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When defenses fail: Investigating an innate immune response in Panamanian Golden Frogs
AdvisorVoyles, Jamie L
Ecology, Evolution and Conservation Biology
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To combat the threat of emerging infectious diseases in wildlife, ecoimmunologists implement a disease triad framework to understand the complex interactions among pathogens, their hosts, and their shared environments. This framework is particularly useful in highly complex disease systems, consisting of multiple hosts and pathogens in heterogenous environments. Due to its high complexity, the disease triad is an ideal framework to analyze the chytridiomycosis panzootic in amphibians. The pathogen behind this panzootic is the fungus Batrachochytrium dendrobatidis (Bd), which has led to the decline of innumerable amphibian species, including the Panamanian golden frog (Atelopus zeteki). I focused on the role of host immunity, to investigate which components of the A. zeteki immune system are most important in defending against Bd. Given that Bd can evade or dampen the acquired immune responses of some amphibians, nonspecific immune defenses are thought to be especially important for amphibian defenses against Bd. In particular, skin secretions constitute a vital component of amphibian innate immunity against skin infections, but their role in protecting A. zeteki from Bd is unknown. I investigated the importance of this innate immune component by reducing the skin secretions from A. zeteki and evaluating their effectiveness against Bd in vitro and in vivo. Following exposure to Bd in a controlled inoculation experiment, I compared key disease characteristics (e.g., changes in body condition, prevalence, pathogen loads, and survival) among groups of frogs that had their skin secretions reduced and control frogs that maintained their skin secretions. Surprisingly, I found that the skin secretions collected from A. zeteki increased Bd growth in vitro. This finding was further supported by infection and survival patterns in the in vivo experiment where frogs with reduced skin secretions tended to have lower pathogen loads and survive longer compared to frogs that maintained their secretions. These results suggest that the skin secretions of A. zeteki are not only ineffective at inhibiting Bd but may enhance Bd growth, possibly leading to greater severity of disease and higher mortality in this highly vulnerable species. These results differ from those of previous studies in other amphibian host species that suggest that skin secretions are a key defense in protecting amphibians from developing severe chytridiomycosis. Therefore, I suggest that the importance of immune components cannot be generalized across all amphibian species or over time. Moreover, the finding that skin secretions may be enhancing Bd growth emphasizes the importance of investigating these immune components in detail, especially for species that are a conservation priority.