Chytrid fungal infections in alpine toads proliferate during winter dormancy

The spread and impact of wildlife pathogens is often seasonal, and identifying the seasons of high impact is critical to biodiversity and public health management. Here, we report new evidence that winter host dormancy, a period generally neglected in terms of pathogen seasonal dynamics, promotes the spread of infection from the globally threatening amphibian fungal pathogen, Batrachochytrium dendrobatidis (Bd). Bd surveillance of Yosemite toads (Anaxyrus canorus) during their first year of life showed that: (a) detectable Bd infections were nearly absent in the tadpole stage and immediately after metamorphosis, (b) Bd prevalence and intensity gradually increased during the first two months of post-metamorphic growth, and (c) Bd prevalence and intensity increased four-fold in toads during the first winter dormancy. High prevalence and intensity of Bd infections after winter dormancy was observed in yearling toads across two consecutive years and was much higher than that observed in aquatically breeding adults during the same timeframe. To our knowledge, this is the first evidence from wild amphibians that Bd proliferates during terrestrial winter dormancy. This discovery calls for a reconsideration of the seasons enabling the proliferation and persistence of Bd and identifies recently metamorphosed hosts as overwinter Bd reservoirs. More generally, the study underscores the importance of host dormancy in pathogen persistence and seasonal infection spread.