Our findings demonstrate a considerably broader geographic range for O. cervipedis, extending into subarctic western North America, at least to latitudes of 66°N. Together with anecdotal reports of Onchocerca sp. in moose from the Yukon Territory (P. Merchant, Pers. Comm.), our results suggest a continuous distribution of O. cervipedis in the northern boreal forest regions of western North America. Additionally, we provide two new host records for O. cervipedis: the Yukon-Alaska moose (A. a. gigas) and Grant’s caribou (R. t. granti), supporting contentions that this parasite is a host generalist [1–11]. Migratory behaviour of some of the host species (e.g., Mulchatna caribou and Alaskan moose), may facilitate range expansion across sometimes vast distances. Due to the opportunistic nature of our sampling and the small sample size we could not evaluate age or sex predilections. In general, adult ungulates are reported to be more frequently infected ; however, calves and fawns are rarely harvested and this may have biased previous conclusions.
The impact of O. cervipedis in moose and caribou remain unknown. Clinical disease attributed to O. cervipedis in deer is most commonly confined to distal limbs and is associated with pain, loss of digits and hoof, swelling, and ulceration with female nematodes protruding through the lesions [8, 9, 33]. Such pathology would certainly affect animal mobility, and interference in normal activities may make clinically affected animals easier prey to both predators and hunters . In contrast, we found O. cervipedis to be fairly common in moose and its occurrence may be characterized as an incidental finding with minimal pathology in ‘healthy’ moose harvested for subsistence at higher boreal latitudes of North America. The nodules in moose and the caribou were typically found along the metatarsus or metacarpus, and caused a mild to moderate local inflammatory reaction. In these hosts, parasite localization may also affect locomotion but the distal limbs and joints are not affected. There are, however, a few reports of clinical disease, with Onchocerca sp. associated with open sores on the legs of moose in the Yukon (Philip Merchant, Pers. Comm.). In general, detection of sick animals in the wild may be difficult. Heavily parasitized animals with reduced mobility may be rapidly eliminated by wolf and bear predation, and therefore, the significance and extent of O. cervipedis in northern Canada and Alaska is difficult to establish.
Related Onchocerca species
Until recently, O. cervipedis was thought to be the only Onchocerca species parasitizing native cervids of North America. Molecular characterization of Onchocerca microfilariae from subcutaneous tissues of white-tailed deer in Northeastern USA revealed that at least one other species is present in native North American cervids . In contrast, five Onchocerca species have been described infecting wild cervids in the Palearctic . This is consistent with a general pattern of greater nematode species diversity in ungulates of Eurasia. This pattern is related to the historical expansion and geographic colonization of ungulate hosts and parasites, including filarioids, from Eurasia into North America over the past 2–3 Myr [e.g., . However, recent studies in North America further suggest that the biodiversity of Onchocerca species from the Nearctic has been underestimated, and deserves more rigorous investigation combining comparative morphological and molecular approaches.
Regarding the broader diversity for species of Onchocerca across the Holarctic, O. cervipedis is reported from Sakha and the Altai region of Russia [see , although conspecificity of Eurasian and North American parasites remains to be determined. Among Palearctic species in cervids, Onchocerca alcis has been described from tendon insertions of the tibia in European moose, and seems to be most closely related to Onchocerca jakutensis (= O. tubingensis), a red deer (Cervus elaphus) parasite . Other species that infect red deer are O. skrjabini, Onchocerca flexuosa, and Onchocerca garmsi. Onchocerca skrjabini is commonly found in reindeer in Sweden. It does not form nodules but is found free in tissues surrounding the tendons of tibio-tarsal and radio-carpal joints, and also in other parts of the body, including muzzle and shoulders . Similarly, O. garmsi does not form nodules and it can be found free in subcutaneous tissues of the sternal region. Onchocerca flexuosa is found in subcutaneous nodules on the back, chest, and abdomen of red deer . The nodular lesions caused by the Nearctic O. cervipedis in moose and reindeer seem to be more similar to those caused by O. flexuosa. Onchocerca flexuosa, however, has also been associated with necrotic foci in the liver, and to a lesser extent in kidney, myocardium, and other tissues in slaughtered Swedish reindeer. Histologically, these foci or granulomas contain larval or adult nematodes, and culture revealed Corynebacterium spp., thus constituting a relevant meat hygiene issue and resulting in discard of affected organs see . A more systemic distribution of O. cervipedis has not been observed nor has the potential for secondary bacterial infection been investigated. If found this may be of food safety relevance for subsistence and other hunters.
Parasite and vector ecology
The only Simuliidae known to be involved in the transmission of O. cervipedis to moose in northern Alberta were Simulium decorum and Simulium venustum. Both S. decorum/noelleri (as they are morphologically indistinguishable) and species within the S. venustum complex are present in the Mackenzie Mountains, NT [37, 38]. Other species that feed on moose and other large ungulates, such as those in the Simulium arcticum complex and Simulium vittatum/tribulatum, also occur in the study area in the NT (D. Currie, unpublished data), but their competency as vectors for O. cervipedis is unknown. The above mentioned species and/or species-complex are also widely distributed in Alaska, and the Yukon [39, 40]. In California, Prosimulium impostor is thought to be the only dipteran vector responsible for transmission of O. cervipedis to Columbian black-tailed deer . Although P. impostor is not reported in our study area, a variety of other species within the genus Prosimulium are present [37, 38].
Filarioid nematodes may be particularly sensitive to climatic changes both through effects on vector abundance and parasite development. In Finland, severe disease outbreaks in reindeer caused by S. tundra have been linked to episodes of unusually warm climatic conditions [15, 16, 19]. Under current climate warming scenarios for northern Canada and Alaska  we might anticipate substantial changes in the ecology, distribution, and abundance and impacts of O. cervipedis, and perhaps other filarioids, in northern ungulates [e.g., [20, 42]. Climate-facilitated range expansion of the parasite may become of particular importance for naive migratory tundra caribou populations. To date, O. cervipedis has not been reported in these caribou, and was not found by our group despite examination of metatarsals from over 500 barrenground caribou (R. t. groenlandicus) across North America between 2007 and 2011 . Sitka black-tailed deer (O. hemionus sitkensis), potentially sympatric with some moose populations in Alaska, may also serve as suitable definitive hosts and should be further investigated.
Some species of Onchocerca parasites of wild cervids are zoonotic. For instance, Onchocerca jakutensis, infecting red deer (Cervus elaphus) in Europe, has been the cause of nodular dermatological disease in humans . This raises the possibility of zoonotic potential in other Onchocerca species associated to wild ungulates, such as O. cervipedis.