The skin I live in: Pathogenesis of white-nose syndrome of bats

Abstract

The emergence of white-nose syndrome (WNS) in North America has resulted in mass mortalities of hibernating bats and total extirpation of local populations. The need to mitigate this disease has stirred a significant body of research to understand its pathogenesis. Pseudogymnoascus destructans, the causative agent of WNS, is a psychrophilic (cold-loving) fungus that resides within the class Leotiomycetes, which contains mainly plant pathogens and is unrelated to other consequential pathogens of animals. In this review, we revisit the unique biology of hibernating bats and P. destructans and provide an updated analysis of the stages and mechanisms of WNS progression. The extreme life history of hibernating bats, the psychrophilic nature of P. destructans, and its evolutionary distance from other well-characterized animal-infecting fungi translate into unique host-pathogen interactions, many of them yet to be discovered.

Fig 1. Distribution and impact of WNS on North American bats.

(A) Map of distribution of WNS in North America (2006–2024). Conf., WNS confirmed, Susp., WNS suspect; Pd+, Pseudogymnoascus destructans detected; Pd?, P. destructans suspect [126]. (B) Little brown bat (Myotis lucifugus) with characteristic macroscopic WNS lesions consisting of white fuzzy-flaky material (i.e., P. destructans hyphae) on the skin of the wing, pinnae, and around the nose. Photo by Heather Kaarakka, Wisconsin DNR. (C) WNS status based on histopathology and quantitative PCR detection of P. destructans [79,127], conservation status (ESA, SARA, IUCN), and population trends (IUCN) of continental bat species of the eastern USA and Canada. ESA, US Endangered Species Act [128]; SARA, Canada Species at Risk Act [129]; IUCN, The International Union for Conservation of Nature Red List of Threatened Species [130]; EN, endangered; VU, vulnerable; NT, near threatened; LC, least concern.

Fig 2. Phylogeny and life cycle of Pseudogymnoascus destructans in hibernacula.

(A) Phylogenetic tree showing the major taxonomic groups with representative taxa of pathogenic ascomycete fungi. Plant pathogens are denoted by blue dots, while animal (including insect) pathogens are marked with orange dots. Note that P. destructans (marked with a bat icon) resides within the class Leotiomycetes that otherwise contains mostly plant pathogens and is not closely related to other consequential pathogens of humans and animals (e.g., Onygenales, Candida spp.). Support values from a maximum likelihood analysis are presented for each major group and the tree is rooted with representative pathogenic fungi from the division Basidiomycota; taxa included in the tree are based on Berbee with some modifications [33]. (B) P. destructans’ life cycle in hibernacula. Transmission of conidia (dormant form of the fungus) from the hibernacula environment to bats occurs while bats are active (euthermia). Conidia germinate and colonize the skin of torpid bats. Subsequent hyphal conidiation leads to shedding of conidia into the environment. Conidia remain viable in the hibernacula environment until the following winter, which perpetuates the cycle of transmission. Figure designed using BioRender (Agreement # BH271RUAM8).

Fig 3. Proposed model of WNS pathogenesis.

Our proposed model of WNS pathogenesis is divided into the following phases: (1) Noninvasive colonization involves superficial colonization of the epidermis (stratum corneum) without recruitment of inflammatory cells regardless of the hibernation phase (torpor, arousal, or emergence). (2) Early invasion consists of non-damaging entry into epidermal keratinocytes (deeper epidermis). Infected keratinocytes release chemokines during the euthermic periods (arousal or emergence) that might lead to recruitment of small numbers of phagocytes into the site of infection. Limited antimicrobial activity by local and newly recruited immune cells is expected at this stage given the intracellular location of P. destructans and the absence of cell damage signaling (e.g., alarmins). (3) Late invasion is characterized by increased P. destructans burden in the epidermis that leads to cell damage and replacement by biofilm-like matrix-embedded fungal clusters (cupping lesions) and up-regulation, during euthermia, of alarmins, chemokines and cytokines known to promote Th17 antifungal responses. The short duration of arousals likely precludes further recruitment of myeloid and memory T cells, which could partially explain the lack of IL17A up-regulation detected in these late invasion sites in aroused bats. Memory Th17 cells are likely recruited after a longer euthermic period (emergence) leading to a more robust antifungal inflammatory response that might lead to IRIS, thrombosis and ischemic necrosis of infected tissue. (4) Clearance and resolution happen if bats survive the hibernation period and the IRIS by eliminating the fungus and repairing the damaged tissue. Figure designed using BioRender (Agreement # XE271RUHMM).

Fig 4. Pathogenic features of P. destructans’ skin invasion.

During early invasion, P. destructans hyphae enter bat keratinocytes by EGFR-mediated endocytosis or active penetration at euthermic- (37°C) or torpor-like temperatures (12°C), respectively, while P. destructans conidia enter bat keratinocytes by EGFR-mediated endocytosis at euthermic-like temperatures. Invading P. destructans blocks apoptosis of infected keratinocytes and 1,8-dihydroxynaphthalene (DHN) melanin coating on conidia inhibits acidification and maturation of fungal containing endosomes favoring intracellular survival. During late invasion, tightly clustered pleomorphic hyphae (h) embedded in electrodense biofilm-like matrix (black arrowheads) replace dead keratinocytes (white arrowheads) forming cupping lesions. “Late invasion” transmission electron microscopy picture taken from the skin of a WNS–positive Myotis lucifugus previously sampled in Isidoro-Ayza and Klein [59]. Notice that superficial hyphae are narrower than those in deeper layers of the skin. Figure designed using BioRender (Agreement # MO271RUM95).