Emerging diseases are a key factor in the disconcerting rate of contemporary amphibian declines. The chytrid fungus, Batrachochytrium dendrobatidis (Bd), ranks among the chief pathogenic challenges to vulnerable amphibian populations. Previous work has indicated that amphibian exposure to dead Bd can induce an adaptive immune response, leading to an acquired resistance. But it is yet unclear if using dead Bd is a feasible immunization strategy. Dose the concentration of dead Bd affect host frog ability to acquire resistance? Does the duration of treatment affect the host frog ability to acquire resistance? Treatment of amphibians with flash-frozen, dead Bd may be the key to protecting susceptible populations, yet it has not yet been established what would be needed to induce acquired resistance to Bd. To determine the dose and duration of exposure necessary to induce resistance, we manipulated the duration and dose of exposure of adult frogs to dead Bd using a 5x5 response surface experimental design in the laboratory that includes the full vertical and horizontal dose and duration gradients and the four corners of the treatment matrix so that we test 13 of the 25 possible treatments. We exposed Cuban tree frogs (Osteopilus septentrionalis) to 5 levels of dead Bd, as well as a control, every other day for 1, 2, 3, or 4 weeks. After the exposures, we exposed all frogs to live Bd (1ml of 1x106 zoospores/ml) and maintained them at 18°C for two weeks in an environmental chamber. Frogs were swabbed and Bd burdens quantified by estimating the density of zoospores using a StepOne Real-Time PCR System. Data were analyzed using GLM with a negative binomial distribution.
Results/Conclusions
Duration of treatment with dead Bd was found to have a significant negative effect on zoospore abundance found on frogs post exposure to live pathogen. Dose of dead Bd was not found to be significant. Longer treatment with dead Bd is shown to reduce zoospore loads. The concentration of dead Bd does not appear to have a significant effect on pathology. The chytrid fungi are implicated in hundreds of extinctions and declines among amphibians. Knowing dose thresholds contributes greatly in the development of a disease mitigation strategy.