Effect of season and temperature on mortality in amphibians due to chytridiomycosis

Objective To investigate the distribution and incidence of chytridiomycosis in eastern Australian frogs and to examine the effects of temperature on this disease. Design A pathological survey and a transmission experiment were conducted. Procedure Diagnostic pathology examinations were performed on free-living and captive, ill and dead amphibians collected opportunistically from eastern Australia between October 1993 and December 2000. We conducted a transmission experiment in the laboratory to investigate the effects of temperature: eight great barred frogs (Mixophyes fasciolatus) exposed to zoospores of Batrachochytrium dendrobatidis and six unexposed frogs were housed individually in each of three rooms held at 17degreesC, 23degreesC and 27degreesC. Results Chytridiomycosis was the cause of death or morbidity for 133 (55.2%) of 241 free-living amphibians and for 66 (58.4%) of 113 captive amphibians. This disease occurred in 34 amphibian species, was widespread around the eastern seaboard of Australia and affected amphibians in a variety of habitats at high and low altitudes on or between the Great Dividing Range and the coast. The incidence of chytridiomycosis was higher in winter, with 53% of wild frogs from Queensland and New South Wales dying in July and August. Other diseases were much less common and were detected mostly in spring and summer. In experimental infections, lower temperatures enhanced the pathogenicity of B dendrobatidis in M fasciolatus. All 16 frogs exposed to B dendrobatidis at 17degreesC and 23degreesC died, whereas 4 of 8 frogs exposed at 27degreesC survived. However, the time until death for the frogs that died at 27degreesC was shorter than at the lower temperatures. Infections in survivors were eliminated by 98 days. Conclusion Chytridiomycosis is a major cause of mortality in free-living and captive amphibians in Australia and mortality rate increases at lower temperatures.