Maintenance of Borrelia burgdorferi among vertebrate hosts: a test of dilution effect mechanisms

A better understanding of the mechanisms through which host diversity can influence reservoir pathogen infection is needed to mitigate disease risk. Efforts may involve computational modeling, especially since it is infeasible to perform large-scale experimental studies of different host composition scenarios in natural settings. We used individual-based models to examine how changes in host diversity characterized by differences in host reservoir competency for both Ixodes scapularis and Borrelia burgdorferi can influence the maintenance of the pathogen and subsequent human Lyme disease risk. We simulated 1440 different host communities, with 10 repetitions each, consisting of varying densities of Peromyscus leucopus (white-footed mouse, 0–50), Tamias striatus (eastern chipmunk, 0–30), Blarina brevicauda (short-tailed shrew, 0–30), Sciurus carolinensis (eastern gray squirrel, 0–15), and Didelphis virginiana (Virginia opossum, 0–2). We then quantified support for three mechanisms (i.e., vector regulation, encounter reduction, and transmission reduction) through which biodiversity–disease relationships occurred using species richness, Shannon H diversity, and host abundance. For each of the dilution effect mechanisms, host abundance was consistently the best-supported predictor of disease risk. In our model, a dilution effect occurred via vector regulation and transmission reduction, where increasing both species richness and host abundance reduced both density of nymphs (DON) and density of infected nymphs (DIN). However, if disease risk is measured solely by calculating nymphal infection prevalence (NIP), it may seem that host diversity amplifies disease risk. Understanding the mechanisms through which the wildlife host community influences pathogen transmission cycles in nature will help foster effective control and reduction of disease risk in humans.