Date of Award
Master of Science
This thesis consists of three sections, all of which are linked to the ecology of infectious disease and the decline of amphibians caused by chytridiomycosis. This thesis represents a detailed analysis of factors that allow the amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), to persist in the Peruvian Andes. Collectively, these three sections elaborate on the current knowledge of the disease, how it persists in an area, as well as recommendations for future disease mitigation. Chapter one is a review of environmental and biological factors that contribute to the persistence of Bd. Here we address the use of biotic and abiotic reservoirs by the pathogen that allow Bd to persist. Biological reservoirs include both amphibian and non-amphibian hosts, thus allowing Bd to persist and proliferate when susceptible host densities recover. More than 40% of all amphibian populations are in decline and more than a third are at risk of extinction making it important to develop conservation measures for endangered and diminishing biodiversity. By understanding disease dynamics between reservoirs and other susceptible hosts, mitigation strategies can be developed to save amphibian populations from loss of genetic diversity or extinction. Chapter two addresses the current distribution and disease status of Bd in the Kosñipata Valley in southeastern Peru. We investigated the post-epizootic distribution and prevalence of Bd by examining trends in Bd prevalence across multiple sample periods. We modeled infection prevalence using logistic regression as a function of species, sample period, reproductive mode, life stage, and elevation and their two-way interactions. The most parsimonious model of prevalence contained two-way interactions between sample period and elevation, and reproductive mode and life stage. Overall we observed that prevalence varied over time; prevalence was higher in 2012-2015 than it was in 2008-2009. The interaction between elevation and sample period revealed that while prevalence decreased with elevation during the wet season, it generally increased with elevation during the dry season. We concluded that the prevalence of Bd in the Kosñipata Valley appears to have stabilized over time, indicating enzootic Bd. Long-term monitoring of infection in hosts is important because temporal patterns in prevalence and infection intensity are associated with changes in population abundance which could lead to future changes in the disease state. Chapter three examines Hypsiboas gladiator as a proposed reservoir species for Bd. We conducted field surveys to determined prevalence and calculated Bd loads across life stages, and calculated zoospore shedding rates of H. gladiator and sympatric species. Results showed that H. gladiator had the highest prevalence and Bd load, followed by Pristimantis toftae and then P. platydactylus. We tried to determine if prevalence and Bd load varied with distance from streams but found no direct evidence to support our claims. We also conducted susceptibility trials of H. gladiator to Bd. Results from our susceptibility trials are inconclusive. Shedding rates were higher in 2014 than 2013 for all species and varied by year. At this time we cannot determine if H. gladiator is a reservoir species for Bd.
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