
scrotal areas. Mast cell tumors usually appear to be discrete masses, but they frequently
extend deep into surrounding tissues.
Epizootiology.
These tumors tend to affect middle-aged dogs but have been observed in dogs ranging
from 4 months to 18 years (Pulley and Stannard, 1990).
Pathologic findings.
Because of the substantial variation in histologic appearance of mast cell tumors,
a classification and grading system described by Patnaik et al. (1986) has become
widely accepted. In this system, grade I has the best prognosis, and grade III the
worst prognosis. Grade I tumors are well differentiated, with round to ovoid uniform
cells. The nuclei are regular, the cytoplasm is packed with large granules that stain
deeply, and mitotic figures are rare to absent. Grade II (intermediately differentiated)
mast cell tumors have indistinct cytoplasmic boundaries with higher nuclear-cytoplasmic
ratios, fewer granules, and occasional mitotic figures. Grade III (anaplastic or undifferentiated)
mast cell tumors have large, irregular nuclei with multiple prominent nucleoli. The
cytoplasmic granules are few, but mitotic figures are much more frequent. In addition
to skin lesions, mast cell tumors have been associated with gastric ulcers. These
lesions are most likely secondary to tumor production of histamine. Histamine stimulates
the H2 receptors of the gastric parietal cells, causing increased acid secretion.
Gastric ulcers have been observed in large numbers (>75%) of dogs with mast cell tumors
(Howard et al., 1969). The ulcers can be found in the fundus, pylorus, and/or proximal
duodenum.
Pathogenesis.
Although all mast cell tumors should be considered potentially malignant, the outcome
in individual cases can be correlated with the histologic grading of the tumor. Grade
III tumors are most likely to disseminate internally. This spread is usually to regional
lymph nodes, spleen, and liver and less frequently to the kidneys, lungs, and heart.
Diagnosis and differential diagnosis.
Mast cell tumors can be distinguished histologically from other round cell tumors
(such as histiocytomas and cutaneous lymphomas) by using toluidine blue, which metachromatically
stains the cytoplasmic granules of the mast cells red or purple.
Treatment.
Initial treatment for mast cell tumors is generally wide surgical excision (1 to 3
cm margins). Even with wide surgical margins, approximately 50% of mast cell tumors
may recur. If the site is not amenable to wide surgical excision, debulking surgery
and radiation therapy may be used. Other alternatives include amputation (if on a
limb) or radiation therapy alone. As an adjunct to surgery, Grier et al. (1990, Grier
et al. (1995) found that deionized water injected into surgical margins reduced tumor
recurrence by hypo-osmotically lysing any mast cells left behind. This technique has
recently been refuted by Jaffe et al. (2000). For systemic mastocytosis, and nonresectable
or incompletely excised mast cell tumors, chemotherapy can be used. Treatment options
would include oral prednisolone, intralesional triamcinalone, and the combination
of cyclophosphamide, vincristine, and prednisolone (Graham and O'Keefe, 1994).
Research complications.
Because of the possibility of systemic histamine release and tumor recurrence, dogs
with mast cell tumors are not good candidates for research studies. Grade I mast cell
tumors may be excised, allowing dogs to continue on study; however, monitoring for
local recurrence should be performed on a regular basis (monthly). Grade II tumors
are variable; animals that undergo treatment should be monitored for recurrence monthly,
and evaluation of the buffy coat should be performed every 3–6 months for detection
of systemic mastocytosis. Because of the poor prognosis for grade III tumors, treatment
is unwarranted in the research setting.
e.
Hemangiosarcomas
Etiology.
Hemangiosarcomas are malignant tumors that originate from endothelial cells.
Clinical signs.
These tumors may arise in the subcutis but are more commonly found in the spleen and
the right atrium. Clinical signs are associated with the site of involvement. Vascular
collapse is frequently observed secondary to rupture and hemorrhage from splenic masses.
Heart failure can be observed secondary to tumor burden or hemopericardium. When found
in the skin, hemangiosarcomas are poorly circumscribed, reddish black masses that
range in size from 1 to 10 cm in diameter. The most common cutaneous sites are the
ventral abdomen, the prepuce, and the scrotum.
Epizootiology: Hemangiosarcomas occur most frequently in 8- to 13-year-old dogs. The
German shepherd dog is most commonly affected.
Pathologic findings.
Grossly, splenic hemangiosarcomas resemble nodular hyperplasia or hematomas (Fig.
7
). The masses are spherical and reddish black and can range in size up to 15–20 cm
in diameter. On cut section the masses may appear reddish gray or black and have cavernous
areas of clotted blood. When the masses are found in the heart, the endocardium may
be covered by a thrombus, giving the 2 to 5 cm tumors a reddish gray or yellow appearance.
Histologically, hemangiosarcomas are composed of immature endothelial cells that form
vascular channels or clefts. These spaces may be filled with blood or thrombi. The
neoplastic cells are elongated with round to ovoid, hyperchromatic nuclei and frequent
mitotic figures.
Fig. 7
Splenic hemangiosarcoma in a random-source research dog.
Pathogenesis.
Hemangiosarcomas can be found in one or many sites. In cases where multiple sites
are involved, it may be impossible to identify the primary tumor. This neoplasia is
highly malignant and spreads easily. Metastasis occurs most frequently to the lungs
but can be found in any tissue.
Diagnosis and differential diagnosis.
Splenic hemangiosarcoma may resemble nodular hyperplasia or some manifestations of
lymphoma. When the heart is affected, other causes of heart failure must be ruled
out. Echocardiography is a valuable tool for identifying the primary lesion. Histopathology
should be used to differentiate dermal hemangiosarcoma from hemangiomas and other
well-vascularized tumors.
Treatment.
Surgery is generally the first choice of treatment for hemangiosarcoma. Dermal tumors
are treated with radical resection, splenic tumors by total splenectomy, and heart
tumors by debulking and pericardiectomy. Because of the high likelihood of metastasis,
adjunct chemotherapy should always be considered.
Research complications.
Dogs with dermal hemangiosarcoma may be cured after complete resection with margins,
but monitoring should be done regularly for recurrence. The other forms of hemangiosarcoma
have a much poorer long-term prognosis, and treatment is typically unwarranted in
the research setting.
5.
Transmissible Venereal Tumors
Etiology.
Also known as infectious or venereal granuloma, Sticker tumor, transmissible sarcoma,
and contagious venereal tumor, the transmissible venereal tumor is transmitted to
the genitals by coitus (Nielsen and Kennedy, 1990). The origin of this tumor is still
unknown but has been described as a tumor of lymphocytes, histiocytes, and reticuloendothelial
cells. Although this tumor has been reported in most parts of the world, it is most
prevalent in temperate climates (MacEwen, 1991).
Clinical signs.
The tumors are usually cauliflower-like masses on the external genitalia, but they
can also be pedunculated, nodular, papillary, or multilobulated. These friable masses
vary in size up to 10 cm, and hemorrhage is frequently observed. In male dogs, the
lesions are found on the caudal part of the penis from the crura to the bulbus glandis
or on the glans penis (Fig. 8
). Less frequently, the tumor is found on the prepuce. Females typically have lesions
in the posterior vagina at the junction of the vestibule and vagina. When located
around the urethral orifice, the mass may protrude from the vulva. These tumors have
also been reported in the oral cavity, skin, and eyes.
Fig. 8
Transmissible venereal tumor on the penis of a dog.
Epizootiology and transmission.
Transmissible venereal tumors are most commonly observed in young, sexually active
dogs. Transmission takes place during coitus when injury to the genitalia allows for
transplantation of the tumor. Genital to oral to genital transmission has also been
documented (Nielsen and Kennedy, 1990). Extragenital lesions are believed to be a
result of trauma prior to exposure to the tumor.
Pathologic findings.
Histologically, cells are arranged in compact masses or sheets. The cells are round,
ovoid, or polyhedral and have large, round hyperchromic nuclei. Also present are large
nucleoli and numerous mitotic figures. The cytoplasm is eosinophilic, and cell borders
are indistinct.
Pathogenesis.
Tumor growth is rapid after implantation but later slows. Metastasis is rare (<5%
of cases) but may involve the superficial inguinal and external iliac lymph nodes
as well as distant sites.
Diagnosis and differential diagnosis.
Transmissible venereal tumors have been confused with lymphomas, histiocytomas, mast
cell tumors, and amelanotic melanomas. Cytology may be of benefit in making a definitive
diagnosis, so impression smears should be made prior to processing for histopathology.
Prevention.
Thorough physical examinations prior to bringing new animals into a breeding program
should prevent introduction of this tumor into a colony.
Control.
Removing affected individuals from a breeding program should stop further spread through
the colony.
Treatment.
Surgery and radiation can be used for treatment, but chemotherapy is the most effective.
Vincristine (0.5–0.7 mg/m2) IV once weekly for 4–6 treatments will induce remission
and cure in greater than 90% of the cases (MacEwen, 1991).
Research complications.
Experimental implantation of transmissible venereal tumors has been shown to elicit
formation of tumor-specific IgG (Cohen, 1972). This response may occur in natural
infections and could possibly interfere with immunologic studies.
6.
Mammary Gland Tumors
Etiology.
Dogs are susceptible to a wide variety of mammary gland neoplasms, most of which are
influenced by circulating reproductive steroidal hormones.
Clinical signs.
Single nodules are found in approximately 75% of the cases of canine mammary tumors.
The nodules can be found in the glandular tissue or associated with the nipple. Masses
in the two most caudal glands (fourth and fifth) account for a majority of the tumors.
Benign tumors tend to be small, well circumscribed, and firm, whereas malignant tumors
are larger and invasive and coalesce with adjacent tissues.
Epizootiology.
Mammary tumors are uncommon in dogs under 5 years of age with the incidence rising
sharply after that. Median age at diagnosis is 10–11 years. Mammary tumors occur almost
exclusively in female dogs, with most reports in male dogs being associated with endocrine
abnormalities, such as estrogen-secreting Sertoli cell tumors.
Pathologic findings.
Based on histologic classification of mammary gland tumors, approximately half of
the reported tumors are benign (fibroadenomas, simple adenomas, and benign mesenchymal
tumors), and half are malignant (solid carcinomas, tubular adenocarcinomas, papillary
adenocarcinomas, anaplastic carcinomas, sarcomas, and carcinosarcomas) (Bostock, 1977).
Extensive discussions of classification, staging, and histopathologic correlations
can be found in MacEwen and Withrow (1991b) and Moulton (1990).
Pathogenesis.
Mammary tumors of the dog develop under the influence of hormones. Receptors for both
estrogen and progesterone can be found in 60–70% of tumors. Futher, Schneider et al.
(1969) showed that the risk of developing mammary tumors increased greatly after the
first and second estrus cycles. Dogs spayed prior to the first estrus had a risk of
0.8%, whereas dogs spayed after the first and second estrus had risks of 8% and 26%,
respectively.
Malignant mammary tumors typically spread through the lymphatic vessels. Metastasis
from the first, second, and third mammary glands is to the ipsilateral axillary or
anterior sternal lymph nodes. The fourth and fifth mammary glands drain to the superficial
inguinal lymph nodes where metastasis can be found. Many mammary carcinomas will eventually
metastasize to the lungs.
Diagnosis and differential diagnosis.
Both benign and malignant mammary tumors must be distinguished from mammary hyperplasia
and mastitis.
Prevention.
Mammary tumors can effectively be prevented by spaying bitches prior to the first
estrus. This is commonly done in the general pet population at 6 months of age. Recently,
the topic of spaying sexually immature dogs (8–16 weeks of age) has received much
attention for the control of the pet population. Kustritz (1999) reviewed the techniques
for anesthesia and surgery, as well as possible pros and cons of spaying at this young
age.
Treatment.
Surgery is the treatment of choice for mammary tumors, because chemotherapy and radiation
therapy have not been reported to be effective. The extent of the surgery is dependent
on the area involved. Lumpectomy or nodulectomy should be elected in the case of small
discrete masses, while mammectomy and regional or total mastectomies are reserved
for more aggressive tumors. At the time of surgery, axillary lymph nodes are removed
only if enlarged or positive on cytology for metastasis. Inguinal lymph nodes should
be removed any time the fourth and fifth glands are excised (MacEwen and Withrow,
1991b).
Research complications.
Because 50% of mammary tumors are benign, treatment may be rewarding, allowing dogs
to continue on study. If removed early enough, malignant masses could yield the same
results. All dogs should be monitored regularly for recurrence and new mammary tumors.
7.
Thyroid Carcinomas
Etiology.
Beagles are among the breeds with the highest prevalence of thyroid carcinomas. Benjamin
et al. (1996) reported a correlation between lymphocytic thyroiditis, hypothyroidism,
and thyroid neoplasia in the beagle.
Clinical signs.
Thyroid carcinomas generally present as palpable cervical masses. Affected animals
may experience dysphagia, dyspnea, and vocalization changes. Precaval syndrome resulting
in facial edema is also observed in some cases.
Epizootiology and transmission.
The mean age of dogs presented with thyroid carcinomas is 9 years, with equal distribution
of cases between the sexes.
Pathologic findings.
Grossly, thyroid carcinomas are multinodular masses, frequently with large areas of
hemorrhage and necrosis. They tend to be poorly encapsulated and invade local structures
such as the trachea, esophagus, larynx, nerves, and vessels. The masses are unilateral
twice as often as bilateral (Capen, 1990). Histologically, thyroid carcinomas are
divided into follicular, papillary, and compact cellular (solid) types (see Capen,
1990, for complete discussion).
Pathogenesis.
Thyroid carcinomas tend to grow rapidly and invade local structures. Early metastasis
is common and occurs to the lungs by invasion of branches of the thyroid vein.
Diagnosis and differential diagnosis.
Nonpainful cervical swellings such as seen with thyroid tumors are also consistent
with abscesses, granulomas, salivary mucoceles, and lymphomas. Often a preliminary
diagnosis can be made by fine-needle aspirate.
Treatment.
Surgery is the treatment of choice for thyroid carcinomas that have not metastasized.
When the tumor is freely movable, surgery may be curative. Surgical excision may be
difficult for tumors that adhere to local structures, requiring excision of the jugular
vein, carotid artery, and associated nerves. When bilateral tumors are observed, preservation
of the parathyroid glands may not be possible. In such cases, treatment for hypoparathyroidism
will be necessary. Both chemotherapy and radiation therapy have been suggested for
extensive bilateral tumors and/or after incomplete excision, but no controlled trials
have been performed (Ogilvie, 1991).
Research complications.
In the research setting, treatment of this tumor may not be rewarding. Only freely
movable tumors can be practically treated without seriously affecting research efforts.
Euthanasia is warranted in the more advanced cases when clinical illness is apparent.
F.
Miscellaneous Diseases
1.
Congenital Disorders
Beagles are subject to many of the inherited and/or congenital disorders that affect
dogs in general. In a reference table on the congenital defects of dogs (Hoskins,
2000b), disorders for which beagles are specifically mentioned include brachyury (short
tail), spina bifida, pulmonic stenosis, cleft palate–cleft lip complex, deafness,
cataracts, glaucoma, microphthalmos, optic nerve hypoplasia, retinal dysplasia, tapetal
hypoplasia, factor VII deficiency, pyruvate kinase deficiency, pancreatic hypoplasia,
epilepsy, GM1 gangliosidosis, globoid cell leukodystrophy, XX sex reversal, and cutaneous
asthenia (Ehlers-Danlos syndrome). In addition, there are defects that affect so many
breeds that the author simply lists “many breeds” for the breeds affected by those
disorders. Thus these defects could also affect beagles and include pectus excavatum,
Polydactyly, radial and ulnar dysplasia, hypoadrenocorticism, entropion, lens coloboma,
factor VIII deficiency (von Willebrand's disease), renal agenesis or ectopia, and
developmental defects of the reproductive and lower urinary tracts.
At a commercial breeder of purpose-bred beagles, the most common birth defects were
umbilical hernia (1.82% of births) and open fontanelle (1.44% of births) (R. Scipioni
and J. Ball, personal communication, 1999). Other defects observed include cleft palate
and cleft lip, cryptorchidism, monorchidism, limb deformity, inguinal hernia, diaphragmatic
hernia, hydrocephaly, and fetal anasarca. Each of these other congenital defects occurred
at less than 1.0% incidence.
2.
Age-Related Diseases
a.
Cataracts and Nuclear Sclerosis
Etiology.
Cataract is an opacification of the lens or the lens capsule. It is the pathologic
response of the lens to illness or injury, because the lens has no blood supply. Cataracts
can be caused by metabolic, inflammatory, infectious, or toxic causes and can be congenital,
juvenile, or degenerative. Nuclear sclerosis is an apparent opacification of the lens
caused by the compression of older lens fibers in the center of the lens (nucleus)
as a consequence of the production of new fibers. Because the nucleus increases in
size as the animal ages, the sclerosis is more apparent in older animals and may be
mistaken as a senile cataract. The ability to see the fundus during ophthalmoscopy
persists with nuclear sclerosis but is obstructed by a true cataract.
Clinical signs.
The first clinical sign is typically the ability to visualize the opaque lens through
the pupil of the dog's eye. Dogs have an impressive ability to tolerate bilateral
lens opacity (especially when development is gradual), and often visual impairment
is detected late in the development of the condition (Helper, 1989). Moderate vision
loss may cause the dog to be hesitant in moving in new surroundings or unable to locate
movable objects (such as a toy). Rapid cataract development can result in a sudden
vision loss, such as can occur with diabetic cataracts.
Epizootiology and transmission.
Certain dog breeds can be predisposed to the development of juvenile or senile cataracts
or to metabolic disorders that result in cataract development, such as diabetes mellitus.
Dogs in studies for diabetes mellitus should be observed regularly for cataract development.
Toxicological studies may also induce formation of cataracts.
Pathogenesis.
Lens fibers respond to all biological or chemical insults by necrosis and liquefaction
(Render and Carlton, 1995), because they have no blood supply with which to recruit
an inflammatory and repair process. Disruption of these fibers by any means, therefore,
leads to opacification. The exact processes by which the varieties of congenital and
juvenile cataracts are produced have not been determined. In diabetic cataracts, the
excess glucose is metabolized to sorbitol and fructose. As these alcohols and sugars
accumulate in the lenticular cells, they produce an osmotic imbalance, which brings
fluid into the cells, causing swelling and degeneration of lens fibers and resultant
opacity (Capen, 1995).
Diagnosis and differential diagnosis.
The ability to visualize the retina and fundus during ophthalmoscopy differentiates
true cataracts from nuclear sclerosis. Dogs with cataracts should be evaluated for
possible causes, especially diabetes mellitus. Diabetes mellitus will typically affect
middle-aged dogs and feature rapid cataract formation, whereas juvenile and senile
cataracts are slow to develop and affect younger and older dogs, respectively. Progressive
retinal atrophy can also cause secondary cataract formation; pupillary light response
is maintained with primary cataracts (even if the lens is completely opaque), whereas
this reflex is obtunded by retinopathy.
Prevention.
Most forms of cataracts cannot be prevented, for their exact etiologic pathogenesis
is unknown. Diabetic cataracts, however, can be prevented by proper regulation of
blood glucose concentrations with insulin therapy and proper diet.
Treatment.
Because dogs do not need to focus visual images as accurately as human beings, proper
lens clarity and function are not necessary for an adequate quality of life. Many
dogs adjust quite well to the visual impairment caused by persistent cataracts. Lens
removal can be performed for dogs seriously affected by cataracts, but this would
not be anticipated for dogs in the research setting. Information on surgical lens
extraction procedures can be found in Helper (1989) or other veterinary ophthalmology
textbooks.
Research complications.
Research complications would be minimal with cataracts, unless the dogs were intended
for use in ophthalmologic or visual acuity–based studies.
b.
Hip Dysplasia
Etiology.
Hip dysplasia is a degenerative disease of the coxofemoral joint. A specific etiology
is unknown, but the development of hip dysplasia has a strong genetic component (Pedersen
et al., 2000), modified by age, weight, size, gender, conformation, rate of growth,
muscle mass, and nutrition (Smith et al., 1995).
Clinical signs.
The initial clinical abnormality caused by hip dysplasia is laxity of the coxofemoral
joint. This may present as a gait abnormality without any indication of lameness or
stiffness. Eventually, affected dogs will have periods of lameness and, in protracted
cases, will be rendered immobile by severe pain.
Epizootiology and transmission.
Hip dysplasia has been seen in most dog breeds, but it typically affects larger breeds
of dogs. In the research setting, it is primarily a condition of random-source large-breed
dogs used for surgical research.
Diagnosis and differential diagnosis.
Hip dysplasia is classically diagnosed by radiography of the pelvis and hip joints.
Radiographic abnormalities consistent with hip dysplasia include shallow acetabula
with remodeling of the acetabular rim, flattening of the femoral head, subchondral
bone sclerosis (caused by erosion of articular cartilage and exposure of underlying
bone), and osteophyte production around the joint (Pedersen et al., 2000). Hip dysplasia
needs to be differentiated from other musculoskeletal or neurological conditions that
can cause unusual gaits and/or lameness. This may be somewhat difficult, because clinical
signs of hip dysplasia may develop before radiographic abnormalities. Radiographic
calculation of the distraction index (DI) to measure joint laxity has proven to be
a good means to predict future hip dysplasia before other radiographic changes are
evident (Smith et al., 1995).
Prevention.
Because of the genetic component, dogs with hip dysplasia should not be used in breeding
colonies. Dogs should be provided a good plane of nutrition but not be allowed to
become overweight. Dogs that were limit-fed at 75% of the food amount eaten by ad
libitum–fed dogs had lower body weights and decreased severity of radiographic lesions
of hip dysplasia (Kealy et al., 1997).
Treatment.
In the stages when clinical signs are episodic, cage rest and analgesics for several
days can be used to treat the symptoms. More advanced cases may require continuous
analgesia. Sectioning of the pectineus muscle or tendon may provide some pain relief
but does not affect the progression of the disease (Pedersen et al., 2000). Surgical
treatments for hip dysplasia include femoral head ostectomy and total hip replacement.
Neither surgical treatment is likely in a research setting.
Research complications.
Long-term studies using large-breed dogs may be affected by the eventual development
of hip dysplasia. In studies where hip dysplasia would be a serious complication or
confounding variable (e.g., orthopedic research), dogs should be radiographed upon
arrival to assess possibility of early coxofemoral joint degeneration and suitability
for use in the study.
c.
Benign Prostatic Hyperplasia
Etiology.
Benign prostatic hyperplasia (BPH) is an age-related condition in intact male dogs.
The hyperplasia of prostatic glandular tissue is a response to the presence of both
testosterone and estrogen.
Clinical signs.
BPH is often subclinical. Straining to defecate (tenesmus) may be seen because the
enlarged gland impinges on the rectum as it passes through the pelvic canal. Urethral
discharge (yellow to red) and hematuria can also be presenting clinical signs for
BPH.
Epizootiology and transmission.
BPH typically affects older dogs (>4 years), although it has been seen as early as
2 years of age.
Pathologic findings.
In its early stages, canine BPH is hyperplasia of the prostatic glandular tissue.
This is in contrast to human BPH, which is primarily stromal in origin. Eventually,
the hyperplasia tends to be cystic, with the cysts containing a clear to yellow fluid.
The prostate becomes more vascular (resulting in hematuria or hemorrhagic urethral
discharge), and BPH may be accompanied by mild chronic inflammation.
Pathogenesis.
BPH occurs in older intact male dogs because increased production of estrogens (estrone
and estradiol), combined with decreased secretion of androgens, sensitizes prostatic
androgen receptors to dihydrotestosterone. The presence of estrogens may also increase
the number of androgen receptors, and hyperplastic prostate glands also have an increased
ability to metabolize testosterone to 5α-dihydrotestosterone (Kustritz and Klausner,
2000).
Diagnosis and differential diagnosis.
BPH is diagnosed in cases of nonpainful symmetrical swelling of the prostate gland
in intact male dogs, with normal hematologic profiles and urinalysis characterized
by hemorrhage, at most. Differential diagnoses include squamous metaplasia of the
prostate, paraprostatic cysts, bacterial prostatitis, prostatic abscessation, and
prostatic neoplasia (primarily adenocarcinoma). These differential diagnoses also
increase in frequency with age and, except for squamous metaplasia, can also occur
in castrated dogs. As such, these conditions do not necessarily abate or resolve when
castration is used for treatment of prostatic enlargement.
Prevention.
Castration is the primary means for prevention of benign prostatic hyperplasia.
Treatment.
The first and foremost treatment for BPH is castration. In pure cases of BPH, castration
results in involution of the prostate gland detectable by rectal palpation within
7–10 days. For most dogs in research studies this is a viable option to rapidly improve
the animal's condition. The alternative to castration is hormonal therapy, primarily
with estrogens. This may be applicable in cases in which the dog is a valuable breeding
male (e.g., genetic diseases), and semen collection is necessary. If the research
study concerns steroidal hormone functions, then neither the condition nor the treatment
is compatible. Newer drugs marketed for human males have also shown promise in treating
canine BPH. Finasteride (Proscar) is a 5α-reductase inhibitor that limits metabolism
of testosterone to 5α-dihydrotestosterone. Treatment at daily doses of 1–5 mg/kg has
been effective in causing prostatic atrophy without affecting testicular spermatogenesis
(Kustritz and Klausner, 2000). Dogs given 1.0 mg/kg were proven to still be fertile.
There are also indications that lower doses may be effective in relieving BPH. Androgen
receptor antagonists (flutamide and hydroxyflutamide) have also been studied in the
dog and found to be effective for treatment of BPH while maintaining libido and fertility
(Kustritz and Klausner, 2000). Unfortunately, both the 5α-reductase inhibitors and
the androgen receptor antagonists are not presently labeled for use in male dogs in
the United States.
Research complications.
BPH can cause complications to steroidal hormone studies, in that the condition may
be indicative of abnormal steroidal hormone metabolism, and neither castration nor
estrogen therapy is compatible with study continuation. It is presently unknown whether
the use of the newer antihyperplastic agents systemically alters physiologic parameters
outside of the prostate itself. The development of tenesmus as a clinical sign may
also affect studies of colorectal or anal function.
3.
Other Miscellaneous Diseases
a.
Juvenile Polyarteritis Syndrome
Etiology.
Juvenile polyarteritis syndrome (JPS) is a painful disorder seen in young beagles
(occasionally reported in other breeds). The lesion consistent with the syndrome is
systemic necrotizing vasculitis. The cause of the vasculitis has not been established,
but it appears to have an autoimmune-mediated component and may have a hereditary
predisposition.
Clinical signs.
Clinical signs of JPS include fever, anorexia, lethargy, and reluctance to move the
head and neck. The dogs tend to extend the neck ventrally. Most dogs seem to be in
pain when touched, especially in the neck region. The syndrome typically has a course
of remissions and relapses characterized by 3–7 days of illness and 2–4 weeks of remission
(Scott-Moncrieff et al., 1992). There may be a component of this condition that is
subclinical, given that a vasculitis has been diagnosed postmortem in beagles that
had no presenting signs.
Epizootiology and transmission.
JPS typically affects young beagles (6–40 months), with no sex predilection.
Pathologic findings.
On gross necropsy, foci of hemorrhage can be seen in the coronary grooves of the heart,
cranial mediastinum, and cervical spinal cord meninges (Snyder et al., 1995). Local
lymph nodes may be enlarged and hemorrhagic. Histologically, necrotizing vasculitis
and perivasculitis of small to medium-sized arteries are seen. These lesions are most
noticeable in the three locations where gross lesions are observed, but they may be
seen in other visceral locations. The perivasculitis often results in nodules of inflammatory
cells that eccentrically surround the arteries (Fig. 9a
). The cellular composition of these nodules is predominantly neutrophils, but it
can also consist of lymphocytes, plasma cells, or macrophages (Snyder et al., 1995).
Fibrinous thrombosis of the affected arteries is also seen (Fig. 9b). A subclinical
vasculitis has also been diagnosed in beagles postmortem; it is not known whether
this subclinical condition is a different disorder or part of a JPS continuum. This
subclinical vasculitis often affects the coronary arteries (with or without other
sites).
Fig. 9
(a) Epicardium and coronary artery from a research beagle that exhibited shifting
leg lameness and nonlocalized pain. The coronary artery is surrounded by rings of
inflammatory cells, consistent with juvenile polyarteritis syndrome. Magnification:
×10. (b) Higher magnification of the coronary artery with perivascular inflammation
shown in (a). Inflammatory cells have infiltrated the tunica media, and fibrinous
thrombosis occludes the arterial lumen. Magnification: ×100.
Pathogenesis.
The initiating factors for JPS are unknown. It was once presumed to be a reaction
to test compounds by laboratory beagles, but this may have been coincident to the
fact that the beagle is the breed most often affected with JPS. Immune mediation of
JPS is strongly suspected, because the clinical signs have a cyclical nature and respond
to treatment with corticosteroids, and the affected dogs have elevated α2-globulin
fractions and abnormal immunologic responses. There may be hereditary predisposition,
given that pedigree analysis of some affected dogs has indicated that the offspring
of certain sires are more likely to be affected, and breeding of two affected dogs
resulted in 1/7 affected pups (Scott-Moncrieff et al., 1992).
Diagnosis and differential diagnosis.
Differential diagnoses include encephalitis, meningitis, injury or degeneration of
the cervical vertebrae or disks, and arthritis. In the research facility,
the disorder may be readily confused with complications secondary to the experimental
procedure, or with postsurgical pain. Beagles with JPS that were in an orthopedic
research study were evaluated for postsurgical complications and skeletal abnormalities
prior to the postmortem diagnosis of systemic vasculitis (authors’ personal experience).
Prevention and control.
No prevention and control measures are known at this time.
Treatment.
Clinical signs can be abated by administration of corticosteroids. Prednisone administered
orally at 1.1 mg/kg, ql2 hr, was associated with rapid relief of clinical symptoms.
Maintenance of treatment at an alternate-day regimen of 0.25–0.5 mg/kg was shown to
relieve symptoms for several months. However, withdrawal of corticosteroid therapy
led to the return of clinical illness within weeks.
Research complications.
Because of the potentially severe clinical signs and the need for immunosuppressive
treatment, JPS is often incompatible with use of the animal as a research subject.
It is unknown whether subclinical necrotizing vasculitis causes sufficient aberrations
to measurably alter immunologic responses.
b.
Hyperplasia of the Gland of the Nictitating Membrane
Etiology.
“Cherry eye” is a commonly used slang term for hyperplasia and/or prolapse of the
gland of the nictitating membrane (third eyelid). This is not considered a congenital
anomaly, but there is breed disposition for this condition, including beagles. A specific
etiology is not known.
Clinical signs.
The glandular tissue of the nictitating membrane protrudes beyond the membrane's edge
and appears as a reddish mass in the ventromedial aspect of the orbit (Fig. 10
). Excessive tearing to mucoid discharge can result, and severe cases can be associated
with corneal erosion.
Fig. 10
Hyperplasia of the gland of the nictitating membrane (“cherry eye”) in a research
beagle.
Pathologic findings.
Typically the glandular tissue is hyperplastic, possibly with inflammation. Rarely
is the tissue neoplastic.
Pathogenesis.
Prolapse of the gland may be a result of a congenital weakness of the connective tissue
band between the gland and the cartilage of the third eyelid (Helper, 1989).
Prevention.
Hyperplasia of the third eyelid cannot be prevented, but dogs that develop this condition
unilaterally should have the other eye evaluated for potential glandular prolapse.
Preventative surgical measures might be warranted.
Treatment.
Corticosteroid treatment (topical or systemic) can be used to try to reduce the glandular
swelling. However, surgical reduction or excision of the affected gland is typically
required to resolve the condition. In the reduction procedure, the prolapsed gland
is sutured to fibrous tissue deep to the fornix of the conjunctiva (Helper, 1989).
If reduction is not possible (as with deformed nictitating cartilage) or is unsuccessful,
removal of the gland can be performed. Such excision is fairly straightforward and
can be done without removal of the nictitating membrane itself. The gland of the third
eyelid is important in tear production; although the rest of the lacrimal glands should
be sufficient for adequate tear production, keratoconjunctivitis sicca is a possible
consequence after removal of the gland of the nictitating membrane.
Research complications.
In most cases, research complications would be minimal, especially if treated adequately.
Either the presence of the hyperplastic gland, or its removal, might compromise ophthalmologic
studies.
c.
Interdigital Cysts
Etiology.
Interdigital cysts are chronic inflammatory lesions (not true cysts) that develop
in the webbing between the toes (Fig. 11
). The cause for most interdigital cysts is usually not identified unless a foreign
body is present. Bacteria may be isolated from the site, but the lesions may also
be sterile (hence the synonym “sterile pyogranuloma complex”).
Fig. 11
Interdigital cyst between the third and fourth digits of the forelimb of a research
beagle.
Clinical signs.
Dogs with interdigital cysts are usually lame on the affected foot, with licking and
chewing at the interdigital space. Exudation may be noticed at the site of the lesion.
The lesion appears as a cutaneous ulcer, usually beneath matted hair, with possible
development of sinus tracts and purulent exudate.
Epizootiology and transmission.
Interdigital cysts are common in a variety of canine breeds, including German shepherds.
Beagles have been affected in the research setting. Interdigital cysts usually occur
in the third and fourth interdigital spaces (Bellah, 1993).
Pathologic findings.
Histopathologically, interdigital cysts are sites of chronic inflammation, typically
described as pyogranulomatous.
Pathogenesis.
Initial development of the cysts is unknown, except for those cases in which a foreign
body can be identified.
Diagnosis and differential diagnosis.
Bacterial culture swabs and radiographs should be taken of the cysts to rule out bacterial
infection, and radiopaque foreign bodies or bony lesions, respectively. A biopsy should
be taken if neoplasia is suspected.
Treatment.
If a foreign body is associated with the lesion, then removal is the first order of
treatment. If biopsy of the site provides a diagnosis of sterile pyogranuloma complex,
then systemic corticosteroid therapy (e.g., prednisolone at 1 mg/kg ql2h) can be initiated
and then tapered once the lesion heals. Interdigital cysts that are refractory to
medical therapy require surgical removal. Excision includes removal of the lesion
and the interdigital web, and a two-layer closure of the adjacent skin and soft tissues
is recommended (Bellah, 1993). The foot should be put in a padded bandage and a tape
hobble placed around the toes to reduce tension when the foot is weightbearing. The
prognosis for idiopathic interdigital cysts is guarded, because the cysts tend to
recur (Bellah, 1993).
Research complications.
Research complications from the cysts are minimal, unless the dogs need to be weight-bearing
for biomechanic or orthopedic studies. Treatment with systemic steroids could be contraindicated
with some experimental designs.