Mortality Events in Yuma Myotis (Myotis yumanensis) Due to White-Nose Syndrome in Washington, USA

The impacts of white-nose syndrome (WNS) on many bat species in eastern North America have been well documented because of the length of time that the causative agent, Pseudogymnoascus destructans (Pd), has been present and the ability to monitor bat hibernacula in that region. However, the disease outcomes for bat species in western North America are less known because of the more recent arrival of Pd and the challenges associated with monitoring hibernating bat populations in parts of the western USA. We report on mortality events involving Yuma myotis (Myotis yumanensis) bats at two locations in King and Benton counties, Washington, USA, that were attributed to WNS during the late winters of 2020-21 and 2024, respectively. All bats that were grossly examined had depleted subcutaneous white adipose tissue, tested positive for the presence of Pd, had histopathologic lesions consistent with WNS, and did not exhibit evidence of other disease processes that may have contributed to death. Mortality was probably higher than what was documented because the locations of the Pd-contaminated hibernacula from which the bats originated were inaccessible or unknown and thus could not be surveyed. These findings indicate that Yuma myotis may be highly susceptible to WNS, and close monitoring is warranted to understand how WNS will affect population trends in this (and other) western bat species.