In bats, the wing area takes up to 85% of the total body surface area (Reeder and Cowles 1951) and is essential for their ability to fly and for important physiological functions like thermo- and osmoregulation (Kluger and Heath 1970, Bakken and Kunz 1988, Bassett and Studier 1988, Thomson and Speakman 1999). The wing consists of two layers of epithelium, separated by a thin layer of blood and lymphatic vessels, delicate nerves, muscles and specialised connective tissues (Quay 1970, Makanya and Mortola 2007). Visual assessment of the health of the wing membrane has been a useful tool for bat biologists (Reichard and Kunz 2009, Francl et al. 2011), particularly since the discovery of the white-nose disease caused by the lethal dermato-pathogen Pseudogymnoascus destructans (Lorch et al. 2011, Warnecke et al. 2013, Wibbelt et al. 2013) that affects the skin and the underlying connective tissue of bats during hibernation. Even with an unaided eye, the skin can be examined for the presence of pathogens and for abnormalities in the colouration, elasticity and obvious physical damage. Reichard and Kunz (2009) described the wing damage index (WDI) which is based on the degree of wing damage including scarring and associated lack of pigmentation, holes, tears and necrotic tissue. We used this approach for two Bulgarian bat populations in order to investigate the presence of strange-looking cutaneous lesions on the wings of Myotis myotis (Borkhausen, 1797) and Myotis blythii (Tomes, 1857). These were proven to be caused by skin mites, Psorergatoides kerivoulae Fain, 1959. The aim of our study was to describe the histopathology of the lesions caused by this parasite and present new data on its distribution.

Sample collection

Sample processing

Over the course of the study, we collected information on the wing condition of 450 bat individuals from two study sites. We identified 16 individuals with lesions prior to the hibernation period, no individuals with signs of infection immediately after the hibernation period and three individuals with lesions one month after the beginning of the active season (end of May 2022). The shape of the lesions was mainly spherical and in some individuals (n = 10), there were multiple lesions on the same wing. The diameter of the lesions varied from 2 mm to 15 mm. Some of the severe cases were with multiple lesions on both wings covering up to 20% of the total wing surface area. Visually the lesions lack elasticity and resemble crumpled paper-like tissue with dark-orange colouration (Fig. 1). This makes it easy for field identification. We observed visual signs of infestation only on the wings of the study animals.

Figure 1.  

Wings of Myotis myotis infested with Psorergatoides kerivoulae. A Lesions manifested as pale-coloured spots on the wing membrane; B Dense infestation of mites was observed in the tissue samples. The darker ovoids are the holes remaining after the mites extraction. Scale bar 1 mm.

Figure 2.  

Psorergatoides kerivoulae from a wing of Myotis myotis. A, B females; C deutonymph; D female, dorsum of idiosoma (phase contrast). Scale bar 100 μm, all to scale.

The microscopic examination of the bat wing sections with normal tissue morphology and with lesions showed the presence of parasitic mites in the latter. There are significant differences in histological characteristics between normal bat skin and the lesions. The bat wing membrane sections with usual morphology were covered with a single layer of cuboidal epithelium, with a focal intracellular brown pigment deposit. The subepithelial tissue was loose, with thin-walled vessels and some striated muscle fibres. In contrast, the degree of damage in some individuals suggested functional impairment of the derma. In the lesions’ sections, a focal transformation to squamous epithelium with superficial keratinisation was observed, around multiple ovoid vesicular structures intradermally, morphologically consistent with parasitic mites. Around the mites’ structures, there was an intense inflammatory infiltrate – lymphocytes, plasmocytes and some eosinophils and neutrophils. There were no muscle fibres observed (Fig. 3B-D).

Figure 3.  

Histological sections of wing samples from Myotis blythii b. A Intact area; B Infested area with two Psorergatoides kerivoulae cross-sectioned; C, D Sections through acari-induced lesions with lymphoplasmacytic infiltrate. H&E staining. Scale bars: 50 μm (A, B), 100 μm (C, D).

The combination of the next features showed that the parasitic mites belong to the genus Psorergatoides Fain, 1959, the species Psorergatoides kerivoulae Fain, 1959: four pairs of strongly reduced dorsolateral shield setae, considered apomorphic of the genus (Fain 1959a, Fain 1959b, Nelson et al. 2017), two setae on femora I–III, only one seta on femur IV, length and width of dorsum ca. 120 μm, length of terminal setae more than 50 μm (Fig. 2).

Out of fifty specimens of Psorergatoides kerivoulae studied, both females (Figs. 2 A, B andD) and deutonymph (Fig. 2 C) were found. The density of the mite population was estimated to be 8 individuals/mm². The mites were located very close to each other (Fig. 1B).

Skin mites are common in mammalian populations (Izdebska and Rolbiecki 2020) and over 20 families of various parasitic mites are associated with bats (Giesen 1990, Izdebska and Rolbiecki 2020). Representatives of the family Psorergatidae are small mono- or oligoxenic skin parasites and may cause skin lesions in the host (Giesen 1990, Izdebska and Krawczyk 2012, Nelson et al. 2017). Only a few literature sources are available about the clinical symptoms associated with mite infestation (Nelson et al. 2017). Although Psorergatidae seems to be common in the host populations (Izdebska and Fryderyk 2012), they are understudied because of their small size and practically asymptomatic presence in the skin in most cases (Izdebska and Fryderyk 2012, Nelson et al. 2017). As a whole, only two species are published for the fauna of the Balkan Peninsula and south-eastern Europe: Psorergates apodemi Fain, Lukoschus & Hallmann, 1966 and P. muricola Fain, 1961 (Beron 2021). Our observation of Psorergatoides kerivoulae is the southernmost record for the species in Europe and the first mention of the genus Psorergatoides in the Balkans. Psorergatoides kerivoulae has an exceptionally wide geographic range and so far, it has been reported from Belgium, DR Congo, Ivory Coast, Malaysia (Borneo), Poland and Australia (Nelson et al. 2017, Beron 2021, Cierocka et al. 2022). The parasite has been associated with the following bat species: Kerivoula cuprosa Thomas, 1912, K. lanosa (Smith, 1847), Myotis bocagei (Peters, 1870), M. macropus (Gould, 1854), M. muricola (Gray, 1846), M. myotis (Borkhausen, 1797), M. mystacinus (Kuhl, 1817), Plecotus auritus (Linnaeus, 1758) (Chiroptera: Vespertilionidae) (Haitlinger 1979, Giesen 1990, Izdebska and Fryderyk 2012, Izdebska and Krawczyk 2012, Nelson et al. 2017, Beron 2021, Cierocka et al. 2022). The presence of mites in the skin lesions of the lesser mouse-eared bats (Myotis blythii) could be considered a new host-parasite association. The mechanisms and transmission pathways of Psorergatidae, usually show a narrow range of host specificity; thus the taxonomic status is questionable and undoubtedly will require future clarification.

The genus Psorergatoides infests Chiroptera hosts and often induces skin lesions and parasitosis which can rarely be lethal. The observed lesions in our studied individuals were relatively large. Some individuals were with multiple lesions with a diameter of around 10 mm. In comparison to other studies, the concentration of mites in our samples was higher (see Baker 2005). We observed infected individuals before hibernation (n = 16), no infected individuals immediately after the end of hibernation and three infected individuals one month after the beginning of the active season. There were evident infectious processes in those individuals and, to our knowledge, our study is the first to describe the histopathology of the infection. One explanation why we were not detecting any signs of mites infestation in the animals sampled directly after the end of hibernation can be related to the lifecycle of the parasites. Similarly to other bat skin mites, they might have reduced reproductive activity during the winter period due to the inactivity of the host and the low temperatures (Lourenço and Palmeirim 2008).

Our study sites are important bat hibernacula with records of the presence of the causative agent of the white-nose disease. A large fraction of the bats from the studied colonies exhibits visual signs of infection with Pseudogymnoascus destructans (Zhelyazkova et al. 2020). Indeed, the skin mites infestation may be an additional stress factor for those populations and more research and conservation efforts are needed. Future research on the way hibernation affects the presence of these skin mites and how the infection affects the activity patterns during the winter season is needed. Another question open for discussion is whether the lesions lead to dehydration and disturbed osmoregulation, both crucial for the survival of the animals. New host-parasite association combined with wounded and damaged skin significantly increases the chance of other infections even with opportunistic pathogens (Robinson et al. 2019). In bats, a wide variety of commensal bacterial and fungal species are known to be enteropathogens and have the potential to cause opportunistic infections (Mühldorfer 2013).

Our study supports the validity of wing damage scoring as a cost-effective way to look at the general health of bats and as a useful tool for discovering pathogens. Damage to the wings does affect the foraging success of bats. Bats suffering from moderate wing damage are less manoeuvrable and have smaller foraging success, which indirectly leads to increased metabolic rate and can cause insufficient body fat storing (Voigt 2013). Lastly, some studies consider the connection between wing morphology and damage and the risk of extinction in bats (Jones et al. 2003, Safi and Kerth 2004). This further emphasised the need for more research efforts related to wing membrane diseases, damage causes and risk factors in bats.

Funding for this study (especially for the laboratory work) was provided by the Bulgarian National Science Fund (project CP-06-N51/9 “Caves as a reservoir for novel and reoccurring zoonoses - ecological monitoring and metagenomic analysis in real-time”). We also would like to acknowledge funding from the EU Framework Horizon 2020 through the COST Action CA18107 ‘Climate change and bats: from science to conservation – ClimBats’ (https://climbats.eu/). Nia Toshkova was funded by the Bulgarian National Science Fund (CP-06-COST/15 from 16.12.2020) and а PhD Fellowship from Karoll Knowledge Foundation. Violeta Zhelyazkova was funded by supported by the Bulgarian Ministry of Education and Science under the National Research Programme “Young scientists and postdoctoral students” (DCM 577/ 17.08.2018). We would also like to acknowledge the volunteers who helped us in the field: Boris Petrov, Petar Velkov, Katrin Dimitrova and Kristin Meshinska; and Alexander Marinov for improving the English of this paper.

The authors have no conflicts of interest to declare. All co-authors have seen and agree with the contents of the manuscript and there is no financial interest to report. We certify that the submission is original work and is not under review at any other publication.