Date Published: August 2, 2017
Publisher: Public Library of Science
Author(s): Jiri Pikula, Sybill K. Amelon, Hana Bandouchova, Tomáš Bartonička, Hana Berkova, Jiri Brichta, Sarah Hooper, Tomasz Kokurewicz, Miroslav Kolarik, Bernd Köllner, Veronika Kovacova, Petr Linhart, Vladimir Piacek, Gregory G. Turner, Jan Zukal, Natália Martínková, Sharon Swartz.
While white-nose syndrome (WNS) has decimated hibernating bat populations in the Nearctic, species from the Palearctic appear to cope better with the fungal skin infection causing WNS. This has encouraged multiple hypotheses on the mechanisms leading to differential survival of species exposed to the same pathogen. To facilitate intercontinental comparisons, we proposed a novel pathogenesis-based grading scheme consistent with WNS diagnosis histopathology criteria. UV light-guided collection was used to obtain single biopsies from Nearctic and Palearctic bat wing membranes non-lethally. The proposed scheme scores eleven grades associated with WNS on histopathology. Given weights reflective of grade severity, the sum of findings from an individual results in weighted cumulative WNS pathology score. The probability of finding fungal skin colonisation and single, multiple or confluent cupping erosions increased with increase in Pseudogymnoascus destructans load. Increasing fungal load mimicked progression of skin infection from epidermal surface colonisation to deep dermal invasion. Similarly, the number of UV-fluorescent lesions increased with increasing weighted cumulative WNS pathology score, demonstrating congruence between WNS-associated tissue damage and extent of UV fluorescence. In a case report, we demonstrated that UV-fluorescence disappears within two weeks of euthermy. Change in fluorescence was coupled with a reduction in weighted cumulative WNS pathology score, whereby both methods lost diagnostic utility. While weighted cumulative WNS pathology scores were greater in the Nearctic than Palearctic, values for Nearctic bats were within the range of those for Palearctic species. Accumulation of wing damage probably influences mortality in affected bats, as demonstrated by a fatal case of Myotis daubentonii with natural WNS infection and healing in Myotis myotis. The proposed semi-quantitative pathology score provided good agreement between experienced raters, showing it to be a powerful and widely applicable tool for defining WNS severity.
Wildlife conservation medicine is currently being challenged by a number of infectious and non-infectious diseases  that can potentially induce mass mortality events . Recently, the health of temperate-region bats has been compromised by a generalist fungal agent that causes white-nose syndrome, Pseudogymnoascus destructans [3–7]. White-nose syndrome (WNS) emerged as a point-source epidemic. Its geographic spread since 2006 has been associated with a major decline in Nearctic bat populations [8–10]. On the other hand, Palearctic bat communities in Europe and Asia appear to tolerate hyperendemic exposure to this virulent pathogen .
The origin of Holarctic bat samples has been described previously [7,11,12]. Briefly, bats were sampled at 20 sites in the Czech Republic, Slovenia, Latvia, Poland, Russia and the USA. Additional Nearctic Myotis septentrionalis males were sampled in Hannibal, Missouri (39.70 N, 91.36 W; USA) from January to March 2015, two hibernation seasons after the first documentation of P. destructans infection.
Prevalence of each WNS pathology grade (Figs 1 and 2) induced by natural P. destructans skin infection in Palearctic and Nearctic bat species varied from 0 to 100% in different bat species (Table 1). Signs of fungal skin-surface colonisation not classified as WNS (in the absence of other lesions) were present in all Nearctic bats and in 89% of Palearctic bats. Prevalence of infection within hair follicles and associated glands was highly variable in all hibernating bat species and for species with multiple investigated individuals ranged from 17 to 67% for the hair follicle infection and from 15 to 67% for the sebaceous gland infection. While wing membrane infection progressed to multiple and/or confluent cupping erosions in most bat species, single cupping erosions only were observed in Miniopterus schreibersii (n = 1), M. bechsteinii (n = 3) and Rhinolophus euryale (n = 1). Fungal hyphae regularly breached the epidermal/dermal interface in M. lucifugus (100%, n = 10) and M. septentrionalis (86%, n = 7) as well as in Palearctic bats (46–100%). Full thickness invasion of the wing membrane occurred more frequently in Palearctic bats (34% on average) and M. septentrionalis (71%) than in M. lucifugus (30%). A high percentage of both Palearctic and Nearctic bats (73 and 76% on average, respectively) showed an inflammatory response to fungal invasion, though this mostly resulted in a considerably lower occurrence of fungal sequestration (25 and 22% on average, respectively; Table 1). While Eptesicus serotinus (n = 1), M. alcathoe (n = 7), Nyctalus noctula (n = 8), Pipistrellus pipistrellus (n = 2), Pipistrellus pygmaeus (n = 2) and Plecotus austriacus (n = 1) were also sampled for histopathology, all were confirmed negative for WNS. Among these, four M. alcathoe were confirmed positive for P. destructans infection using qPCR (S2 Table).
Fuelled by no reports of mass die-offs from Europe [36,37], the most erroneous conclusion concerning white-nose syndrome is the belief that P. destructans infection causes no harm to Palearctic bats. Pseudogymnoascus destructans infection is known to induce complex physiological and transcriptional effects in hibernating bats long before the onset of the clinical signs indicative of late-stage WNS [19,38]. Palearctic and Nearctic bats are exposed to similar fungal loads, resulting in equivalent focal skin-tissue invasiveness pathognomonic for WNS lesions [7,11–13,39,40]. It would be reasonable, therefore, to expect both morbidity effects (not always recognisable in the field) and mortality across the distribution range of P. destructans. Here, we show the full range of WNS skin pathology in Palearctic and Nearctic bats. Moreover, we document a case of P. destructans infection in a European M. daubentonii bat progressing to fatality due to extensive skin necrosis and infarction. The differential outcome of WNS in Eurasia and North America appears to be associated with some form of tolerance mechanism . Our data, however, suggests that Palearctic bat tolerance to WNS infection may be limited by severity of wing damage.