COS 259-4 - Temporal variation in pathogen virulence for an emerging wildlife disease

Abstract: Emerging infectious diseases have had severe impacts on wildlife populations. Introduced pathogens that are optimized to a host population preceding an introduction to new landscapes can lead to mismatched virulence in naïve host populations. This can result in dramatic shifts in pathogen virulence following introduction to novel populations. Generally, trade-off theory would predict pathogen fitness to be maximized at an intermediate virulence where pathogen replication is balanced with host survival and subsequent transmission. However, other factors may also shape pathogen evolution such as host resistance, competition between strains, and environmental conditions. White-nose syndrome is caused by a fungal pathogen, Pseudogymnoascus destructans, that was introduced to North America in 2006 and has caused precipitous declines in multiple hibernating bat species. Transmission of P. destructans to susceptible hosts occurs through a highly prevalent environmental reservoir and bats are frequently infected upon arrival to their hibernation sites, suggesting that the effect of transmission from infected to susceptible hosts on pathogen virulence evolution may be weak. Additionally, the introduced isolate was clonally reproducing with little genetic variability andn changes in pathogen virulence over time have yet to be explored. To examine these changes, we performed an in vivo host challenge experiment by experimentally infecting Eptesicus fuscus (big brown bats) with an isolate from a site in North America where population declines historically occurred and a contemporary isolate collected from the same site 14 years after invasion. To evaluate virulence between treatments, bats were hibernated in environmental chambers for 110 days, epidermal swabs were collected to determine infection intensity (via qPCR), pathogen tissue invasion was examined through histology, and the amount of physiological disruption was quantified through the frequency of bat torpor arousals. To further investigate temporal patterns of virulence, we performed an in vitro investigation of the growth rates of isolates from multiple sites across on the landscape. We found shifts in pathogen virulence and observed a reduction in virulence across a temporal scale in North American isolates collected within the same site. Additionally, our in vitro experiment revealed a reduction in growth rate over time across the landscape. Combining host and culture experiments we conclude that P. destructans virulence has reduced over time suggesting that transmission is not the primary driver of virulence evolution.