The emergence of infectious disease rarely ends in the complete extinction of host species. Frequently, the level of virulence in a severe disease system shifts such that hosts and pathogens can persist in a shared environment. However, the mechanistic underpinnings of these transitions are not well understood. The emergence of the disease, chytridiomycosis in the amphibian communities of western Panama provides a compelling system to investigate such shifts in host-pathogen interactions. The pathogen that causes chytridiomycosis, Batrachochytrium dendrobatids (Bd), is renown for its ability spread rapidly into amphibian communities and cause extremely high levels of mortality, which has set the stage for critical research on the coevolutionary dynamics following Bd emergence.
We are testing the hypothesis that Bd pathogenicity has shifted since initial chytridiomycosis epizootic events, which occurred approximately a decade ago in western Panama. Specifically, we are resurveying multiple historic outbreak sites to characterize amphibian community structure and infection patterns. In addition, we are collecting Panamanian Bd isolates from multiple time points: ten years ago, at the most severe point of an epizootic event, and 1-2 years ago (i.e., “Historic” and “Contemporary” isolates). We are testing for differences in phenotypic characteristics, genotypes and pathogenicity in naive amphibian hosts.
Although community structures have been altered and amphibian diversity remains relatively low, we provide evidence that some amphibian species that were putatively driven to extinction by Bd are rebounding in western Panama. In addition, our results suggest that current infection patterns in Panama do not fit the classic models for persistence in an enzootic host-pathogen system. Specifically, our findings indicate that, although Bd infection is maintained across seasons, prevalence has decreased markedly in all sites and all species tested. Moreover, our laboratory inoculation experiments suggest that there are no significant differences among the Historic and Contemporary Bd isolates in phenotypic characteristics or in any of our virulence response variables, including prevalence, infection intensity, body mass or mortality. Thus, it appears that some amphibian species are persisting— and are even rebounding— even though Bd is present across seasons and despite the fact that Contemporary Bd isolates are as pathogenic as isolates collected during the initial Bd outbreaks. We discuss a variety of possible mechanisms that could help explain these shifts in disease dynamics in the iconic tropical amphibian communities of western Panama.