New diseases are emerging at an increasing rate, and in numerous cases are having devastating effects on wildlife species. The amphibian chytrid fungus (Batrachochytrium dendrobatidis; Bd) causes the disease chytridiomycosis that since its emergence has resulted in the extinction or serious decline of hundreds of amphibian species worldwide. In California?s Sierra Nevada, chytridiomycosis has caused the near-extinction of the once-common mountain yellow-legged frog. During summer 2012, the largest remaining mountain yellow-legged frog population will suffer a die-off event caused by the recent arrival of Bd in the area. In mountain yellow-legged frog populations the arrival of Bd in a previously uninfected population typically results in frog population extinction, and the goal of the proposed study is to change the outcome to long-term frog population persistence. This disease intervention will take the form of a field experiment implemented during the Bd-caused frog die-off. In this experiment the effectiveness of two treatments, (1) an antifungal drug, and (2) the augmentation of antifungal bacteria that occur naturally on frog skin, will be assessed at the scale of an entire frog population. This experiment will also provide an opportunity to describe the mechanisms underlying treatment effectiveness, including the role of the frog immune system, the microbial community present on the skin of frogs, and rapid evolution in frogs. Based on results from previous laboratory and small-scale field trials it is expected that antifungal drug and bacterial augmentation treatments will increase frog survival relative to frogs that are left untreated. Results from the proposed study will have important implications for conservation efforts aimed at wildlife species worldwide that are threatened by emerging diseases (e.g., amphibians, bats, apes). The study will also provide training opportunities for undergraduate and graduate students, with every effort made to include those from underrepresented groups to the maximum extent possible.
The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) is an important pathogen of amphibians that has recently spread worldwide, causing the extinction of more than 100 species to date. In California’s Sierra Nevada mountains Bd is spreading into Bd-naïve populations of the mountain yellow-legged frog (Rana muscosa, Rana sierrae) and has caused the extirpation of hundreds of populations during the past decade. The major goal of this project was to conduct a field disease intervention experiment during a Bd outbreak in Kings Canyon National Park to test the hypothesis that anti-Bd treatments could be used to change the outcome from frog population extirpation to long-term persistence. During the proposed intervention, frogs were to be treated with the antifungal drug Itraconazole, the symbiotic skin bacterium Janthinobacterium lividum, both, or neither. In previous small-scale lab and field trials, Itraconazole and J. lividum treatment significantly increased frog survival, most likely by allowing sufficient time for the frog immune response against Bd to be activated. In addition, we sought to gain insights into the still poorly understood immune response by our study frog species against Bd using immunological and genomics approaches. The specific objectives were as follows: (1) Conduct a field disease intervention experiment during a Bd outbreak in Kings Canyon National Park to test the hypothesis that anti-Bd treatments could be used to change the outcome from frog population extirpation to long-term persistence, and (2) gain insights into the still poorly understood immune response by our study frog species against Bd using immunological and genomics approaches. Results from our detailed description of Bd-frog dynamics in several frog populations early in the Bd invasion process indicate that the arrival of Bd into a naïve frog population does not always result in an epizootic (i.e. epidemic in wildlife populations). Instead, some invasions fail, and Bd prevalence and load subsequently return to low levels. We also found that in contrast to previous treatments conducted with adult frogs, our treatment of juvenile frogs with Itraconazole and J. lividum did not increase long-term survival of treated frogs, although it did alter infection dynamics and increase survival over the first field season. These results suggest that these treatments are effective only when they act in concert with a frog-acquired immune response. Our genetics investigations are ongoing and are producing important insights into the conditions under which this poorly understood immune response is effective against Bd. Mitigating the effects of Bd on amphibians is the focus of an intensive global research effort. Our (1) experiments testing the efficacy of Bd treatments and frog immunization in reducing Bd-caused frog mortality and increasing the probability of frog population persistence despite ongoing Bd infection, and (2) genetics investigations of the frog immune response against Bd, are producing important results that are directly relevant to this effort. The comparison of our results with those obtained by research groups working in other study systems will provide a more comprehensive understanding of the conditions under which particular mitigation strategies are likely to be most effective. The results of our study have implications for the field of immunology. Although it is generally assumed that the amphibian immune system is relatively incapable of mounting an effective immune response against Bd, the African clawed frog model system was recently used to provide the first evidence that frogs could in fact develop an acquired immune response against this pathogen. Our research is investigating this immune response in a non-model species and should provide additional insights into the strength and effectiveness of the frog acquired immune response against Bd. Our project has involved postdoctoral scholars, Ph.D. students, and undergraduates, all of whom have gained valuable experience in field and laboratory methods. At San Francisco State University, this project supported underrepresented minorities. We communicated our results with the National Park Service (NPS) and we are training NPS staff on the methods used to conduct field Bd treatments. These skills will allow the NPS to implement future disease interventions designed to prevent the Bd-caused extirpation of mountain yellow-legged frog populations. Many emerging diseases in humans have their origins in wildlife. Our study helps improve our understanding of wildlife-disease dynamics and helps develop methods to mitigate the effects of wildlife diseases have implications for human health.
