All hosts are not created equal: Variation in the responses of amphibians to the fungal pathogen, Batrachochytrium dendrobatidis

Background/Question/Methods

The emergence and spread of infectious diseases has had profound impacts on many ecological systems. Disease is implicated in the loss of biological diversity, including the worldwide decline of amphibian populations. Predicting disease dynamics and ecosystem-level outcomes requires a basic understanding of host-pathogen interactions. While most pathogens can infect multiple host species, there may be considerable variation in the response to infection among different species. Hosts employ a variety of strategies against infection, including resistance (responses that limit parasite burden) or tolerance (responses that limit pathogen-induced damage to host health). Amphibians represent an excellent system for characterizing and categorizing variation in host responses to pathogen infection. The fungal pathogen, Batrachochytrium dendrobatidis (Bd) infects a wide-range of amphibian species and has highly variable effects on host health and survival. We investigated the effects of Bd infection on three different amphibian host species (Pseudacris regilla, Anaxyrus boreas, and Rana cascadae) at two different life history stages (larval and post-metamorphic) and across four pathogen inoculation intensities in standardized laboratory experiments. We monitored host survival throughout the experiment and sampled individuals at day 8 and day 15 of the experiment for quantitative estimates of infection load.

Results/Conclusions

We found differences in absolute survival, rates of mortality, and infection load across species, life history stages, pathogen inoculation treatments, and sampling time-points. As predicted, survival was reduced and infection load was greater in metamorphic amphibians, as compared with larvae. Among larval amphibians, Anaxyrus boreas displayed the highest infection load, and infection tended to increase over time. In metamorphic amphibians, Pseudacris regilla exhibited the highest infection load, and infection tended to decrease over time. In both larvae and metamorphs, Rana cascadae showed the lowest overall infection load. Interestingly, responses to experimental pathogen inoculation treatments were not necessarily dose dependent. Metamorphic R. cascadae exhibited reduced survival when exposed to higher pathogen inoculation treatments but did not display differences in infection load across treatments. Conversely, A. boreas showed significant variation in infection load but no differences in survival across pathogen inoculation treatments. Only P. regilla exhibited both reduced survival and an increase in infection load as experimental inoculation dose increased. Our results suggest a high degree of variation in responses to a conservation-relevant pathogen and provide evidence that differences in tolerance and resistance to Bd may underlie observed variation in disease susceptibility among amphibian species.