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.

From eDNA to decisions using a multi-method approach to restoration planning in streams

“…After evaluating sites of interest for the threat of non-native predators with eDNA and VES, we identified and prioritized feasible R. boylii recovery actions for each site. This assessment was done in consultation with management agency stakeholders, including U.S. National Park Service staff that have conducted California red-legged frog ( Rana draytonii ) translocations to Yosemite Valley 4 , 24 , 44 , as well as U.S. Forest Service staff that have conducted monitoring at R. boylii -occupied sites.…”

“…Non-native predator eradication programs—such as those removing non-native fish and bullfrogs—have been successful in restoring some Sierra Nevada aquatic habitats 39 , 40 . These eradication initiatives have laid the groundwork for a surge in ranid reintroduction efforts 4 , 7 , 24 , 41 – 44 , presenting an important opportunity to closely examine restoration methods and prioritization.…”

Drosophila melanogaster as a model arthropod carrier for the amphibian chytrid fungus Batrachochytrium dendrobatidis

“…Inoculum consisted of a cocktail of four pathogenic Bd strains isolated from Rana sierra or R . muscosa in the California Sierra Nevada mountain range: TST77, CJB4, CJB5-(2), and CJB7 [ 29 , 30 ]. Concentrations of Bd were counted using a hemocytometer and Bd zoospore viability was visually confirmed via microscopy.…”

Safe harbor: translocating California red‐legged frogs to a climate refuge in Yosemite National Park

“…populations can also provide insurance against pathogens such as the amphibian chytrid fungus (Batrachochytrium dendrobatidis; Bd) which is present in the Valley (De Castro & Bolker, 2005;Adams, Bushell, & Grasso, 2022), and to which R. draytonii is susceptible (Adams et al, 2020). We incorporated an ex situ component of the reintroduction program at the San Francisco Zoo, including a Bd inoculation study (Adams, Bushell, & Grasso, 2022) and optimization of husbandry techniques for R. draytonii, which had not been reared in captivity before. The captive-reared frogs have subsequently contributed significantly to the establishment of an introduced population in Yosemite Valley.…”