Developing field & diagnostic methods to survey for chytridiomycosis in Tasmanian frogs

About the report

Chytridiomycosis is an infectious disease caused by the amphibian chytrid fungus (Batrachochytrium dendrobatidis). B. dendrobatidis is recognised as a major threatening process for amphibian populations worldwide. In Australia the fungus is associated with frog declines and is directly implicated in the extinction of several frog species in Southeast Australia (Anon 2003a).

During the spring and summer of 2004-05 a total of 56 frog habitats in Tasmania were assessed for chytridiomycosis using field survey techniques targeting the tadpole stages supported by chytrid-specific laboratory testing.

The survey confirmed the presence of the chytrid fungus in a number of frog habitats close to major Tasmanian cities and towns. Tadpoles of five species of Tasmanian frog (including one endemic species) were shown to be carriers of the fungus. Using a hand lens, the presence of depigmentation and asymmetry in jaw sheaths and loss of tooth rows in live tadpoles was strongly correlated with chytrid infection. Field surveys of up to 60 tadpoles at each survey site in combination with the Taqman chytrid PCR test were useful diagnostic tools for the detection of chytridiomycosis at targeted amphibian habitats.

The detection of the chytrid infection in remote wetlands at high altitude (> 800 m) on the Tasmanian Central Plateau is of particular concern. Declines in the range and abundance of the endemic Tasmanian Tree Frog, Litoria burrowsae and the Green & Gold Frog, Litoria raniformis are considered to be directly linked to the establishment and spread of chytridiomycosis in Tasmania.

Tadpole populations in several peri-urban wetlands and private suburban frog ponds were shown to have chytrid infection. The prolonged time interval between the entry & establishment of chytrid in Tasmania (possibly the late 1970's) and this baseline survey suggests that in the intervening years the fungus has transferred locally to other frog habitats through natural ecological processes and by anthropogenic means.

Combined with appropriate hygiene protocols to prevent anthropogenic spread of chytrid infection, this survey protocol has application in baseline and follow up monitoring surveys for chytrid infection in frog populations. In the course of the survey a range of sampling methods were tested and refined to improve its application more universally.