Batrachochytrium dendrobatidis and the Decline and Survival of the Relict Leopard Frog

Abstract

Epizootic disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd) is a major driver of amphibian declines, yet many amphibians declined before the pathogen was described. The Relict Leopard Frog, Rana onca (=Lithobates onca), was nearly extinct, with the exception of populations within a few geothermal springs. Growth of Bd, however, is limited by high water temperature, and geothermal springs may have provided refuge during outbreaks of chytridiomycosis. We conducted field surveys and laboratory experiments to assess the susceptibility of R. onca to Bd. In the field, we found Bd at one of the two areas where remnant populations of R. onca still occur, but not in the other. In the laboratory, we infected juvenile frogs from these two areas with two hypervirulent Bd isolates associated with declines in other ranid species. In our experiments, these Bd isolates did not affect survivorship of R. onca and most infections (64%) were cleared by the end of the experiments. We propose that R. onca either has inherent resistance to Bd or has recently evolved such resistance. These results may be important for conservation efforts aimed at establishing new populations of R. onca across a landscape where Bd exists. Resistance, however, varies among life stages, and we also did not assess Bd from the local environment. We caution that the resistance we observed for young frogs under laboratory conditions may not translate to the situation for R. onca in the wild.

Amphibian resistance to chytridiomycosis increases following low‐virulence chytrid fungal infection or drug‐mediated clearance

“…Since virulence of Bd is thought to decrease with increased passages (transfers) in culture media (Langhammer et al, 2013), serial passages could be used to develop attenuated Bd vaccines; although, Bd virulence may depend on subculturing methods (Voyles et al, 2014). Indeed, the low-virulence of SLL in R. onca observed in a previous study (Jaeger et al, 2017) and in this current study could be due to the high passage number.…”

“…We had previously confirmed the absence of Bd in this population using quantitative real-time PCR (qPCR) of swabs collected from 60 adult frogs using methods described below. We housed and maintained R. onca and R. pipiens in individual containers at 19°C following protocols previously described (Jaeger et al, 2017). We focused our efforts on frogs rather than tadpoles because the immune systems of amphibians at these distinct life stages are drastically different (Rhoo et al, 2019), and chytridiomycosis resistance or tolerance at the tadpole stage may not translate to adult stages (Waddle et al, 2019).…”

“…We used the Section Line Lake (SLL) Bd strain as our hypovirulent Bd inoculation. This isolate has demonstrated high virulence in a laboratory study on Rana cascadae (Piovia-Scott et al, 2015), but was found to display hypovirulence towards R. onca at a later transfer history (transfer 14) when the strain may have attenuated (Jaeger et al, 2017). For this study, we did not attempt to quantify or characterize immune mechanisms associated with frogs inoculated with SLL, so we simply refer to these frogs as 'inoculated'.…”

Immunological Aspects of Chytridiomycosis

“…Under certain circumstances, such as in particular species or with repeated priming, it may be possible for this adaptive immune response to function protectively and reduce mortality [163]. Circumstantial evidence concerning experimental animals suggests that adaptive immune memory may be involved in animals that recover from chytridiomycosis after 4-6 weeks post exposure or in animals that persist with low-level burdens indefinitely (as reported by [133,158,[164][165][166]). However, examples of improved survival associated with repeated immune priming may alternatively be due to increased reactivity of the innate response, known as an innate immune memory [167].…”

Impacts of temperature on immune performance in a desert anuran (Anaxyrus punctatus)

“…(2015) found that Bd can persist in Amargosa toads ( Anaxyrus nelsoni ) even during the spring and summer, when temperatures can exceed 30°C. Additionally, imperiled Mojave Desert species such as the relict leopard frog ( Rana onca ) can contract and succumb to Bd infection (Jaeger et al., 2017; Waddle et al., 2019).…”