Summary1. Amphibian chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), is responsible for the greatest disease-driven loss of vertebrate biodiversity in recorded history. Understanding drivers of host susceptibility to this cutaneous disease is hindered by gaps in our knowledge of the host-pathogen relationship. One such overlooked aspect of susceptibility is variation in skin maintenance processes, particularly skin turnover via routine sloughing. It has been suggested that sloughing plays a role in immune defence, by removing skin-associated microbes. Thus, skin sloughing may play an important role in the pathogenesis of chytridiomycosis. 2. To determine the relationship between skin sloughing and disease progression, we exposed adult Australian green tree frogs (Litoria caerulea) to a local Bd strain and monitored sloughing rates and individual infection loads on a naturalistic cycling temperature regime (15-23°C). 3. We determined sloughing rates in real-time by using an array of infrared video cameras to film frog behaviour and monitored infection load before and after sloughing by swabbing and analysis with quantitative PCR. 4. We found that sloughing rate increased with Bd infection load in infected frogs, but sloughing itself did not affect Bd load on the ventral skin surface. Furthermore, Bd infection did not affect the duration of characteristic sloughing behaviour, and sloughing retained rhythmicity even at high infection loads. 5. Although an increased sloughing rate might be considered advantageous for Bd-infected animals, it does not appear to curb the progression of disease and may actually contribute to the loss of physiological homoeostasis seen in terminally ill frogs by further inhibiting water and electrolyte transport across the skin. 6. By measuring sloughing rates directly for the first time, our results shed light on how Bd interacts with the physiological processes of the skin and indicate that variation in skin sloughing frequency may play a role in the observed inter-and intraspecific variation in susceptibility to disease.

The fungal pathogen Batrachochytrium dendrobatidis (Bd) has been implicated in amphibian population declines globally. Given that Bd infection is limited to the skin in post-metamorphic amphibians, routine skin sloughing may regulate infection. Skin sloughing has been shown to reduce the number of cultivatable microbes on amphibian skin, and Bd infection increases skin sloughing rates at high loads. However, it is unclear whether species specific differences in skin sloughing patterns could regulate Bd population growth on the skin, and influence subsequent infection dynamics. We exposed five Australian frog species to Bd, and monitored sloughing rates and infection loads over time. Sloughing reduced Bd load on the ventral skin surface, in all five species, despite wide variation in susceptibility to disease. In the least susceptible species, an increase in sloughing rate occurred at lower infection loads, and sloughing reduced Bd load up to 100%, leading to infection clearance. Conversely, the drop in Bd load with sloughing was only temporary in the more susceptible species. These findings indicate that the ability of sloughing to act as an effective immune defence is species specific, and they have implications for understanding the pattern of Bd population growth on individual hosts, as well as population-level effects.

The skin is the first line of defence in preventing the establishment of pathogens and associated infections in frogs. Regular sloughing of the outer layer can reduce the abundance of cultivable cutaneous microbes in green tree frogs which has ramifications for our understanding of cutaneous pathogens like the amphibian chytrid fungus.

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