Chytridiomycosis, an infectious disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd), poses an imminent conservation threat. The global emergence of Bd has led to mass mortality events in many species, even resulting in ~90 extinctions. Thus, novel management tools are urgently needed. Exposure to filtrate from Bd cultures can induce partial immunity against live Bd, suggesting the potential for vaccination. However, little is known about immunity to Bd, especially regarding strain specificity. We hypothesized that induced resistance could be specific to the vaccinating strain, and therefore provide weaker protection against other strains. To evaluate strain specificity of this induced immune phenotype, we conducted a 3x2 factorial experiment in which we crossed vaccination and live infection treatments with two Bd strains that differ in pathogenicity. Specifically, we exposed adult Cuban tree frogs, Osteopilus septentrionalis, to either filtrate isolated from a California strain, the global pandemic strain (Panama), or an artificial spring water sham-control. After twelve days of vaccination, we then exposed frogs to 9 x 105 zoospores of live Bd from either the California or Panama strain. After 16 days, we swabbed the frogs and used qPCR to quantify infection status and zoospore count.
Results/Conclusions
We analyzed the Bd load data using a zero-inflated model with vaccination and live exposure treatments as fixed factors and negative binomial errors using the R package “pscl”. We found a significant interaction between vaccination strain and infection strain on Bd loads. Specifically, frogs vaccinated with the California-isolated strain and then exposed to the Panama-isolated strain had lower loads than frogs vaccinated with the Panama strain or the sham treatment. These results suggest that the magnitude of induced resistance is sensitive to the identities of the vaccinated and infecting strains. In addition, these results contradict the hypothesis that vaccination provides strain-specific protection. Follow up experiments are needed to determine the mechanisms underlying resistance phenotypes and the asymmetric cross-resistance observed here.