Initiation of Batrachochytrium dendrobatidis infection in the absence of physical contact with infected hosts - a field study in a high altitude lake

Understanding transmission is a critical prerequisite for predicting disease dynamics and impacts on host populations. It is well established that Batrachochytrium dendrobatidis (Bd), the amphibian fungal pathogen responsible for chytridiomycosis, can be transmitted directly, through physical contact with an infected host. However, indirect pathways of transmission remain poorly investigated. We conducted a five-week long field infection experiment at a high altitude mountain lake in the French Pyrenees to investigate Bd transmission pathways in larval midwife toads Alytes obstetricans. Uninfected naive tadpoles were co-housed either with infected tadpoles (direct and indirect transmission) or with uninfected ones (indirect transmission only). We found that physical contact with an infected host is not necessary for initial infection with Bd and that all tadpoles became infected after only four weeks. However, physical contact with infected tadpoles led to a faster spread within a tadpole group and resulted in higher Bd loads and subsequently higher mortality. Our findings clearly demonstrate that in A. obstetricans, Bd can quickly spread in a population even without physical contact. Our experiment therefore stresses the importance of indirect transmission of Bd zoospores in infected lakes for disease dynamics, especially when a reservoir species such as A. obstetricans is present.