Effects of intrinsic and extrinsic factors on the prevalence of the fungus Batrachochytrium dendrobatidis (Chytridiomycota) in stream tadpoles in the Atlantic Forest domain

Recent evidence indicates that infectious diseases are one of the main causes of amphibian decline worldwide. Chytridiomycosis is an emergent disease caused by the waterborne fungus Batrachochytrium dendrobatidis ( Bd ). The infectious stage of this pathogen comprises motile zoospores that encyst in amphibian host to form a zoosporangium, from which new zoospores are subsequently released into the water. Therefore, the aquatic environment facilitates Bd transmission, suggesting that tadpoles could be more prone to infection than adults. However, unlike adults, infection during the amphibian larval stage is rarely lethal. This leads to a high prevalence of Bd in tadpoles, consequently acting as reservoirs for this pathogen. With this study, we aim to understand the effects of intrinsic and extrinsic factors on the prevalence of Bd in an assemblage of stream tadpoles in the Atlantic Forest, south-eastern Brazil. For Bd detection, we inspected individuals’ mouthparts searching for a specific pattern of depigmentation that has been widely associated with infection by this pathogen in tadpoles. Specimens presenting depigmentation compatible with the described patterns within the literature were classified as Bd -positive. We found 38.7% (209 out of 540) Bd -positive tadpoles. Slow-developing tadpoles presented a higher Bd prevalence compared to fast-developing tadpoles. Also, tadpoles inhabiting streams with lower temperatures and slower water flow presented high Bd prevalence regardless of their developmental duration. Our results suggest that slow-developing tadpoles can potentially act as effective Bd reservoir, and that cooler, slow-water streams could favour the pathogen maintenance in the environment.