Biodiversity and the dilution effect in disease ecology

Many infectious diseases of humans are caused by pathogens that reside in n onhuman animal reservoirs and are transmitted to humans via the bite of an arthropod vector Most vectors feed from a variety of host species that diff er dramatically in their reservoir competence; that is, their probability o f transmitting the infection from host to vector. We explore a conceptual m odel of what we termed the "dilution effect," whereby the presence of verte brate hosts with a low capacity to infect feeding vectors (incompetent rese rvoirs) dilute the effect of highly competent reservoirs, thus reducing dis ease risk. Using Lyme disease as an example, we demonstrate the presence an d estimate the magnitude of the dilution effect for local sites in eastern New York State. We found that the prevalence of Lyme disease spirochetes, B orrelia burgdorferi, in field-collected Ixodes ticks (37.6% and 70.5% for n ymphal and adult stages, respectively) was dramatically lower than expected (similar to 90% and >95% for nymphal and adult stages, respectively) if ti cks fed predominantly on highly competent reservoirs, white-footed mice (Pe romyscus leucopus) and eastern chipmunks (Tamias striatus). We inferred the role of additional host species using an empirically based model that inco rporated data on tick burdens per host, relative population densities of ho sts, and reservoir competence of each host. Assuming an empirically realist ic reservoir competence of 5% for non-mouse and non-chipmunk hosts, we dete rmined that alternative hosts must provide 61% and 72% of larval and nympha l meals, respectively. Using computer simulations, we assembled simulated h ost communities that differed in species richness, evenness, and net intera ctions between alternative hosts and mice. We found that increasing species richness (but not evenness) reduced disease risk. Effects were most pronou nced when the most competent disease reservoirs were community dominants an d when alternative hosts had a net negative influence on the dominance of m ice as a host for ticks. Our results highlight a critical role of biodivers ity and host community ecology in the transmission of vector-borne zoonotic diseases that in turn has important consequences for human health.