Effectiveness of antifungal treatments during chytridiomycosis epizootics in populations of an endangered frog

Abstract

The recently-emerged amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) has had an unprecedented impact on global amphibian populations, and highlights the urgent need to develop effective mitigation strategies. We conducted in-situ antifungal treatment experiments in wild populations of the endangered mountain yellow-legged frog during or immediately after Bd-caused mass die-off events. The objective of treatments was to reduce Bd infection intensity (“load”) and in doing so alter frog-Bd dynamics and increase the probability of frog population persistence despite ongoing Bd infection. Experiments included treatment of early life stages (tadpoles and subadults) with the antifungal drug itraconazole, treatment of adults with itraconazole, and augmentation of the skin microbiome of subadults with Janthinobacterium lividum, a commensal bacterium with antifungal properties. All itraconazole treatments caused immediate reductions in Bd load, and produced longer-term effects that differed between life stages. In experiments focused on early life stages, Bd load was reduced in the 2 months immediately following treatment and was associated with increased survival of subadults. However, Bd load and frog survival returned to pre-treatment levels in less than 1 year, and treatment had no effect on population persistence. In adults, treatment reduced Bd load and increased frog survival over the entire 3-year post-treatment period, consistent with frogs having developed an effective adaptive immune response against Bd. Despite this protracted period of reduced impacts of Bd on adults, recruitment into the adult population was limited and the population eventually declined to near-extirpation. In the microbiome augmentation experiment, exposure of subadults to a solution of J. lividum increased concentrations of this potentially protective bacterium on frogs. However, concentrations declined to baseline levels within 1 month and did not have a protective effect against Bd infection. Collectively, these results indicate that our mitigation efforts were ineffective in causing long-term changes in frog-Bd dynamics and increasing population persistence, due largely to the inability of early life stages to mount an effective immune response against Bd. This results in repeated recruitment failure and a low probability of population persistence in the face of ongoing Bd infection.

In situ treatment of juvenile frogs for disease can reverse population declines

“…Our results differ from previous field studies that did not find evidence of persistent effects of itraconazole treatment on apparent survival or Bd infection (Bosch et al 2015; Hudson et al 2016; Geiger et al 2017; Knapp et al 2022 [this study found persistent effects of treatment for adults, but not for earlier life stages]). Our treatment targeted a specific period (shortly after metamorphosis) when Cascades frogs are highly susceptible to Bd and experience dramatic increases in Bd prevalence (Hardy et al 2015).…”

“…We detected dramatic reductions in Bd infection intensity after a single course of treatment with itraconazole and a dramatic increase in apparent overwinter survival of itraconazole‐treated frogs compared to controls, presumably due to a reduction in chytridiomycosis‐related mortality. This treatment effect is consistent with previous field‐based itraconazole treatment studies in Cascades frogs (Hardy et al 2015) and other amphibian species (e.g., Hudson et al 2016; Geiger et al 2017; Knapp et al 2022). It is likely that itraconazole treatment enhanced apparent survival by helping to maintain infection intensity below a critical threshold for disease‐induced mortality (Vredenburg et al 2010) during the overwintering period, which is when Bd infection and Bd ‐related mortality peak for many species (Rachowicz & Briggs 2007; Kinney et al 2011; Stevenson et al 2013; Brannelly 2014; Rumschlag & Boone 2018).…”

“…We found that treating recently metamorphosed frogs with itraconazole can be an effective management tool for maintaining the viability of at‐risk amphibian populations threatened by chytrid fungal pathogens. Previous studies have emphasized that the effects of treatment are likely to be short term, requiring frequent, ongoing treatment to achieve long‐term management goals (Langwig et al 2015; Garner et al 2016; Knapp et al 2022). Our study suggests the potential for longer‐term effects of treatment – also observed for adult mountain yellow‐legged frogs by Knapp et al (2022) – which may allow for the development of intermittent treatment plans to achieve long‐term management goals.…”

“…We expect treatment of hosts to be most beneficial when host populations have a long‐lived life stage that is relatively resistant to disease and when treatment can be targeted at times and places where environmental conditions and host life history lead to enhanced disease susceptibility. In situations where all life stages are highly susceptible to disease (e.g., Sierra Nevada Yellow‐legged frogs), treatment may not be an effective disease mitigation strategy, and management efforts should focus on other approaches such as expanding populations that are coexisting with Bd in an enzootic state (Knapp et al 2022). However, many amphibian species experience reduced Bd susceptibility in later life stages (Fisher et al 2009; Rollins‐Smith et al 2011; Sauer et al 2020), and in these contexts, in situ treatment of susceptible early life stages may amplify the effects of other management actions, such as habitat manipulation.…”

“…While eradication of the pathogen is possible (Bosch et al 2015), it is unlikely to be feasible in most situations, leading to a renewed focus on facilitating coexistence with Bd (Scheele et al 2019; Brannelly et al 2021). In situ treatment of amphibian hosts with antifungal chemicals, such as itraconazole, has emerged as one of the more reliable interventions for reducing Bd infections in individuals (e.g., Hudson et al 2016; Geiger et al 2017; Knapp et al 2022). However, these studies have generally found only short‐term benefits of this intervention, leading many researchers to conclude that benefits to populations will be minimal unless treatments are sustained (Garner et al 2016; Fisher & Garner 2020).…”