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Chytridiomycosis and the Mountain Yellow Legged Frog: studies of physiological factors that influence disease in Rana muscosa.

  • Author(s): Stice, Mary Jennifer;
  • Advisor(s): Hayes, Tyrone B;
  • et al.

Declines in Rana muscosa within the last ten years have been so severe that this species is threatened with extinction. The fungal pathogen Batrachochytrium dendrobatidis (Bd) has emerged as a major contributor to declines in the already small and fragmented populations of this species. Adults, juveniles and larvae continue to be found within affected populations from year to year, albeit in smaller numbers. The work presented within this dissertation asked whether immune responses could be enhanced against Bd and examined the effect of physiological factors on Bd prevalence and related mortality in R. muscosa in lab and field caught animals. I studied the possibility of protecting individuals against Bd using immunizations and examined the beneficial effects of sub-clinical Bd infections and survival between life stages. My work also included examining interactions between seasonal changes in endocrine function related to reproduction and prevalence of Bd.

In laboratory studies I found that immunizations with killed Bd prior to live exposure in juvenile R. muscosa frogs was not protective. No difference in how quickly animals became infected nor in mortality between groups, was found. However, exposure to Bd during the larval stage, at sub-infectious doses, was protective in those same animals when re-exposed to Bd as sub-adults. This work provides two points for further investigation; infectious dose may be a determining factor in the pathogenesis of Bd and protective responses developed in larvae that are capable of resolving or building a memory response to infection with Bd.

Within field populations of R. muscosa I detected seasonal fluctuations in Bd prevalence and the hormones involved in the stress and reproduction, corticosterone and testosterone. I found no association between either hormone and Bd prevalence within and between populations This finding refutes the immunocompetence handicap hypothesis claim that these two hormones are immunosuppressive in all vertebrates and that disease and breeding are positive correlated with each other The strongest predictors of infection

prevalence and intensity were month and site of collection or population sampled. This supports work showing Bd is constrained by temperature within the environment but further suggests that factors unique to sites are important in modulating disease.

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