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Abstract
To evaluate and compare the risk of emerging vector-borne diseases (VBDs), a Model
for INTegrated RISK assessment, MINTRISK, was developed to assess the introduction
risk of VBDs for new regions in an objective, transparent and repeatable manner. MINTRISK
is a web-based calculation tool, that provides semi-quantitative risk scores that
can be used for prioritization purposes. Input into MINTRISK is entered by answering
questions regarding entry, transmission, establishment, spread, persistence and impact
of a selected VBD. Answers can be chosen from qualitative answer categories with accompanying
quantitative explanation to ensure consistent answering. The quantitative information
is subsequently used as input for the model calculations to estimate the risk for
each individual step in the model and for the summarizing output values (rate of introduction;
epidemic size; overall risk). The risk assessor can indicate his uncertainty on each
answer, and this is accounted for by Monte Carlo simulation. MINTRISK was used to
assess the risk of four VBDs (African horse sickness, epizootic haemorrhagic disease,
Rift Valley fever, and West Nile fever) for the Netherlands with the aim to prioritise
these diseases for preparedness. Results indicated that the overall risk estimate
was very high for all evaluated diseases but epizootic haemorrhagic disease. Uncertainty
intervals were, however, wide limiting the options for ranking of the diseases. Risk
profiles of the VBDs differed. Whereas all diseases were estimated to have a very
high economic impact once introduced, the estimated introduction rates differed from
low for Rift Valley fever and epizootic haemorrhagic disease to moderate for African
horse sickness and very high for West Nile fever. Entry of infected mosquitoes on
board of aircraft was deemed the most likely route of introduction for West Nile fever
into the Netherlands, followed by entry of infected migratory birds.
West Nile virus (WNV) was first detected in the Western Hemisphere in 1999 during an outbreak of encephalitis in New York City. Over the next 5 years, the virus spread across the continental United States as well as north into Canada, and southward into the Caribbean Islands and Latin America (1). This article highlights new information about the epidemiology and transmission dynamics of human WNV disease obtained over the past 5 years of intensified research. Epidemiology WNV is transmitted primarily by the bite of infected mosquitoes that acquire the virus by feeding on infected birds. The intensity of transmission to humans is dependent on abundance and feeding patterns of infected mosquitoes and on local ecology and behavior that influence human exposure to mosquitoes. Although up to 55% of affected populations became infected during epidemics in Africa, more recent outbreaks in Europe and North America have yielded much lower attack rates (1,2). In the area of most intense WNV transmission in Queens, New York, in 1999, ≈2.6% of residents were infected (most of these were asymptomatic infections), and similarly low prevalence of infection has been seen in other areas of the United States (3,4). WNV outbreaks in Europe and the Middle East since 1995 appear to have caused infection in 1,000 potentially WNV-viremic blood donations were identified, and the corresponding blood components were sequestered. Nevertheless, 6 WNV cases due to transfusion were documented in 2003, and at least 1 was documented in 2004, indicating that infectious blood components with low concentrations of WNV may escape current screening tests (19). One instance of possible WNV transmission through dialysis has been reported (20). WNV transmission through organ transplantation was also first described during the 2002 epidemic (15). Chronically immunosuppressed organ transplant patients appear to have an increased risk for severe WNV disease, even after mosquito-acquired infection (16). During 2002, the estimated risk of neuroinvasive WNV disease in solid organ transplant patients in Toronto, Canada, was approximately 40 times greater than in the general population (16). Whether other immunosuppressed or immunocompromised patients are at increased risk for severe WNV disease is uncertain, but severe WNV disease has been described among immunocompromised patients. WNV infection has been occupationally acquired by laboratory workers through percutaneous inoculation and possibly through aerosol exposure (21,22). An outbreak of WNV disease among turkey handlers at a turkey farm raised the possibility of aerosol exposure (17). Dynamics of Transmission: Vectors WNV is transmitted primarily by Culex mosquitoes, but other genera may also be vectors (23). In Europe and Africa, the principal vectors are Cx. pipiens, Cx. univittatus, and Cx. antennatus, and in India, species of the Cx. vishnui complex (6,24). In Australia, Kunjin virus is transmitted primarily by Cx. annulirostris (11). In North America, WNV has been found in 59 different mosquito species with diverse ecology and behavior; however, 40%. Field studies during and after WNV outbreaks in several areas of the United States have confirmed that house sparrows were abundant and frequently infected with WNV, characteristics that would allow them to serve as important amplifying hosts (23,25,37). The importance of birds in dispersing WNV remains speculative. Local movements of resident, nonmigratory birds and long-range travel of migratory birds may both contribute to the spread of WNV (38,39). Although WNV was isolated from rodents in Nigeria and a bat in India, most mammals do not appear to generate viremia levels of sufficient titer to contribute to transmission (24,40–42). Three reptilian and 1 amphibian species (red-ear slider, garter snake, green iguana, and North American bullfrog) were found to be incompetent as amplifying hosts of a North American WNV strain, and no signs of illness developed in these animals (43). Viremia levels of sufficient titer to infect mosquitoes were found after experimental infection of young alligators (Alligator mississippiensis) (44). In Russia, the lake frog (Rana ridibunda) appears to be a competent reservoir (45). Nonmosquitoborne WNV transmission has been observed or strongly suspected among farmed alligators, domestic turkeys in Wisconsin, and domestic geese in Canada (17,46,47). Transmission through close contact has been confirmed in both birds and alligators in laboratory conditions but has yet to be documented in wild vertebrate populations (23,36,44). Control of WNV Transmission Avoiding human exposure to WNV-infected mosquitoes remains the cornerstone for preventing WNV disease. Source reduction, application of larvicides, and targeted spraying of pesticides to kill adult mosquitoes can reduce the abundance of mosquitoes, but demonstrating their impact on the incidence of human WNV disease is challenging because of the difficulty in accounting for all determinants of mosquito abundance and human exposure. One study indicated that clustering of human WNV disease in Chicago varied between mosquito abatement districts, suggesting that mosquito control may have some impact on transmission to humans (14). Persons in WNV-endemic areas should wear insect repellent on skin and clothes when exposed to mosquitoes and avoid being outdoors during dusk to dawn when mosquito vectors of WNV are abundant. Of insect repellents recommended for use on skin, those containing N,N-diethyl-m-toluamide (DEET), picaridin (KBR-3023), or oil of lemon eucalyptus (p-menthane-3,8 diol) provide long-lasting protection (48). Both DEET and permethrin provide effective protection against mosquitoes when applied to clothing. Persons' willingness to use DEET as a repellent appears to be influenced primarily by their level of concern about being bitten by mosquitoes and by their concern that DEET may be harmful to health, despite its good safety record (49). To prevent transmission of WNV through blood transfusion, blood donations in WNV-endemic areas should be screened by using nucleic acid amplification tests. Screening of organ donors for WNV infection has not been universally implemented because of concern about rejecting essential organs after false-positive screening results (50). Pregnant women should avoid exposure to mosquito bites to reduce the risk for intrauterine WNV transmission. Future Directions WNV disease will likely continue to be a public health concern for the foreseeable future; the virus has become established in a broad range of ecologic settings and is transmitted by a relatively large number of mosquito species. WNV will also likely continue to spread into Central and South America, but the public health implications of this spread remain uncertain. Observations thus far in North America indicate that circulation of other flaviviruses, such as dengue, viral mutation, and differing ecologic conditions may yield different clinical manifestations and transmission dynamics. Over the next few years, research efforts might well be focused in several areas. Research into new methods to reduce human exposure to mosquitoes is crucial and can help prevent other mosquitoborne illnesses. This should include development of new methods to reduce mosquito abundance, development of new repellents, and behavioral research to enhance the use of existing effective repellents and other personal protective measures against mosquito bites. A better understanding of the dynamics of nonmosquitoborne transmission is essential to prevent disease among infants of infected mothers and recipients of blood transfusions and transplanted organs. Currently available prevention strategies such as the dissemination of knowledge and products for personal protection from mosquito exposure and the application of existing techniques for reducing mosquito abundance in communities at risk of WNV transmission need to be vigorously implemented. National and international surveillance for WNV transmission will be important to monitor spread of the virus and the effect of control strategies. Finally, further research into the ecologic determinants of WNV transmission, including climatic factors and dynamics of reservoir and vector populations, could help in determining geographic areas of higher risk for WNV disease.
This cohort descriptive study summarizes the epidemiological, clinical, and laboratory characteristics of the Rift Valley fever (RVF) epidemic that occurred in Saudi Arabia from 26 August 2000 through 22 September 2001. A total of 886 cases were reported. Of 834 reported cases for which laboratory results were available, 81.9% were laboratory confirmed, of which 51.1% were positive for only RVF immunoglobulin M, 35.7% were positive for only RVF antigen, and 13.2% were positive for both. The mean age (+/- standard deviation) was 46.9+/-19.4 years, and the ratio of male to female patients was 4:1. Clinical and laboratory features included fever (92.6% of patients), nausea (59.4%), vomiting (52.6%), abdominal pain (38.0%), diarrhea (22.1%), jaundice (18.1%), neurological manifestations (17.1%), hemorrhagic manifestations (7.1%), vision loss or scotomas (1.5%), elevated liver enzyme levels (98%), elevated lactate dehydrogenase level (60.2%), thrombocytopenia (38.4%), leukopenia (39.7%), renal impairment or failure (27.8%), elevated creatine kinase level (27.3%), and severe anemia (15.1%). The mortality rate was 13.9%. Bleeding, neurological manifestations, and jaundice were independently associated with a high mortality rate. Patients with leukopenia had significantly a lower mortality rate than did those with a normal or high leukocyte count (2.3% vs. 27.9%; odds ratio, 0.09; 95% confidence interval, 0.01-0.63).
The transmission of infectious diseases is an inherently ecological process involving interactions among at least two, and often many, species. Not surprisingly, then, the species diversity of ecological communities can potentially affect the prevalence of infectious diseases. Although a number of studies have now identified effects of diversity on disease prevalence, the mechanisms underlying these effects remain unclear in many cases. Starting with simple epidemiological models, we describe a suite of mechanisms through which diversity could increase or decrease disease risk, and illustrate the potential applicability of these mechanisms for both vector-borne and non-vector-borne diseases, and for both specialist and generalist pathogens. We review examples of how these mechanisms may operate in specific disease systems. Because the effects of diversity on multi-host disease systems have been the subject of much recent research and controversy, we describe several recent efforts to delineate under what general conditions host diversity should increase or decrease disease prevalence, and illustrate these with examples. Both models and literature reviews suggest that high host diversity is more likely to decrease than increase disease risk. Reduced disease risk with increasing host diversity is especially likely when pathogen transmission is frequency-dependent, and when pathogen transmission is greater within species than between species, particularly when the most competent hosts are also relatively abundant and widespread. We conclude by identifying focal areas for future research, including (1) describing patterns of change in disease risk with changing diversity; (2) identifying the mechanisms responsible for observed changes in risk; (3) clarifying additional mechanisms in a wider range of epidemiological models; and (4) experimentally manipulating disease systems to assess the impact of proposed mechanisms.
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History
Date
received
: 24
June
2021
Date
accepted
: 19
October
2021
Page count
Figures: 7,
Tables: 3,
Pages: 29
Funding
Funded by:
Ministerie van Landbouw, Natuur en Voedselkwaliteit (NL)
Award ID: KB-12-009.01-001
Funded by:
Ministerie van Landbouw, Natuur en Voedselkwaliteit (NL)
Award ID: BO-20-009-026
Funded by:
funder-id http://dx.doi.org/10.13039/501100001830, Wageningen University and Research Centre;
Award ID: KB-33-001-006-WBVR
Funded by:
funder-id http://dx.doi.org/10.13039/100014803, European Food Safety Authority;
Award ID: NP/EFSA/ALPHA/2016/13-CT01
Funded by:
European Food Safety Authority (IT)
Award ID: NP/EFSA/ALPHA/2017/10
Funded by:
funder-id http://dx.doi.org/10.13039/100014803, European Food Safety Authority;
Award ID: PO/ALPHA/2019/06
The development of MINTRISK was funded by the Dutch Ministry of Agriculture, Nature
and Food Quality (KB-12-009.01-001), Wageningen University & Research (KB-33-001-006-WBVR)
and the European Food Safety Authority (NP/EFSA/ALPHA/2016/13-CT01; NP/EFSA/ALPHA/2017/10;
PO/ALPHA/2019/06). The case study on vector-borne diseases was funded by the Dutch
Ministry of Agriculture, Nature and Food Quality (BO-20-009-026). URL Dutch Ministry
of Agriculture, Nature and Food Quality:
https://www.rijksoverheid.nl/ministeries/ministerie-van-landbouw-natuur-en-voedselkwaliteit URL Wageningen University & Research:
https://www.wur.nl/en.htm URL European Food Safety Authority:
https://www.efsa.europa.eu/en The funders had no role in study design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Categories
Subject:
Research Article
Subject:
Medicine and Health Sciences
Subject:
Epidemiology
Subject:
Medical Risk Factors
Subject:
Medicine and Health Sciences
Subject:
Medical Conditions
Subject:
Infectious Diseases
Subject:
Vector-Borne Diseases
Subject:
Biology and Life Sciences
Subject:
Veterinary Science
Subject:
Veterinary Diseases
Subject:
Medicine and Health Sciences
Subject:
Medical Conditions
Subject:
Infectious Diseases
Subject:
Disease Vectors
Subject:
Biology and Life Sciences
Subject:
Species Interactions
Subject:
Disease Vectors
Subject:
Medicine and Health Sciences
Subject:
Epidemiology
Subject:
Infectious Disease Epidemiology
Subject:
Medicine and Health Sciences
Subject:
Medical Conditions
Subject:
Infectious Diseases
Subject:
Infectious Disease Epidemiology
Subject:
Social Sciences
Subject:
Economics
Subject:
Economic Analysis
Subject:
Economic Impact Analysis
Subject:
People and places
Subject:
Geographical locations
Subject:
Europe
Subject:
European Union
Subject:
Netherlands
Subject:
Medicine and health sciences
Subject:
Medical conditions
Subject:
Tropical diseases
Subject:
Neglected tropical diseases
Subject:
Rift Valley fever
Subject:
Medicine and health sciences
Subject:
Medical conditions
Subject:
Infectious diseases
Subject:
Viral diseases
Subject:
Rift Valley fever
Subject:
Medicine and health sciences
Subject:
Medical conditions
Subject:
Infectious diseases
Subject:
Zoonoses
Subject:
Rift Valley fever
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