Amphibian resistance to chytridiomycosis increases following low‐virulence chytrid fungal infection or drug‐mediated clearance
Authors: Waddle Anthony W. , Rivera Rebeca , Rice Hannah , Keenan Emma C. , Rezaei Ghazal , Levy Joshua E. , Vasquez Yesenia S. , Sai Marlai , Hill Jessica , Zmuda Alexandra , Lambreghts Yorick , Jaeger Jef R. 
Published: August 2021

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 co‐housing 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 imparting long‐term 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.

Synthesis and Applications. Translocations are an important management tool for many amphibian conservation programs. For amphibian species that have experienced declines due to the disease chytridiomycosis [caused by the pathogen Batrachochytrium dendrobatidis (Bd)], the success of translocations may be limited due to the persistent threat of the disease in the wild. We completed a series of experiments aimed at increasing amphibian resistance to chytridiomycosis using low‐virulence, transmissible Bd inoculations and Bd infection followed by clearance with an antifungal (itraconazole). Our low‐virulence Bd inoculations were transmissible, did not cause disease and provided long‐term resistance against chytridiomycosis. Similarly, infection with Bd and clearance with itraconazole led to increased resistance to chytridiomycosis in subsequent exposures. The use of a low‐virulence Bd inoculation could be further developed into a transmissible vaccine, propagating the protective benefits beyond individuals that can be directly inoculated. At this stage, we advise that using the alternative approach of infecting frogs with Bd from translocation sites followed by clearance of infections with itraconazole may be sensible for increasing host resistance to chytridiomycosis.