Fur lice in Arctic foxes in Svalbard: prevalence, intensity and impacts on health status and demographics

In November 2019, bloodsucking lice were identified in Arctic foxes in Svalbard for the first time, coinciding with similar findings in Nunavut, Canada. Genetic analysis revealed that lice from both regions were 100 % identical, suggesting the presence of a previously undescribed species distinct from dog lice. The origin of the lice remains uncertain, but historical evidence of lice in Canadian Arctic foxes from the 1990’s supports the hypothesis that they may have been introduced to Svalbard by migrating Arctic foxes. The prevalence of lice in Arctic foxes in Svalbard has increased since the 2019–2020 trapping season, initially affecting an estimated 10 % of the population and rising to 76 % in the 2021–2022 season. Morphological and molecular analyses, along with observations of abnormal fur loss patterns in infested foxes, highlight the impact of lice infestations. These species-specific ectoparasites, which are highly dependent on their hosts, cause pruritis and fur damage, and in severe cases, can lead to anaemia. The study aimed to collect all Arctic fox carcasses trapped during the 2022–2023 season to investigate the impact of fur lice by documenting the prevalence, abundance, and distribution of lice, changes in fur quality and skin pathology, and develop and establish methods and protocols for the evaluation of the intensity of lice infestation. The project also conducted a pilot study to test camera traps on Arctic fox den sites to provide a non-invasive method for monitoring lice infections. During the 2022–2023 trapping season, a total of 36 Arctic foxes were captured, with fur lice detected in 16 individuals, resulting in an overall prevalence of 41.7 %. This was a decline from the previous year’s prevalence of 76 % but remains higher than earlier seasons. Lice were mainly found in the neck and shoulder areas but were distributed across the entire body in heavily infested foxes. Heavier lice burdens in the pelts were related to lower body weights. Infestations were associated with significant fur damage, including discoloration and shortened fur, with infected foxes showing these changes more frequently than uninfected ones. The total lice burden ranged from 0 to 10 984 per pelt, with an average of 3 161. Egg density was higher than other life stages (eggs, larvae and adults), and the highest lice concentrations were found in the neck and shoulders. Histopathological analysis of the skin showed chronic irritation caused by lice, though there was no evidence of inflammation or immune response. Hair follicles appeared healthy and undamaged and hair loss was attributed to mechanical damage from scratching and biting, driven by tactile irritation rather than hypersensitivity. Camera traps at five breeding dens captured over 24 000 images, with 409 showing Arctic foxes. Signs of lice infestation were evident in 279 of these images, documenting infestation at all dens. Infestation severity developed throughout the season (from January–February) and peaked in late spring (May–June). Despite this, foxes with no sign of lice on the fur were also observed on the dens. The observed seasonal trend in louse infections is noteworthy. Although the trapped fox dataset did not show a clear correlation between infection intensity and trapping dates, the trapping ends in mid-March, well before the peak seasonal changes observed in May and June through camera trap evidence. Thus, prevalence and abundance estimates based on trapped foxes may underestimate the impact of lice on the population. Based on these findings and the literature we hypothesize that the recent introduction of fur lice to Arctic foxes in Svalbard, a previously naïve population, may lead to a gradual development of partial immunity. Over time, younger naive individuals or older foxes with weakened immune systems may become more susceptible to heavy infestations compared to healthy adults with prior exposure to lice. Immunity could also explain why some uninfected foxes coexist with infected individuals at the same breeding dens despite close physical contact. Another aspect related to the seasonal molt is the finding that seasonal shedding of winter fur in spring in other species significantly reduces louse infection intensity. Similarly, the seasonal molt of Arctic foxes from dense winter fur to summer fur in May probably plays a role in reducing louse prevalence and abundance. Hairs with attached eggs are shed during molting, decreasing the overall infection burden. So far, we have no information about fur lice in Arctic foxes during summer. Continued monitoring is recommended to better understand the dynamics of lice infestations, their health impacts, and potential development of immunity in the Arctic fox population.

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