| Batrachochytrium dendrobatidis (Bd), a fungus pathogenic to amphibians, is an ideal system in which to study the mechanisms of epidemic fadeout, especially with respect to changes in pathogen virulence over time. Through survey work conducted in western Panama from 2012 to 2013, in the Richards-Zawacki (Tulane) and Voyles (New Mexico Tech) labs have found that while mortality rates are still high and many amphibian species have disappeared, some populations of susceptible hosts are persisting many years after the arrival of Bd. For example, harlequin frogs (Atelopus varius) and common rocket frogs (Colostethus panamensis) experienced severe declines due to Bd outbreaks in Panama in 2004 - 2006 and were thought to be locally extinct. However, surviving populations of both species have been recently found in the same areas that were first exposed to Bd between 8 and 10 years ago.;We are comparing pathogenicity over time using a combination of in vitro phenotypic assays and in vivo inoculation experiments with historic (during epidemic) and contemporary (8-10 years post-epidemic) Bd isolates. Specifically, we are testing the hypothesis that Bd has decreased in pathogenicity over time since the Panamanian epidemic, consistent with epidemic fadeout. To test this hypothesis I inoculated Australian green tree frogs (Litoria caerulea) with epidemic and post-epidemic isolates and measured survival, body condition and phenotypic changes to test for a shift in pathogenicity. I also quantified the following Bd traits for isolates from both timepoints: timing of zoospore release, maximum zoospore density, and duration of zoospore activity in culture. Changes in these traits, are hypothesized to affect pathogenicity and transmission.;We found higher mortality and higher pathogen loads in frogs inoculated with the post-epidemic (2012-2013) isolates of Bd than with the historic isolates. This suggests that rather than attenuating in pathogenicity, Bd may have become more pathogenic over time. We also found differences in life cycle timing and zoospore production between old and new Bd isolates that could be linked to changes in pathogenicity. |
