Print PageAmphibian biodiversity is currently facing a severe crisis that has already caused declines in 42% of species and threatens as much as 32% with extinction. Remarkably, an emerging infectious disease, chytridiomycosis, is directly linked to the recent extinction or serious decline of hundreds of amphibian species and is increasingly proposed as a primary threat to amphibians. Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatitus (Bd), has been described as the agent that has caused the greatest loss of vertebrate biodiversity due to disease in recorded history. The list of amphibian species driven to extinction by Bd is growing (>200) yet not all infected species are susceptible and many appear to harbour the pathogen with no apparent negative effects.
Bd proliferates in epidermal cells of post-metamorphic amphibians and in the mouthparts of tadpoles, and is capable of killing individuals rapidly via the disruption of skin function. Like all chytridiomycota, Bd has an aquatic, flagellated zoospore stage. This zoospore infects the keratinized tissues of amphibians and then encysts, goes through asexual reproduction and releases more propagules into the water. Because of this unique natural history, amphibian hosts with aquatic lifestyles are expected to be particularly susceptible to the pathogen yet many direct developing species that never approach water are also susceptible to Bd. Why? This study seeks to determine whether amphibian traits are associated with fungal disease risk and asks, "Can we predict what species may be vulnerable to chytridiomycosis?"
Results/Conclusions
I use a well-documented case of chytridiomycosis in two species of frogs in California to illustrate the difficulty in answering this question. The Sierra Nevada Mountain yellow-legged frog species complex (Rana sierrae and R. muscosa) have been severely affected by Bd. In these species, Bd occurs in both the enzootic and epizootic state. Infected host population stability depends on Bd infection intensity levels. Animals that reach high pathogen loads succumb while those that do not reach high loads survive. It is still unknown why epizootic events occur in some areas and not in others but several factors are possible. Many species are resilient to the pathogen and either do not become susceptible to it or have some level of population-level resistance (e.g. Rana catesbaeiana, Xenopus leavis). Species or populations that are non-susceptible to Bd are presumably either protected by poor abiotic growing conditions for Bd, immunological response, populations of other suitable hosts, low virulence of Bd strain, or bacterial epibiotic symbionts.
