In an era of bio-terrorism and emerging novel pathogens including AIDS, Ebola, SARS and avian influenza, the need to more closely address disease dynamics is clear. In the past 25 years, the world has seen an unprecedented increase in the number of emerging infectious diseases throughout the world, and this has implications for both human and animal health. Wild animal populations can be very useful in investigating zoonotic diseases and human epidemiology. Determination of the prevalence of macro- and microparasites in a wildlife model system could provide useful insight into how these pathogens flow through and persist in a natural population of animals. We can evaluate the demographics and try to address questions such as: Are there key hosts responsible for a disproportionate amount of the transmission? Are there hot spots of transmission? How does this pathogen flow through the population?
Peromyscus leucopus (the white-footed mouse) provides a unique opportunity to investigate these patterns in the wild because of its ease of handling, high trappability, and inter-annual population dynamics. This allows us to intensively monitor individual interactions between reservoir hosts, both spatially and temporally, and it is these interactions that are key in understanding transmission dynamics of directly transmitted diseases. Additionally, by trapping every two weeks we can closely monitor or track infection and determine any peak transmission periods. These large and replicated monitoring areas we use encompass the home range of many individuals; thus, we can investigate our questions at both the individual and population level. These characteristics make the system conducive to manipulations whereby we can perturb these populations away from equilibrium and disrupt transmission.
The goals of this thesis were to first identify the parasites and outline the seasonal demographics, distribution, and vital rates of the infected and uninfected hosts and we accomplished this with both intensive and extensive trapping efforts. Then, in 2004, we sought to manipulate the populations to see the response to additional food in the form of periodical cicadas or sunflower seeds. In the third year (2005) we investigated the influence of both habitat quality and helminth removal. The 2004 results indicated that P. leucopus does respond to a springtime pulse of protein rich cicadas by increasing in density, but they do not respond to the carbohydrate rich seeds as is typical in the autumn. In 2005, parasite removal led to increased size, breeding, and body condition of animals and caused a reversal of the mid-summer breeding hiatus, while increased habitat quality did not appear to influence any vital rates or demographic characteristics.
There is growing evidence that parasites play an important role in shaping population dynamics, and that chronic parasite infection can influence host fitness by reducing breeding output or perhaps by shaping seasonal breeding strategies. We have shown that there are significant impacts of nematode worms on the body condition, mass, growth, and breeding of P. leucopus, and that parasites can account for the mid-summer breeding hiatus commonly observed in female mice. These results imply that parasites may play a more important role in the vital rates and temporal dynamics of P. leucopus than resource abundance. We also show that the dominant parasite in the community (Pterygodermatities peromysci), exhibits a random distribution amongst the mouse population and this coupled with the impact upon summer breeding should lead to destabilization of the relationship between parasite and host. Further studies are needed determine if and what role parasites play a role in the unstable dynamics of P. leucopus.