Chytridiomycosis in southwest Australia: historical sampling documents the date of introduction, rates of spread and seasonal epidemiology, and sheds new light on chytrid ecology
Ken Aplin1 and Peter Kirkpatrick2
1. Dept Terrestrial Vertebrates, Western Australian Museum, Francis St Perth Western Australia, Australia 6000.
2. School of Veterinary Science, Murdoch University, Western Australia, Australia; formerly Western Australian Museum.
Following initial detection of chytridiomycosis in Perth frogs in August 1998, more than 4000 samples have been examined histologically, many drawn from the archive collections of the WA Museum. Sampling has focussed on the mesic southwestern corner of the state, with minimal effort on other regions including the sub-tropical Kimberley.
Chytridiomycosis was detected in 17 species including both hylid and myobatrachid frogs. Contemporary disease outbreaks were recorded at numerous localities within the forested southwest and metropolitan regions, but also extending far east, north and inland into seasonally-arid woodland and heath habitats. Batrachochytrium was also detected at one locality in the central Kimberley, the first from the subtropical northwest.
The earliest evidence of chytridiomycosis in W.A. comes from localities near the south coast in 1985, with delayed arrival in Perth in 1989, and continued spread through the 1990s. A total of 612 negatives spanning 1952-1984 and a wide geographic area argue convincingly against Batrachochytrium being a native pathogen.
Analysis of infection prevalence data shows a regular seasonal cycle, with peak values of 20-50% during late Winter - Spring, falling to 1-3% over Summer - Autumn. Variation in peak infection rates between years also hints at a possible climatic influence. Peak infection levels are typically higher for semi-aquatic species, but may still attain 10% even for highly terrestrial forms.
Estimates of annual mortality run as high as 50% in some species, but others appear unaffected and may act as carriers. Although no southwestern frog species has yet suffered catastrophic decline, some local extinctions have occurred, and many species may be under long-term threat.