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

White-nose syndrome (WNS) is a cutaneous infection caused by the fungus Pseudogymnoascus destructans (Pd; Blehert et al. 2009; Lorch et al. 2011). Since it was introduced to North America from Eurasia around 2006, Pd has spread across much of the US and southern Canada, causing massive population declines in hibernating bat species (Blehert et al. 2009; Leopardi et al. 2015). The impacts of WNS have been well documented for some species in eastern North America (O’Keefe et al. 2019; Hoyt et al. 2021), with at least three bat species (Myotis lucifugus, Myotis septentrionalis, and Perimyotis subflavus) experiencing overall population declines >90% (Cheng et al. 2021; Hoyt et al. 2021). However, little information is available on how the disease impacts bats in western North America, where Pd has more recently invaded (Lorch et al. 2016). Differences in environmental conditions, bat host species, and hibernation ecology between eastern and western North America have made it difficult to predict and document the impacts of WNS in western bat populations (Weller et al. 2018). Here we report on three mortality events associated with WNS in Yuma myotis (Myotis yumanensis) that occurred at two sites in Benton and King counties, Washington, US.

Beginning in late January 2020, mortality of Myotis sp. bats was observed at a site near North Bend in King County, Washington (coordinates are not provided as bat roost locations are considered sensitive data). The site has several bat houses that bats have used as transitional roosts in the spring after their emergence from hibernation but before their occupation of summer roosts. Bats do not typically egress from hibernacula until March in Washington (Hayes and Wiles 2013). Bat carcasses were primarily found below one of the bat houses and were collected by the landowner and state agency personnel and stored frozen. Mortality continued through late February 2020, with 45 bat carcasses being found. No unusual weather events were reported during this time, and temperatures were relatively normal for the time period (National Weather Service 2024). The bats were suspected to be either little brown bats (M. lucifugus alascensis) or Yuma myotis; the two species are difficult to distinguish morphologically (Rodhouse et al. 2008). This event was initially reported by Blejwas et al. (2023), but neither the bat species affected nor the cause of mortality were confirmed. Pseudogymnoascus destructans had been initially detected in King County in 2016 (Lorch et al. 2016), and we had confirmed WNS at this specific site in 2017. However, large mortality events in bats associated with this disease had not been documented in Washington; previously, most dead bats with WNS in Washington were found as individual carcasses on the landscape. In early March 2021, mortality was once again observed at the site, and carcasses were collected and stored frozen. A total of 28 bats from the 2020 event and nine bats from the 2021 event were sent to the U.S. Geological Survey–National Wildlife Health Center (NWHC) for diagnostic evaluation. A subset of these bats was subjected to various laboratory analyses.

In 2024, a separate mortality event in bats occurred at a site near Richland in Benton County, Washington. Carcasses were found in and around outbuildings on the site. Dead bats were first noted in early January, followed by a period of approximately 1 mo in which no carcasses were found. Mortality recurred starting on 6 February, with multiple dead bats being found each week through early March 2024. Carcasses were then found sporadically until late May 2024. Bats do not typically arrive to the site until April (Hayes and Wiles 2013). Based on historical data, relatively typical temperatures were reported for the area during the time period in which the bats were found (National Weather Service 2024). The site is known to harbor a large colony (approximately 4,100) of Yuma myotis (Hayes and Wiles 2013), but the carcasses could not be definitively identified to species based on morphological characteristics. A total of 43 dead bats was collected by local contract biologists; eight carcasses were stored frozen and sent to NWHC for diagnostic evaluation. We had detected Pseudogymnoascus destructans on live bats (without associated mortality) at the site in spring 2023, but we had not confirmed WNS at that time.

To determine the bat species involved in the mortality events, we extracted DNA from the wing skin of 36 and eight of the bat carcasses from King and Benton counties, respectively, and we amplified and sequenced a portion of the mitochondrial cytochrome-b (cytb) gene as described previously (Lorch et al. 2024a). We examined 19 (King County) and eight (Benton County) bats grossly under ultraviolet (UV) light for the presence of characteristic yellow-orange fluorescence on the patagia, which is often seen on bats with WNS (Turner et al. 2014); we also swabbed the muzzle and wings of these same bats to screen them for the presence of Pd using a quantitative PCR (qPCR) assay specific for the pathogen (Muller et al. 2013). We examined 18/19 bats from King County and 3/8 bats from Benton County internally for the presence of white fat stores and to identify possible signs of trauma. We collected internal organs from eight bats from King County for histopathologic examination; the post-mortem condition of the eight carcasses from Benton County was too poor for assessment of the internal organs. We collected wing, muzzle, and ear skin from 11/19 (King County) and all eight (Benton County) bats to look for microscopic lesions characteristic of WNS and determine the severity of infection using a previously described scoring system (Meteyer et al. 2009; Reeder et al. 2012).

Skin swabs from all bats (100%) were positive for the presence of Pd by qPCR; 18/19 (95%) and 8/8 (100%) bats from King and Benton Counties, respectively, exhibited yellow-orange fluorescence on the patagia. The area of the patagia that fluoresced ranged from a few scattered foci in some bats to extensively covering most of the membrane in others (Fig. 1). The patagia of the one bat for which no fluorescence was noted was extremely autolyzed, which may have affected the UV screening method. Lesions characteristic of WNS (i.e., cupping erosions) were present in all bats for which skin was examined microscopically, and severity scores ranged from 2 to 4 (on a scale of 0–4, where 0=no evidence of WNS and 4=severe wing membrane damage due to WNS; Reeder et al. 2012; Table 1). The combination of histopathologic lesions and Pd detection confirmed a WNS diagnosis in all examined bats.

Bats for which an internal examination was performed had no grossly visible subcutaneous fat stores; this has been a common finding in bats that have died of WNS (Meteyer et al. 2009). Gross lesions were not observed on the internal organs of bats from King County, except that 6/8 (75%) had reddened lungs. However, microscopic lesions in the lungs were not appreciated in histopathology. Similarly, no microscopic lesions were seen in other internal organs that could be attributed to the cause of death. Most bats had parasites in the gastrointestinal tract, but these were considered incidental findings.

Our findings were consistent with WNS as the cause of death. However, challenges in interpreting histopathology due to autolysis of some internal organs meant that other concomitant etiologies could not be completely ruled out.

Most of the bats from the King County mortality events (15/19, 79%) were female. The bats from Benton County were not sexed. All bats from the King County site that were subjected to species identification by DNA barcoding had 100% nucleotide identity across the 733-base-pair portion of the cytb gene that was analyzed. One bat from the Benton County site shared 100% sequence identity with the King County specimens. Sequences from the remaining seven bats from Benton County were identical to one another but differed by a single nucleotide polymorphism from the other specimens. The sequences were most similar to Yuma myotis data deposited in GenBank (93.2–100% identity) and did not closely match sequence data for little brown bats (84.3–86.8% identity). Representative sequences were deposited in GenBank for each of the two genotypes under accession numbers PP806138 and PP806139.

Our investigations demonstrated that WNS was capable of causing host behavioral changes (i.e., early egress from winter hibernacula) and mortality events in Washington, US, similarly to what has been observed in the eastern US (Blehert et al. 2009; Foley et al. 2011). Life history aspects of bat populations in coastal areas of the Pacific Northwest region of the US differ from those in other portions of the northern US in ways that might be expected to affect WNS-related mortality. Notably, bats in the former region generally hibernate in smaller congregations (Perkins et al. 1990). These bats also experience milder aboveground winter temperatures, which facilitate winter activity for some bat species (Falxa 2007), although Yuma myotis were not reported as being active in winter by Falxa (2007). These factors probably affect pathogen exposure, disease transmission, and pathogen growth (i.e., pathogen burden) on the host. Despite this, Pd does appear capable of causing mortality events in Washington, US.

We did not observe evidence that the bats we examined succumbed to a disease process other than WNS. We cannot rule out other factors (e.g., hypothermia or starvation) as the proximate cause of death; however, WNS was probably the ultimate cause of mortality in these cases. The degree of UV fluorescence of the skin and WNS severity scores assessed microscopically indicated that the Pd infections were extensive and comparable to those that have been observed in other bats known to have died of the disease (Reeder et al. 2012).

Since Pd was first detected in western North America in 2016 (Lorch et al. 2016), little has been published on the impacts of WNS on western bat populations. Differences in winter ecology have made population monitoring efforts more difficult in the western than in the eastern US. Although bats that presumed to have died of WNS in Washington state have been found as individual carcasses scattered across the landscape (Blejwas et al. 2023), our results show that when bats are concentrated, mortality events due to WNS can occur in the Pacific Northwest. Furthermore, our findings indicate that Yuma myotis may be a species that is highly susceptible to WNS and at potential risk of population declines from the disease. More thorough monitoring of bat populations using methods such as summer roost counts and acoustic surveys could be used to better assess the impacts that WNS is having on bat species in this region.

We thank Chris Anderson and Mike Smith (Washington Department of Fish and Wildlife), Alex May (Seattle Public Utilities), and Justin Wilde and Cole Lindsey (Department of Energy contractors for Hanford Mission Integration Solutions) for reporting mortality events and collecting carcasses for diagnostic evaluation. We also thank staff at the US Geological Survey–National Wildlife Health Center, especially Emily Banks, Ariel Leon, Dominic Marr, and Tyler McLaughlin, for assistance with necropsy and sample collection, and Maximillian Reynolds and Magdalena Twarowski for conducting molecular analyses. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government.

Additional metadata associated with this study are available at https://doi.org/10.5066/P1J8VFWZ (Lorch et al. 2024b).