Amphibian resistance to chytridiomycosis increases following low virulence chytrid fungal infection or drug-mediated clearance

Amphibian biodiversity is experiencing ongoing declines due in part to the
infectious disease, chytridiomycosis. Efforts to mitigate the effects of
the causal agent of chytridiomycosis, Batrachochytrium dendrobatidis (Bd),
in the wild have not been wholly effective. Translocations are an
important management tool for amphibians, and immunizations represent a
possible strategy for preparing amphibians for release across a landscape
where Bd exists. We evaluated the utility of using an isolate of Bd that
was shown to be hypovirulent to the relict leopard frog (Rana onca) as a
transmissible inoculum for promoting chytridiomycosis-resistance. We
conducted a cohousing experiment to determine if the isolate we used could
be passed between R. onca without increasing in virulence. We then
 followed with an experiment where frogs that were exposed to the
hypovirulent isolate were then challenged with a virulent Bd isolate. In
other experiments, we evaluated whether Bd infections followed by
clearance with itraconazole (an antifungal) could increase resistance to
chytridiomycosis in R. onca and Rana pipiens (northern leopard frog). We
found that our hypovirulent Bd inoculation was transmissible between
hosts, did not cause chytridiomycosis, and was effective at increasing
chytridiomycosis-resistance. Rana onca inoculated with the hypovirulent Bd
isolate had lower pathogen burdens and were 55 times more likely to
survive infections by a virulent Bd isolate than non-inoculated frogs. For
both species, prior exposure to Bd followed by infection clearance with
itraconazole resulted in significantly increased survivorship and lower
pathogen burdens as compared to controls that had no prior Bd exposure.
Rana onca that were previously exposed to Bd were more than 15 times more
likely to survive infections. Previously exposed R. pipiens survived in
higher proportions than controls, but with weaker statistical support.