Epidemiology of the amphibian pathogen Batrachochytrium dendrobatidis, across multiple spatial scales

Abstract

Emerging infectious diseases are increasingly recognized as key threats to wildlife. Batrachochytrium dendrobatidis (Bd), the causative agent of chytridiomycosis, has been implicated in mass mortalities, population declines, and local and global extinctions of many species of amphibians around the world. As such, it is currently the largest infectious disease threat to biodiversity. Understanding the distribution and spatial dynamics of Bd is crucial to predicting spread to new geographic areas, revealing the history of infection, and developing appropriate management strategies. One of the most striking features of Bd is the variability in outcome of infection that has been observed within a species, among populations. By identifying and comparing differences in variables that co-vary between populations exhibiting different infection characteristics, we can start to disentangle the mechanisms allowing for parasite persistence and proliferation. However, infection dynamics operate across nested levels of biological organization: within-host processes underlie among-host processes within a population. As such, this thesis works within the classical themes of spatial epidemiology to consider: 1) the distribution of Bd and the evidence for spatial heterogeneity in both the prevalence and intensity of infection, and 2) the role of individual- and population-level traits in defining infection outcome. The research presented, identifies that Bd functions endemically within Rana pipiens populations in Ontario. Outbreaks of chytridiomycosis are not observed, but infection dynamics show significant interannual fluctuations related to stable geographic factors and local climatic nuances experienced at particular host life history stages. However, Rana pipiens also display variation in resistance to the pathogen, mediated by thermoregulation, dispersal behaviour, and phenotypic properties. Comparisons between host populations show variation in skin-associated bacterial communities, which may mediate susceptibility to chytridiomycosis. These bacterial communities are found to vary across latitude and between sites experiencing different levels of anthropogenic disturbance. Additionally, individual level traits, such as amphibian body temperature and body size are reported to influence bacterial community. Hence, this research highlights the importance of considering context-dependent individual- and populationlevel environmental heterogeneity, when attempting to predict the infection risk of Bd.