Abstract
Recent infectious disease models illustrate a suite of mechanisms that can result in lower incidence of disease in areas of higher disease host diversity–the ‘dilution effect’. These models are particularly applicable to human zoonoses, which are infectious diseases of wildlife that spill over into human populations. As many recent emerging infectious diseases are zoonoses, the mechanisms that underlie the ‘dilution effect’ are potentially widely applicable and could contribute greatly to our understanding of a suite of diseases. The dilution effect has largely been observed in the context of Lyme disease and the predictions of the underlying models have rarely been examined for other infectious diseases on a broad geographic scale. Here, we explored whether the dilution effect can be observed in the relationship between the incidence of human West Nile virus (WNV) infection and bird (host) diversity in the eastern US. We constructed a novel geospatial contrasts analysis that compares the small differences in avian diversity of neighboring US counties (where one county reported human cases of WNV and the other reported no cases) with associated between-county differences in human disease. We also controlled for confounding factors of climate, regional variation in mosquito vector type, urbanization, and human socioeconomic factors that are all likely to affect human disease incidence. We found there is lower incidence of human WNV in eastern US counties that have greater avian (viral host) diversity. This pattern exists when examining diversity-disease relationships both before WNV reached the US (in 1998) and once the epidemic was underway (in 2002). The robust disease-diversity relationships confirm that the dilution effect can be observed in another emerging infectious disease and illustrate an important ecosystem service provided by biodiversity, further supporting the growing view that protecting biodiversity should be considered in public health and safety plans.
Highlights
The dilution effect is an outcome of a potentially broadly applicable set of mathematical models that could help explain human risks of contracting vector-borne zoonoses, i.e., infectious diseases that are spread among host animals by a vector and threaten to spill over into the human population [1,2,3,4,5,6,7,8,9]
We identified all neighboring pairs of counties east of the Mississippi river where we could obtain human West Nile virus (WNV) and Breeding Bird Survey (BBS) data in both 1998 and in 2002, and where one county reported no human infection in 2002 and the neighbor reported at least one human case
From these BBS survey data we constructed indices of species richness and Shannon-Weiner evenness of non-passerines, passerines, and all species combined, the total number of birds sampled on a route, the relative and absolute abundance of four avian families known to be competent viral hosts: Corvidae, crows and jays [16,22,23]; Fringillidae, finches [24]; Passeridae, Old World sparrows [15,16,21,25,27]; Turdidae, thrushes and robins; and the relative and absolute abundance of one species recently implicated in human epidemics of WNV, the American robin Turdus migratorius [10,21,28]
Summary
The dilution effect is an outcome of a potentially broadly applicable set of mathematical models that could help explain human risks of contracting vector-borne zoonoses, i.e., infectious diseases that are spread among host animals by a vector and threaten to spill over into the human population [1,2,3,4,5,6,7,8,9]. In situations of increasing host diversity, a reduction in the probability of transmission of the disease from infected hosts to vectors (transmission reduction) [10], a reduction in the rate of encounters between hosts and infected vectors (encounter reduction), a reduction in the number of susceptible hosts (susceptible host regulation), a reduction in infected vector density (vector regulation), and a faster disease recovery rate among infected hosts (recovery augmentation) [1] All of these mechanisms have a complementary augmentation (e.g., encounter augmentation) or reduction (i.e., recovery reduction) mechanism that can give rise to a positive relation between diversity and disease incidence in certain situations. Meeting these demands can help inform and refocus more effective public health, conservation, and bioterrorism-preparedness strategies by identifying ways in which wildlife (host) community structure can be utilized to minimize the health, ecological, and economic consequences of emerging infectious diseases, whether these diseases emerge naturally or are introduced deliberately
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