Physiological effects of chytridiomycosis, a cause of amphibian population declines

Abstract

Emerging infectious diseases (EIDs) of wildlife can have devastating impacts on biodiversity. The fungal disease, chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd) is implicated with amphibian population declines the world over. As is the case with many EIDs of wildlife, pathogenesis of chytridiomycosis is somewhat unclear. Pathogenesis involves disruption of cutaneous ion uptake, decreased plasma ions, and asystolic cardiac arrest, as well as seemingly unrelated effects on leukocytes, skin shedding, and appetite. In this dissertation I, along with the help of many collaborators, suggest that infection-induced decreases in plasma ions initiate a stress response which may mediate some of the deleterious effects observed during disease development. In chapters one and two, physiological parameters were monitored during an outbreak and a controlled infection, respectively, of Bd in a laboratory colony of Litoria caerulea. Taken together, it was observed that prior to becoming diseased, infected frogs experienced decreased plasma sodium and potassium, appetite, and body mass, as well as increased standard metabolic rate and skin shedding. When infected frogs became diseased, they contained even fewer plasma ions, as well as increased plasma corticosterone (CORT; a stress hormone) and altered white blood cell profiles. These individuals also had continued elevated standard metabolic rate and decreased body condition. In chapter three, it was determined that CORT increases standard metabolic rate in L. caerulea, representing the first time this effect has been observed in an anuran amphibian. Collectively, this dissertation suggests that stress physiology plays a role in chytridiomycosis.