PARENT SESSION
Session #11: The role of parasites and infectious diseases in ecology. Organized by: A. Dobson, L. Real, R. Ostfeld, and R. Holt.
Tuesday, August 7, 2001. Madison Ballroom A
The dilution effect in disease ecology: the search for host X.
Ostfeld, Richard1, Keesing, Felicia1,2, LoGiudice, Kathleen1, Schmidt, Kenneth1,3, 1 2 3
ABSTRACT- We have developed both conceptual and mathematical models of "The Dilution Effect", whereby high species diversity in the community of vertebrate hosts for disease vectors dilutes the impact of the most competent reservoirs for the disease agent and reduces disease transmission. In the case of our model system -- Lyme disease, the main vector is the blacklegged tick, and the principal natural reservoirs are the white-footed mouse and eastern chipmunk. The presence of a diverse assemblage of vertebrate hosts for ticks deflects tick meals away from mice and chipmunks, resulting in lower infection prevalence in the tick population. In addition, most of the species added to increasingly diverse vertebrate communities have the potential to directly reduce rodent density, further reducing infection prevalence and population density of ticks. We have termed the diverse assemblage of non-mouse, non-chipmunk hosts for ticks "Host X" to reflect our ignorance about the roles played by individual vertebrate species. Here we describe our efforts to determine which non-rodent species may be most important in diluting the impact of highly reservoir-competent rodents. Characteristics necessary to produce a strong dilution effect include: (1) the potential to feed a substantial portion of the tick population; and (2) low probability of infecting ticks with the Lyme disease pathogen (i.e. low reservoir competence). Preliminary analyses indicate that ground-nesting and ground-foraging birds and Blarina shrews are poor candidates for status as "Host X". Most of the birds (e.g., veeries, ovenbirds, American robins, woodthrush) host relatively few ticks, and Blarina is a relatively competent reservoir for Lyme bacteria. In contrast, opposums and Sorex shrews may be excellent candidates because they combine heavy tick loads with low competence. Our research emphasizes the importance of understanding the community context of the maintenance and transmission of infectious disease.
KEY WORDS: Lyme disease, tick population, dilution effect, pathogen