Understanding the dynamics and determinants of recovery in the mountain chicken frog (Leptodactylus fallax) following its near extinction due to infectious disease

The Mountain Chicken Frog (Leptodactylus fallax, MCF) is a giant amphibian, endemic to the Lesser Antilles. Once an abundant wild food resource on its home islands of Dominica and Montserrat, the MCF became Critically Endangered and illegal to harvest in the 2000’s following an outbreak of the infectious disease chytridiomycosis.

Chytridiomycosis is caused by a fungal pathogen, Batrachochytrium dendrobatidis (Bd). Bd has driven population declines in over 500 amphibian species and is a major threat to amphibian biodiversity and the MCF’s survival. Susceptibility to Bd infection can vary within amphibian host species, across habitats, and through time. Indeed, a small MCF population recovery was observed in 2014, with some MCFs even able to survive or clear Bd infection. The mechanisms enabling the survival of this population and its apparent tolerance to Bd infection are unclear, but also crucial to understand in order to conserve the MCF and aid wider amphibian conservation efforts. Changes in infectious disease dynamics occur with changes in one or more of: the host, the pathogen, and their environment.

My thesis presented a series of investigations into potential host- and pathogen-related determinants of the MCF’s susceptibility to or survival from the Bd epizootic, complementing existing studies which focus on environmental determinants. The first two host-related studies in the thesis involved a reconstruction of the demographic history of the MCF and a characterisation of key immune loci in the major histocompatibility complex (MHC). Historic MCF population reductions, identified by the demographic models, probably led to the depleted MHC diversity identified by the immunogenetic study. Low MHC diversity potentially explains the susceptibility of most MCFs to lethal Bd infection. Meanwhile, three MHC-I alleles were identified at an increased in frequency in Bd-tolerant MCFs, linking MHC-I diversity to Bd-tolerance for the first time. The increased frequency of these alleles in Dominica also explains why more MCFs survived there than in Montserrat. This thesis then carried out a novel characterisation of the MCF’s skin microbiome, a key innate immune system component. This study revealed that the skin microbiome of surviving wild MCFs has potentially Bd-protective features, but these features were not explicitly tested against Bd infection. Replenishing microbiome diversity in Bd-naïve MCFs did not protect them from Bd infection, raising doubts around the utility of the skin microbiome in determining MCF survival from Bd infection. The last host-related study quantified remaining genetic diversity in captive MCFs and produced breeding recommendations which will influence future population recovery dynamics in the MCF. A final study into pathogen changes across the Bd epizootic revealed that the same, hypervirulent Bd lineage was implicated in the MCF’s decline.

As such, determinants of MCF survival or susceptibility to Bd infection are likely host- rather than pathogen-related. Following these studies, new avenues for research have been opened which will improve our understanding of amphibian chytridiomycosis. Conservation strategy is now more fully informed and better able to work towards mitigating Bd in the wild, a fundamental step in recovering the MCF.

This study was conducted by Nina White, who was a student at Cardiff University and ZSL. This project won Gold in the BIAZA Research Awards category in 2023.