Endemicity of chytridiomycosis features pathogen overdispersion

1. Pathogenscan be critical drivers of the abundance and distribution of wild animal populations. The presence of an overdispersed pathogenload distribution between hosts (where few hosts harbour heavy parasite burdens and light infections are common) can have an important stabilizing effect on host– pathogendynamics where infection intensity determines pathogenicity. This may potentially lead to endemicity of an introduced pathogenrather than extirpation of the host and/or pathogen. 2. Overdispersed pathogenload distributions have rarely been considered in wild animal populations as an important component of the infection dynamics of microparasitessuch as bacteria, viruses, protozoa and fungi. 3. Here we examined the abundance, distribution and transmission of the model fungal pathogen Batrachochytrium dendrobatidis(Bd, cause of amphibian chytridiomycosis) between wild-caught Litoria rheocola(common mist frogs) to investigate the effects of an overdispersed pathogenload distribution on the host population in the wild. We quantified host survival, infection incidence and recovery probabilities relative to infectious burden, and compared the results of models where pathogen overdispersioneither was or was not considered an important feature of host– pathogendynamics. 4. We found the distribution of Bd load between hosts to be highly overdispersed. We found that host survival was related to infection burden and that accounting for pathogen overdispersionallowed us to better understand infection dynamics and their implications for disease control. In addition, we found that the pattern of host infections and recoveries varied markedly with season whereby (i) infections established more in winter, consistent with temperature-dependent effects on fungal growth, and (ii) recoveries (loss of infection) occurred frequently in the field throughout the year but were less likely in winter. 5. Our results suggest that pathogen overdispersionis an important feature of endemic chytridiomycosisand that intensity of infection determines disease impact. These findings have important implications for our understanding of chytridiomycosisdynamics and the application of management strategies for disease mitigation. We recommend quantifying individual infectious burdens rather than infection state where possible in microparasiticdiseases.