Environmental Detection of the Amphibian Chytrid Fungus in Water Bodies Predicts Host Infection Along a Deforestation Gradient

Understanding pathogen dynamics during environmental life stages is vital for comprehending wildlife diseases, especially for those with a free-living phase. In amphibians affected by the chytrid fungus Batrachochytrium dendrobatidis (Bd), most studies have focused on host-pathogen interactions, with less emphasis on Bd's environmental stage. We tested whether the distribution of Bd in natural aquatic environments can predict host infection patterns. We sampled four tropical amphibian species across eight rainforest landscapes with varying habitat loss, testing whether environmental and host Bd detection varied along gradients of habitat change. Using a high-capacity water filtration method coupled with digital and real-time PCR detection assays, we quantified Bd in water and amphibian samples. Our results revealed a strong positive correlation between Bd DNA concentrations in water samples and infection loads on amphibian skin samples. Forest cover and habitat split were the primary predictors of Bd distribution in both free-living and host-associated forms. We identified Bd-GPL and Bd-Asia-2/Brazil lineages across our study landscapes. Our study introduces and validates a robust protocol for detecting Bd in environmental samples, with the potential to enhance monitoring and inform management strategies. Moreover, our work contributes novel, well-replicated spatial data on Bd associations between hosts and their environments.