Winter Tick (Dermacentor albipictus)–Associated Dermatitis in a Wild Elk (Cervus canadensis) in Pennsylvania, USA

On 28 March 2017, a yearling male elk (Cervus canadensis) was found dead in Clearfield County, Pennsylvania, US. On necropsy, the elk was in poor nutritional condition and had extensive alopecia throughout the ventral and lateral portions of the neck and the dorsal thorax associated with severe tick infestations. Skin in these areas had multiple areas of ulceration and was multifocally covered by thick, tan to red crusts (Fig. 1). Portions of skin and brain were collected for histologic examination, fixed in 10% neutral buffered formalin, and processed routinely. Samples were embedded in paraffin, sectioned at 5 µm, and stained using H&E. Histologically, the affected area of skin associated with the tick infestation had severe chronic-active dermatitis with hyperkeratosis and crust formation (Fig. 2). These areas were characterized by multifocal epidermal ulceration with serocellular crusts containing degenerate neutrophils and bacterial colonies; the subjacent dermis was infiltrated by neutrophils, lymphocytes, plasma cells, and eosinophils. Skin scrapes collected from the affected skin were examined cytologically, and no mites or eggs were identified. No other gross lesions were identified. No microscopic lesions were identified in the brain. Immunohistochemistry for chronic wasting disease was performed using previously described techniques, and protease-resistant prion protein was not detected in the tissues examined (Spraker et al. 2002). Briefly, sections of obex and retropharyngeal lymph nodes were fixed in 10% neutral buffered formalin and processed routinely. Samples were embedded in paraffin, sectioned at 5 µm onto positively charged glass slides, and stained with a monoclonal antiprion protein antibody.

A small subset of ticks was collected for 3–5 min from the ventral and lateral regions of the neck. Ticks were submitted to the Northeast Wildlife DNA Laboratory (East Stroudsburg, Pennsylvania, USA) and the Southeastern Cooperative Wildlife Disease Study (Athens, Georgia, USA) for morphologic identification. Morphologic characteristics were observed via microscopy to determine species, sex, and life stage. All 190 ticks examined were morphologically identified as adult Dermacentor albipictus (Keirans and Litwak 1989). Tick DNA was extracted, and a portion of the 16S ribosomal RNA gene was amplified using primers 16S–1 (5′-CCGGTCTGAACTCA-GATCAAGT) and 16S+2 (5′-TTGGGCAA-GAAGACCCTATGAA) as described previously (Gleim et al. 2014). The 16S ribosomal RNA sequence confirmed the tick identity as D. albipictus (GenBank no. AY375429). Of the tick individuals studied, 66.8% (127/190) were female, and 33.2% (63/ 190) were male. This was expected because adult winter ticks are most prevalent from mid-January to April (Addison and McLaughlin 1988). No other tick species were identified from sampled areas.

The wild elk in Pennsylvania was found dead with severe alopecia and dermatitis associated with D. albipictus. Presumably, the heavy tick infestation resulted in severe anemia and the mortality of this elk, as no other gross or microscopic findings were identified, and the clinical syndrome was consistent with a severe presentation of wintertick in moose (Alces alces; Wünschmann et al. 2015). Because it is a one-host tick, D. albipictus attaches to a host as a larva and then remains on that host for about 6 mo as it feeds and molts through each life stage, eventually mating as an adult and falling off to lay eggs in the environment. Infestations on moose during peak winter tick season can range from a few hundred to over 100,000 ticks per host (Samuel and Welch 1991). Severe infestations can lead to premature winter coat loss, anemia, weight loss, and death (Mooring and Samuel 1999). While less common, sporadic reports of similar clinical syndromes associated with winter tick have been reported in other cervid species, including elk in Wyoming (Samuel et al. 1991). A study on winter tick removal grooming by elk in Canada found the highest levels of elk hair loss occurred in March and April, with 55% of cows displaying loss of hair (Mooring and Samuel 1998). Winter tick infestations on elk have been noted in areas such as Kentucky, where elk are used as a stock population for reintroduction in other eastern states, creating the risk for the dissemination of winter ticks (Slabach et al. 2018). Dermacentor albipictus dissemination has already been suspected in both Oklahoma and Yukon, Canada, potentially caused by the relocation of infested elk (Patrick and Hair 1975; Leo et al. 2014). Furthermore, winter ticks have been found on white-tailed deer (Odocoileus virginianus) populations in areas where elk frequent, contributing to overall winter tick numbers and potentially affecting the risk for infestation on elk (Leo et al. 2014; Lockwood et al. 2018).

The seasonality of winter tick prominence during fall and winter was noted by Lockwood et al. (2018), who found higher numbers of D. albipictus on both white-tailed deer and elk in Kentucky compared to three other tick species. Similarly, Slabach et al. (2018) noted winter ticks as the most prevalent species on 263 elk in Kentucky. Environmental conditions favorable for high winter tick numbers include low snowfall, a relatively warm fall and early winter, and early snowmelt in late winter and early spring, all of which were present in Pennsylvania during this season (Wilton and Garner 1993). To our knowledge, there have been no other published reports of severe dermatitis and potential mortality associated with winter tick in eastern elk. In a study published by Williams et al. (2002), genetic analysis of a Pennsylvania elk herd was characterized by high levels of inbreeding, with low levels of unique and rare alleles present. The reduced biologic fitness due to inbreeding depression and a lack of genetic diversity makes the Pennsylvania elk population more susceptible to decline and possibly to pathogens. Winter tick infestations are known to have severe health implications, contributing to death in infested ungulates (Wünschmann et al. 2015). If incidences of severe winter tick infestations on Pennsylvania elk were to increase, it could potentially result in a population decline. Additionally, wildlife management administrators should account for possible winter tick infestations when considering the use of elk in translocation efforts. Recommended future studies should include monitoring winter tick infestations on elk, identifying the risk associated with population declines of Pennsylvania elk as a result, and examining habitat and land-use associated factors.