Epidermal cell death in frogs with chytridiomycosis.
BACKGROUND
Amphibians are declining at an alarming rate, and one of the major causes of decline is the infectious disease chytridiomycosis. Parasitic fungal sporangia occur within epidermal cells causing epidermal disruption, but these changes have not been well characterised. Apoptosis (planned cell death) can be a damaging response to the host but may alternatively be a mechanism of pathogen removal for some intracellular infections.


METHODS
In this study we experimentally infected two endangered amphibian speciesPseudophryne corroboreeandLitoria verreauxii alpinawith the causal agent of chytridiomycosis. We quantified cell death in the epidermis through two assays: terminal transferase-mediated dUTP nick end-labelling (TUNEL) and caspase 3/7.


RESULTS
Cell death was positively associated with infection load and morbidity of clinically infected animals. In infected amphibians, TUNEL positive cells were concentrated in epidermal layers, correlating to the localisation of infection within the skin. Caspase activity was stable and low in early infection, where pathogen loads were light but increasing. In animals that recovered from infection, caspase activity gradually returned to normal as the infection cleared. Whereas, in amphibians that did not recover, caspase activity increased dramatically when infection loads peaked.


CONCLUSIONS
Increased cell death may be a pathology of the fungal parasite, likely contributing to loss of skin homeostatic functions, but it is also possible that apoptosis suppression may be used initially by the pathogen to help establish infection. Further research should explore the specific mechanisms of cell death and more specifically apoptosis regulation during fungal infection.
One Health Research Group, College of Public Health, Medical and Veterinary Sciences, James Cook University , Townsville , QLD , Australia.
Laura A
LA
Brannelly
One Health Research Group, College of Public Health, Medical and Veterinary Sciences, James Cook University , Townsville , QLD , Australia.
Alexandra A
AA
Roberts
One Health Research Group, College of Public Health, Medical and Veterinary Sciences, James Cook University, Townsville, QLD, Australia; Faculty of Veterinary and Agricultural Sciences, University of Melbourne, Melbourne, Victoria, Australia.
Lee F
LF
Skerratt
One Health Research Group, College of Public Health, Medical and Veterinary Sciences, James Cook University, Townsville, QLD, Australia; Faculty of Veterinary and Agricultural Sciences, University of Melbourne, Melbourne, Victoria, Australia.
Lee
L
Berger
Apoptosis; Caspases; Chytridiomycosis; TUNEL; Wildlife disease