Factors Affecting Infectivity and Pathogenicity of Batrachochytrium dendrobatidis in Amphibians.

Wildlife diseases are a current threat to biodiversity, and amphibians have suffered the greatest disease-induced declines and extinctions in history. Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), has contributed to recent pandemic population declines and extinctions of amphibians. My dissertation focused on factors that affect infectivity and pathogenicity of Bd in amphibian hosts. I demonstrated variation in critical aspects of Bd biology between Bd isolates from different climates, Panama and the USA. Panamanian Bd had larger thalli, higher overall zoospore production, and longer duration of zoospore production than Bd from the USA. I also demonstrated time-dependent rates of zoospore production between the isolates. Thus, there are differences among Bd isolates and these differences help explain patterns of Bd occurrence in nature. I examined how the same two Bd isolates affected host susceptibility to exposure and virulance of resultant infections. I found that Panamanian Bd was more infectious but not more virulent and that USA hosts were not susceptible to infection by either isolate. Thus, hosts respond differently to different Bd isolates. I examined how epidermal trauma affected the infectivity and pathogenicity of Bd in Anaxyrus fowleri. Surprisingly, I found that epidermal trauma made Bd less infectious and pathogenic to this species. Thus, epidermal trauma, a common condition of wild amphibians, could actually reduce the impacts of Bd in individual amphibians. Finally, I examined how artificial selection imposed by culture conditions affected the infectivity and pathogenicity of Bd. I observed that one of our Bd isolates was showing signs of attenuation (artificially induced morphological and physiological changes). In a two part design I, 1.) reisolated a low pathogenicity Bd isolate from a moribund host, then 2.) compared infectivity and pathogenicity of the parent and daughter isolates. I found that passing an attenuated Bd isolate through an amphibian host partially restored infectivity but not pathogenicity. Through my dissertation research, I have shown that there are specific host-pathogen lineage interactions and that researchers should carefully design experiments so that results of controlled host-pathogen exposure studies more accurately reflect natural Bd-amphibian interactions