Emerging Infectious Disease and the Imperiled Relict Leopard Frog

The disease chytridiomycosis, caused by the aquatic fungal pathogen Batrachochytrium dendrobatidis (Bd), has emerged as a major contributing factor for worldwide amphibian declines. Although relatively recently described, the impacts from the disease this pathogen causes have been definitively tied to amphibian declines, including some that occurred decades ago. In some cases, declines of individual species occurred with little documentation and are thus poorly understood. The relict leopard frog (Rana onca = Lithobates onca) has experienced such a decline and by the latter part of the 20th century only occurred in two general areas in southern Nevada. Recent research has found Bd within the historical range of the species and that the species shows evidence of potential resistance to chytridiomycosis. Those authors, however, noted that the Bd strains used were not from the local environment and they speculated on possible attenuation. I addressed these concerns by challenging an anuran species known to be susceptible to chytridiomycosis to one of the previously used Bd isolates (SLL) that showed hypovirulence towards R. onca. I also performed a disease transmission experiment intended to increase virulence in SLL towards R. onca in an attempt to elucidate the possibility of attenuation. In other experiments, I isolated Bd from anurans in the local environment, and then used these isolates to challenge juvenile R. onca, as well as an earlier life-stage thought to be more vulnerable to chytridiomycosis. My results indicate that the SLL isolate is still virulent toward a susceptible host species, but R. onca continued to appear resistant toward this particular isolate. My challenge experiments using local isolates of Bd, however, showed that R. onca is susceptible to chytridiomycosis from two Bd isolates found in southern Nevada, as well as an isolate I acquired from a commercial vender. I found that frogs from a currently Bd infected area cleared infections and survived in much higher proportions than those from a Bd-free area. This population-level effect, however, was dependent on life-stage and recently metamorphosed frogs from both areas showed low survivorship when challenged with Bd