Decline in the prevalence of chytridiomycosis in frog populations in North Queensland, Australia

In the early 1990s stream-associated amphibian populations in tropical upland North Queensland experienced severe declines resulting in extinction of three species, local elimination of four species, marked reductions in one species and apparently no declines in other species. Chytridiomycosis, a disease due to the amphibian chytrid fungus, Batrachochytrium dendrobatidis, was the likely cause of this epidemic. We conducted a monitoring study for chytridiomycosis in four species of frogs in North Queensland from October 1998 to October 2002 by collecting specimens in the field and using histology of removed digits to diagnose chytridiomycosis. Chytridiomycosis was diagnosed in 112 (7.1%) of the 1 578 specimens and prevalence was significantly associated with season and altitude, with higher prevalences in winter and above 300 metres altitude. A multivariate model adjusting for potential confounding effects arising from the sampling process demonstrated a significant decline in the time trend of prevalence of chytridiomycosis. The study supports the hypothesis that B. dendrobatidis becomes endemic after the initial epidemic wave. Since the surviving species of stream-associated frog, Litoria genimaculata, has increased to pre-decline numbers, the decline in prevalence of chytridiomycosis is evidence of a changed pathogen-host relationship. The reasons for this change are speculative but could be due to an increase in innate host resistance in response to selection pressure by B. dendrobatidis or to lower rainfall associated with an El Niño effect. These findings justify management strategies that assist susceptible amphibian species to survive an initial epidemic wave of chytridiomycosis