Batrachochytrium dendrobatidis in boreal chorus frogs: Life stage and seasonal variation in the prevalence of an endemic pathogen

Background/Question/Methods

Infectious diseases are emerging as a significant threat to wildlife. The amphibian disease, chytridiomycosis, exemplifies the negative effects of infectious disease on wildlife populations. This disease, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), is linked to amphibian decline and extinction worldwide. Yet, there is considerable variation in population-level responses to Bd introductions ranging from host extinction to long-term persistence with Bd. Our research focuses on identifying the factors that enable populations of susceptible hosts to persist with this pathogen. Detailed descriptions of Bd dynamics in populations of susceptible amphibians are a first step toward hypotheses for how these infected populations persist. 

Results/Conclusions

We monitored temporal variation in Bd prevalence in boreal chorus frogs (Pseudacris maculata), a species known to be susceptible to chytridiomycosis. From 2009-2010, 60 adult frogs were swabbed annually for the presence of Bd from each of 20 ponds. In 2010, an additional 30 metamorphic frogs were swabbed from 15 ponds. Bd persisted for both years in 17 of the 20 ponds, demonstrating host and pathogen persistence across two breeding seasons. Bd prevalence in breeding adults varied from 0%-100% within ponds. Breeding was a key period for Bd transmission among adults with Bd prevalence in individual ponds in 2010 increasing by an average of 17% (SD: 20%; limits: 20% - 53%) during the breeding season. Bd prevalence in summer-emerging metamorphic frogs was significantly lower than adults sampled from the same ponds in spring (paired t-test_df=14, p = 0.0001) with 11 of the 15 ponds testing Bd negative. Despite high Bd prevalence, no mass-mortalities were observed.

Several hypotheses for the persistence of chorus frogs with Bd emerge from these results. The first and simplest explanation is that chorus frogs may not suffer from chytridiomycosis in the field. If adults do not suffer increased mortality as a result of Bd infection, the population could easily persist with Bd. Alternatively, if chorus frogs are dying from chytridiomycosis, then the pronounced reduction in Bd prevalence in metamorphic frogs may play an important role in the persistence of boreal chorus frogs with Bd. Future studies will examine the susceptibility of adult, larval, and metamorphic chorus frogs to chytridiomycosis, and are designed to uncover the mechanisms causing this seasonal reduction in Bd prevalence in metamorphic frogs.