To treat or not to treat? Experimental pathogen exposure, treatment, and release of a threatened amphibian

Abstract Reintroductions may mitigate the effects of severe amphibian declines, but chytridiomycosis—the often‐deadly disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd)—can hinder these efforts. Exposing amphibians to the pathogen ex situ may improve disease outcomes post‐release. We experimentally inoculated federally threatened California red‐legged frogs (Rana draytonii) ex situ, cleared Bd infections, and then released them to the wild for monitoring in an area where Bd is present. Ex situ, previously exposed frogs had lower Bd loads than previously uninfected frogs, and all exposure groups, including previously uninfected frogs, naturally reduced their Bd loads in two independent experiments. After release to the wild, we observed no differences in field‐contracted Bd infection among exposure groups. Treating post‐metamorphic R. draytonii ex situ may not confer additional post‐release benefit to this species in terms of chytridiomycosis burden. Our results, however, do not necessarily mirror Bd susceptibility throughout the amphibian life cycle, with other source populations that may have different Bd infection histories, or responses to different Bd strains. These results provide a clearer understanding of post‐release disease responses of a threatened, Bd‐susceptible species with prior pathogen exposure. Following ex situ experiments with in situ applications can provide a more comprehensive understanding of threat outcomes for declining species in reintroduction programs and strengthen critical links between ex situ and in situ conservation partners