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.