Beyond the swab: ecosystem sampling to understand the persistence of an amphibian pathogen

Abstract/Summary

Understanding the ecosystem-level persistence of pathogens is essential for predicting
and measuring host-pathogen dynamics. However, this process is often masked, in
part due to a reliance on host-based pathogen detection methods. The amphibian
pathogens Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal) are
pathogens of global conservation concern. Despite having free-living life stages, little is
known about the distribution and persistence of these pathogens outside of their
amphibian hosts. We combine historic amphibian monitoring data with contemporary
host- and environment-based pathogen detection data to obtain estimates of Bd
occurrence independent of amphibian host distributions. We also evaluate differences
in filter- and swab-based detection probability and assess inferential differences arising
from using different decision criteria used to classify samples as positive or negative.
Water filtration-based detection probabilities were lower than those from swabs but
were >10%, and swab-based detection probabilities varied seasonally, declining in the
early fall. The decision criterion used to classify samples as positive or negative was
important; using a more liberal criterion yielded higher estimates of Bd occurrence than
when a conservative criterion was used. Different covariates were important when
using the liberal or conservative criterion in modeling Bd detection. We found evidence
of long-term Bd persistence for several years after an amphibian host species of
conservation concern, the boreal toad (Anaxyrus boreas boreas), was last detected.
Our work provides evidence of long-term Bd persistence in the ecosystem and
underscores the importance of environmental samples for understanding and
mitigating disease-related threats to amphibian biodiversity.