Evaluation of a filtration-based method for detecting Batrachochytrium dendrobatidis in natural bodies of water

Infectious diseases are emerging as a significant threat to wildlife. The resulting increased effort to monitor wildlife diseases is driving the development of innovative pathogen monitoring techniques, including many polymerase chain reaction (PCR)-based diagnostics. Despite the utility of these PCR-based techniques, there is still much to be learned about their ability to accurately detect target pathogens in nature. We assessed the diagnostic sensitivity of a PCR-based water filtration technique to detect the directly transmitted aquatic fungal pathogen Batrachochytrium dendrobatidis (Bd) by comparing the results of 4 repeated filter sampling events from 20 ponds to those of skin swabs from ca. 60 boreal chorus frogs Pseudacris maculata from each pond. Filters failed to detect Bd in 31 to 77% of the swab-positive ponds, depending on the time of sampling. However, after 3 repeated sampling events, filtration of small volumes of water (ca. 600 ml) correctly identified 94% of the ponds that tested Bd positive with swabbing, with the highest rates of detection occurring after breeding but before larvae reached metamorphosis. Our results are a case study demonstrating the importance of timing and resampling for the detection of an aquatic microbial pathogen, Bd, from water. This will be a useful technique for monitoring Bd, but additional data are needed to test the degree to which our findings are species or population specific. Future studies need to examine the sensitivity of this technique in other habitats and species that host Bd. These studies will aid in the development of cost-effective monitoring regimes for Bd and potentially other aquatic pathogens.