Asymmetric cross-strain protection for amphibians exposed to a fungal-metabolite prophylactic treatment

Chytridiomycosis, an infectious disease of amphibians caused by the fungal
pathogen Batrachochytrium dendrobatidis (Bd), poses an imminent
conservation threat. The global spread of Bd has led to mass mortality
events in many amphibian species, resulting in at least 90 species’
extinctions to date. Exposure to Bd metabolites (i.e., non-infectious
antigenic chemicals released by Bd) partially protects frogs during
subsequent challenges with live Bd, suggesting its use as a prophylactic
treatment and potential vaccine. However, we do not know whether Bd
metabolite exposure protects against strains beyond the one used for
treatment. To address this knowledge gap, we conducted a 3x2 experiment
where we exposed adult Cuban treefrogs, Osteopilus septentrionalis, to one
of three treatments (Bd metabolites from California-isolated strain
JEL-270, Panamá-isolated strain JEL-419, or an artificial spring water
control) and then challenged individuals with live Bd from either strain.
We found that exposure to Bd metabolites from the California-isolated
strain significantly reduced Bd loads of frogs challenged with the live
Panamá-isolated strain, but no other treatments were found to confer
protective effects. These findings demonstrate asymmetric cross-protection
of a Bd metabolite prophylaxis and suggests that work investigating
multiple, diverse strains is urgently needed.