Increasing urbanization in both developing and developed countries could promote the
emergence of a new set of neglected tropical diseases (NTDs)
In 2014, the United Nations (UN) launched an important report on global trends in
urbanization [1]. The study found that beginning in 2007, for the first time in human
history, more people live in urban than rural areas, with estimates that by 2050 approximately
two-thirds of the world’s population will be urbanized [1, 2]. As shown in Fig 1,
the Western Hemisphere (especially North America) exhibits the highest percentage
of urban dwellers, in addition to Australia and selected other areas, but the UN report
also predicts some important trends in Africa and Asia.
10.1371/journal.pntd.0005308.g001
Fig 1
Global urbanization map showing the percentage of urbanization by country in 2006.
Public domain image available here: https://commons.wikimedia.org/wiki/File:Urbanized_population_2006.png.
For example, it was found that much of the future increase in urban populations will
happen in Asia and Africa, where by the year 2050, 64% (up from 48% in 2014) and 56%
(up from 40% in 2014) will be urbanized, respectively [1]. China, India, and Nigeria
will comprise almost 40% of the future expansion in urban populations from now until
2050 [1]. During this period, new “megacities”—cities with populations that exceed
10 million people—will be formed. Most of the new megacities will be found in Asia,
Africa, and Latin America where NTDs are highly endemic [1], including Mumbai, Delhi,
and Kolkata (India), Dhaka (Bangladesh), Kinshasa (Democratic Republic of Congo),
Lagos (Nigeria), Luanda (Angola), Dar es Salaam (Tanzania), Bogota (Colombia), and
Lima (Peru) [1]. By 2030, the UN estimates that there will be 41 such megacities globally
[1].
We can predict that urbanization will produce both positive and negative effects on
the urban populations in the Global South. On the positive side, urbanization is often
linked to culture, commerce and economic productivity, greater life expectancy, higher
levels of education and literacy, increased access to social services, better access
to health care, and an overall higher quality of life [1]. At the same time, rapid
urbanization can also fail to sustain healthy populations when it outstrips clean
water reserve and sewage management systems or when urban poverty produces unhealthy
diets and diminished physical activity, low-quality housing, and environmental degradation,
together with exposure to air, noise, and other forms of pollution [1– 3]. Crowding
can also be an important factor. Through such mechanisms, the effects can be devastating
in terms of disease transmission and even thwart Sustainable Development Goals (SDGs).
In recognition of these factors, in 2010, WHO chose urbanization as its theme for
World Health Day [2]. Rapid global urbanization over the next few decades has potentially
important implications for the rise of NTDs, as well as NTD and noncommunicable disease
(NCD) comorbidities. While an important feature of NTDs is their disproportionate
impact on populations living in rural poverty, there are selected diseases that primarily
affect the poor in urban settings (Box 1) [4]. Described below are some of the major
NTDs emerging in urban environments over the last three years.
Box 1. High prevalence and incidence NTDs emerging in urban environments
Arbovirus infections transmitted by Aedes aegypti
Dengue
Chikungunya
Zika virus infection
Canine rabies
Leptospirosis, cholera, and typhoid fever
Schistosomiasis and soil-transmitted helminthiases
Chagas disease and leishmaniasis
Intestinal protozoan infections
Neglected virus infections: Arboviruses and rabies
Aedes aegypti is an urban-dwelling mosquito, specifically adapted to humans and responsible
for the transmission of dengue, chikungunya, yellow fever, and Zika virus infection.
The dramatic emergence of Zika virus infection in Brazil in 2015, struck the crowded
and impoverished northeastern city of Recife particularly hard [5], and there are
concerns about Zika now traveling to additional New World and even Old World cities
[6]. Since the end of 2013, chikungunya is also now affecting some of those same cities
in the Americas, while, according to the Global Burden of Disease Study 2015 (GBD
2015), the number of global dengue cases has increased from approximately 33 million
to 80 million incident cases annually over the last decade [7]. Similarly, urban rabies
transmitted from dogs remains an important cause of mortality, although there has
been almost a 50% decline over the last decade, with 17,400 deaths in 2015 according
to the GBD 2015 [8]. While there are no global data comparing urban versus rural incidence
rates of canine-transmitted rabies, in Delhi, India, urban slums were recently shown
to exhibit higher dog bite incidence rates compared with rural slums, with the majority
of both populations not receiving rabies postexposure prophylaxis [9].
Neglected bacterial infections: Leptospirosis, cholera, and typhoid fever
Leptospirosis has emerged as an important urban bacterial zoonosis from rat and dog
urine, especially in the favelas of Brazil’s cities, such as Salvador where it is
an important yet underreported cause of acute febrile illness [10]. The spatio-temporal
determinants of infection there have been studied extensively [11]. Urban leptospirosis
has also been reported in Nairobi [12]. Although not currently incorporated into the
GBD 2015, a recent effort to determine the global burden of leptospirosis estimates
approximately 1 million annual cases resulting in almost 3 million disability-adjusted
life years [13]. However, the study did not report the percentage of cases found in
urban versus rural environments. Similarly, there are multiple reports of cholera,
typhoid fever, and other enteric infection outbreaks in urban slums and in the settings
of poor urban planning or following urban natural disasters [14–18]; however, there
are no published global burden data that differentiate the urban and rural outbreaks.
Neglected parasitic infections
Urban schistosomiasis and ascariasis (as well as other soil-transmitted helminthiases)
have been reported from Africa [19–21] and Latin America [22, 23]. Interestingly,
the Global Atlas of Helminth Infection found that ascariasis and trichuriasis transmission
is highest in peri-urban rather than either urban or rural settings [24]. Some urban
communities can also sustain lymphatic filariasis (LF) transmission [25], although
it was shown recently that rural to urban migrations due to the conflicts in Sierra
Leone and Liberia could not sustain LF transmission [26]. An outbreak of urban Chagas
disease has also been reported from Venezuela and Peru [27, 28]. Urban zoonotic visceral
leishmaniasis from dogs has been reported from Argentina and elsewhere in the Americas,
with low-quality housing, crowding, and dog ownership representing some of the key
risk factors [29, 30]. In India and elsewhere, vivax malaria also represents an important
neglected parasitic infection, which has been refractory to control measures [31].
Giardiasis and other enteric protozoan infections have also been shown to cluster
in urban environments [21, 32].
NTD and NCD comorbidities
Yet another phenomenon we might expect to see with increasing frequency in the new
urban megacities is the increasing overlap of NTDs with NCDs. For example, in India
a new high mortality has been seen in dengue patients with underlying hypertension
and diabetes [33]. As NCDs expand in poor countries due to tobacco and lifestyle changes,
we can expect to see further examples of such NTD comorbidities.
Overall, there is a dearth of information about the urban transmission of NTDs and
very few disease burden estimates that distinguish urban versus rural modes of transmission.
As global urbanization continues to increase, there is going to be an urgent need
for such studies. By 2050, with most of the global population living in cities, we
will need to better understand how NTDs and other poverty-related neglected diseases
flourish in urban environments.
The UN is beginning to shape new public policies for global urbanization, which include
programs for balanced urban growth and spatial distribution, sustainability, and timely
collection of data required for urban planning [1]. The findings of significant and
serious NTDs in urban areas mean that these diseases will also need to be considered
as urban areas and megacities strive to meet their SDGs. Arbovirus infections, leptospirosis,
cholera, and typhoid fever, vector-borne parasitic infections such as schistosomiasis,
Chagas disease, leishmaniasis, and vivax malaria, and NTD–NCD comorbidities each represent
the product of urban planning breakdowns and unchecked growth. Without adequate public
health measures and research and development for new drugs, diagnostics, and vaccines,
we can expect that these diseases will continue to thwart sustainable urban growth
in the coming decades.