Effect of Host Community Composition on the Diversity and Abundance of Lyme Disease
State University of New York at Stony Brook, 2006 - Host-parasite relationships - 123 pages
Evolutionary history and current ecology influence the nature of polymorphic loci, the most prominent area of study by population geneticists. We used the polymorphic outer surface protein C (ospC) locus to investigate the selective pressures and the population structure of Borrelia burgdorferi , the causal agent of Lyme disease. The first chapter of this dissertation demonstrated that the ospC polymorphism is maintained by a multiple niche polymorphism, a form of balancing selection. This was discovered by assessing the alleles transmitted from four mammalian species to the ticks feeding on them. The alleles in a tick depends on the vertebrate species on which it had fed. These data indicate that the environment of B. burgdorferi is heterogeneous, in the form of different vertebrate species, and that no B. burgdorferi strain, defined by the ospC allele, has the highest fitness in all environments---the defining characteristics of a multiple niche polymorphism. In the second chapter we used an inverse modeling approach that included transmission probabilities, the proportion of ticks infected with a strain after feeding on a species, to investigate the affect of the vertebrate community composition on the abundance of each strain. The distribution and abundance of each strain is a function of the relative abundance and the transmission probabilities of each species. However, at least one vertebrate species that feeds and infects numerous ticks had not been identified. The final chapter used a similar inverse modeling approach with the addition of data from other species of potential disease hosts. It was determined that white-footed mice, the presumed "primary" disease host for B. burgdorferi, infects only 25% of the population of infected ticks. This model explains the limited reduction in the prevalence of infected ticks due to a mouse-vaccination program. Shrew species from two genera are responsible for feeding and infecting ∼55% of the infected ticks. Collectively, more than 90% of all infected ticks fed on a mouse, chipmunk, or a shrew. Thus, the majority of the interactions that affect the distribution and abundance of B. burgdorferi, and thus human Lyme risk, occurred with one of these species.
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