Microscopic Aquatic Predators Strongly Affect Infection Dynamics of a Globally Emerged Pathogen

Research on emerging infectious wildlife diseases has placed particular emphasis on host-derived barriers to infection and disease. This focus neglects important extrinsic determinants of the host/pathogen dynamic, where all barriers to infection should be considered when ascertaining the determinants of infectivity and pathogenicity of wildlife pathogens [1–3]. Those pathogens with free-living stages, such as fungi causing catastrophic wildlife declines on a global scale [4], must confront lengthy exposure to environmental barriers before contact with an uninfected host [5–8]. Hostile environmental conditions therefore have the ability to decrease the density of infectious particles, reducing the force of infection and ameliorating the impact as well as the probability of establishing an infection [9]. Here we show that, in nature, the risk of infection and infectious burden of amphibians infected by the chytrid fungus Batrachochytrium dendrobatidis (Bd) have a significant, site-specific component, and that these correlate with the microfauna present at a site. Experimental infections show that aquatic microfauna can rapidly lower the abundance and density of infectious stages by consuming Bd zoospores, resulting in a significantly reduced probability of infection in anuran tadpoles. Our findings offer new perspectives for explaining the divergent impacts of Bd infection in amphibian assemblages and contribute to our understanding of ecosystem resilience to colonization by novel pathogens. [Display omitted] •Microorganism counts correlate inversely with B. dendrobatidis prevalence in nature•Rotifers and ciliates ingest zoospores of B. dendrobatidis•Infection rates of anuran tadpoles are reduced in the presence of aquatic microfauna•Bd-consuming microorganisms hold promise as a biocontrol and field mitigation tool Schmeller et al. show that the death of amphibians exposed to a lethal and globally distributed fungal pathogen is being prevented at some locations by microscopic predators. These aquatic microorganisms either block or weaken infections by consuming the fungal spores that infect amphibians and cause the disease chytridiomycosis.