Abstract
Batrachochytrium dendrobatidis (Bd), a cutaneous amphibian fungus that causes the lethal disease chytridiomycosis, has been implicated as a cause of many amphibian declines. Bd can tolerate low temperatures with an optimum thermal range from 17–24°C. It has been shown that Bd infection may result in species extinction, avoiding the transmission threshold presented by density dependent transmission theory. Prevalence of Bd during autumn and winter has been shown to be as low as 0% in some species. It is currently unclear how Bd persists in field conditions and what processes result in carry-over between seasons. It has been hypothesised that overwintering tadpoles may host Bd between breeding seasons. The Great Barred Frog (Mixophyes fasciolatus) is a common, stable and widespread species in Queensland, Australia, and is known to carry Bd. Investigation into Bd infection of different life stages of M. fasciolatus during seasonally low prevalence may potentially reveal persistence and carry-over methods between seasons. Metamorphs, juveniles, and adults were swabbed for Bd infection over three months (between March and May, 2011) at 5 sites of varying altitude (66 m–790 m). A total of 93 swabs were analysed using Polymerase Chain Reaction (PCR) real-time analysis. PCR analysis showed 6 positive (1 excluded), 4 equivocal and 83 negative results for infection with Bd. Equivocal results were assumed to be negative using the precautionary principle. The 5 positive results consisted of 4 emerging (Gosner stage 43–45) metamorphs and 1 adult M. fasciolatus. Fisher's exact test on prevalence showed that the prevalence was significantly different between life stages. All positive results were sampled at high altitudes (790 m); however prevalence was not significantly different between altitudes. Infection of emerging metamorphs suggests that individuals were infected as tadpoles. We hypothesise that M. fasciolatus tadpoles carry Bd through seasons. Thus, Mixophyes fasciolatus may act as disease reservoirs at multiple life stages.
Highlights
Amphibian species world-wide have experienced rapid population declines and extinction in the recent decades
The 4 equivocal results consisted of 1 adult and 3 metamorphs; observing the precautionary principle, all equivocal results are being treated as false positives and were excluded from analyses
For Batrachochytrium dendrobatidis (Bd) infection to lead to declines, the number of zoospores present on an individual needs to be greater than approximately ten thousand zoospore equivalents [43]
Summary
Amphibian species world-wide have experienced rapid population declines and extinction in the recent decades. The extinction rate between 1980 and 2004 may be up to 200 times greater than the historical rates of previous centuries [1]. Many of these declines and extinctions have occurred in undisturbed and protected habitats [2,3,4]. In these cases the anthropogenic means usually cited for species extinctions—including habitat disturbances, fragmentation and the introduction of exotic pests and predators—fail to satisfactorily explain the level of declines observed [2,5]. Species and populations of species show differing levels of susceptibility and some species persist without obvious declines when infected with Bd [11,12]
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