Extreme climatic events modulate chytrid infection across the landscape

The disease chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd), Bd ), has swept through amphibian communities worldwide. As a fungal pathogen of ectothermic hosts, Bd occurrence is influenced by environmental factors; however, the effect of extreme climatic events on infection dynamics is unclear. We examined the influence of environmental conditions on Bd infection patterns across populations of Fleay's barred frog ( Mix- ophyes fleayi), ), a recovering species in Australia. We conducted surveys four times over 10 months at 16 streams across the species' complete geographical distribution. Three consecutive survey periods coincided with unprecedented drought and heatwaves, region-wide bushfires, and subsequent heavy rainfall. We used generalized linear mixed models and conditional inference trees to evaluate the impact of compound extreme climatic events on host-Bd Bd dynamics. A total of 509 post-metamorphic M. fleayi were sampled, and proportion of infected individuals ranged from 0 to 85.3 % (95 % CI: 0.69-0.95) per stream. We found seasonal variation in prevalence, with minimum values in early summer during prolonged drought and intensifying two months later after flooding rainfall. We found Bd prevalence spiked when long-term cumulative rainfall increased, particularly in frogs with low body condition. High Bd loads likely to impact survival occurred mainly in streams at higher elevations under increased rainfall periods. Our results showed that extreme and back-to-back climatic events, such as flooding rains following severe drought, may heighten the risk of infection and thus mortality in this threatened amphibian. These findings suggest management policies for Bd should consider the impact of major weather events in destabilizing the host-pathogen relationship, leading to potential disease outbreaks.