The Influence of Temperature on Chytridiomycosis In Vivo

Julia M. Sonn, Scott Berman, Corinne L. Richards-Zawacki

Chytridiomycosis, an amphibian disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), is an ideal system for studying the influence of temperature on host-pathogen relationships because both host and pathogen are ectothermic. Studies of Bd in culture suggest that optimal growth occurs between 17 and 23°C, and death of the fungus occurs above 29 or below 0°C. Amphibian immune systems, however, are also temperature dependent and often more effective at higher temperatures. We therefore hypothesized that pathogen load, probability of infection and mortality in Bd-exposed frogs would peak at a lower temperature than that at which Bd grows best in vitro. To test this, we conducted a study where Bd- and sham-exposed Northern cricket frogs (Acris crepitans) were incubated at six temperatures between 11 and 26°C. While probability of infection did not differ across temperatures, pathogen load and mortality were inversely related to temperature. Survival of infected hosts was greatest between 20 and 26°C, temperatures where Bd grows well in culture. These results demonstrate that the conditions under which a pathogen grows best in culture do not necessarily reflect patterns of pathogenicity, an important consideration for predicting the threat of this and other wildlife pathogens.