Population genetics as a tool to elucidate pathogen reservoirs: Lessons from Pseudogymnoascus destructans, the causative agent of White-Nose disease in bats

Emerging infectious diseases pose a major threat to human, animal, and plant health. The risk of species-extinctions increases when pathogens can survive in the absence of the host. Environmental reservoirs can facilitate this. However, identifying such reservoirs and modes of infection is often highly challenging. In this study, we investigated the presence and nature of an environmental reservoir for the ascomycete fungus Pseudogymnoascus destructans, the causative agent of White-Nose disease. Using 18 microsatellite markers, we determined the genotypic differentiation between 1,497 P. destructans isolates collected from nine closely situated underground sites where bats hibernate (i.e., hibernacula) in Northeastern Germany. This approach was unique in that it ensured that every isolate and resulting multi-locus genotype was not only present, but also viable and therefore theoretically capable of infecting a bat. The distinct distribution of multi-locus genotypes across hibernacula demonstrates that each hibernaculum has an essentially unique fungal population. This would be expected if bats become infected in their hibernaculum (i.e., the site they spend winter in to hibernate) rather than in other sites visited before they start hibernating. In one hibernaculum where both the walls and the hibernating bats were sampled at regular intervals over five consecutive winter seasons (1,062 isolates), higher genotypic richness was found on walls compared to bats and multi-locus genotypes showed a stable frequency over multiple winters. This clearly implicates hibernacula walls as the main environmental reservoir of the pathogen, from which bats become re-infected annually during hibernation.