Skin sloughing in susceptible and resistant amphibian hosts regulates infection with a fungal pathogen

Amphibians worldwide are threatened by the fungal pathogen Batrachochytrium dendrobatidis (Bd), which in post-metamorphic animals only infects the skin, and causes the potentially lethal disease chytridiomycosis. Amphibians... [ view full abstract ]

Amphibians worldwide are threatened by the fungal pathogen Batrachochytrium dendrobatidis (Bd), which in post-metamorphic animals only infects the skin, and causes the potentially lethal disease chytridiomycosis. Amphibians regularly shed or slough their skin, and this behavior has been associated with a reduction in the cultivatable microbes on the skin surface. It has been demonstrated that Bd exhibits host-dependent growth patterns, with Bd growing epibiotically in species less susceptible to chytridiomycosis. Therefore, we hypothesized that sloughing would more effectively remove Bd zoospores in less susceptible species. To test this hypothesis, five Australian frog species, Litoria caerulea, Limnodynastes peronii, Lim. tasmaniensis, Platyplectrum ornatum, and Lechriodus fletcheri, were exposed to Bd, and their sloughing rates and infection loads were monitored over time. Utilizing a methodology to remove any artifacts from the swabbing itself, we found that sloughing reduced Bd load on the ventral skin surface, in all five species, despite wide ranging variation in susceptibility to Bd infection and subsequent disease and mortality. In less susceptible species, sloughing reduced Bd load up to 100%, leading to infection clearance. The drop in Bd load was only temporary in susceptible species, potentially due to the invasive growth of Bd in skin layers underlying the stratum corneum in these species. If less susceptible species are able to clear themselves of Bd infection via the routine process of skin shedding, amphibian sloughing may be a more important immune defense than previously thought. In addition, this finding has implications for understanding the pattern of Bd growth on individual hosts, and how this may translate to population-level effects.