Sores of boreal moose reveal a previously unknown genetic lineage of parasitic nematode within the genus Onchocerca

Abstract

Long-standing reports of open sores on the hind legs of moose (Alces alces) have been recorded in Alaska (as well as Canada, Europe, and Michigan), eliciting concerns about causes and infection. We used histological and genomic methods to investigate the sores from 20 adult moose on the Kenai Peninsula, Alaska. We paired this with thermal imagery and molt scoring of adult moose to further describe sore formation and understand its timing. Severe, ulcerative and eosinophilic dermatitis was found in all moose with sores present, and microfilariae within intraepidermal pustules were additionally found in four samples. Genetic analysis of sores from moose revealed a previously unknown genetic lineage of Onchocerca. Adult moose molt and lose their barrier of protection against flies in June and July during peak fly activity, leaving them vulnerable and allowing the development of sores. In summary, our results indicate that the cause for the sores on the hindleg of moose is a previously unknown genetic lineage of Onchocerca, probably transmitted by black flies, in timing with the molt cycle of adult moose. These sores leave moose exposed to pathogens, making them vulnerable, and challenging their health and fitness.

Trapped between food, heat, and insects: Movement of moose (Alces alces) and exposure to flies in the boreal forest of Alaska

“…The costs of tolerating flies may extend into winter because flies can expose moose to parasites. We recently described the effect of filarial nematodes on moose in the Kenai Peninsula, Alaska (Benedict et al., 2024 ; Benedict, Barboza, et al., 2023 ; Benedict, Thompson, et al., 2023 ). Legworm ( Onchocerca sp.…”

“…Legworm ( Onchocerca sp. ), likely transmitted by black flies, likely causes the open sores on the legs of adult moose and led to a decrease in serum protein (Benedict et al., 2024 ; Benedict, Barboza, et al., 2023 ), while Setaria yehi , likely transmitted by mosquitoes, led to morbidity and mortality in calves (Benedict, Thompson, et al., 2023 ). Both mosquitoes and black flies are tolerated by moose and make up the majority of flies in the boreal forests of the Kenai Peninsula, creating repeated spring and summer exposure.…”

“…We deployed Vertex plus‐4 GPS collars with 15 min fixes (Vectronics, Berlin, Germany) on moose during May 2021 while animals were immobilized for other research (Benedict, Barboza, et al., 2023 ). Collars had a 99.99% success rate, erroneous fixes were disregarded.…”

“…During the summer, adult moose shed their winter coat for thermoregulation; concurrently fly abundances increase, allowing biting flies to penetrate their thin coat (Benedict, Barboza, et al., 2023 ). As molt occurs, round sores with severe eosinophilic and ulcerative dermatitis have been seen to progressively appear on the hind legs above the tibio‐tarsal joint, on moose in North America (Benedict, Barboza, et al., 2023 ; Lankester & Samuel, 2007 ; Murie, 1934 ). The sores are likely caused by legworm ( Onchocerca sp.…”

“…The sores are likely caused by legworm ( Onchocerca sp. ), carried by black flies (Diptera: Simuliidae) (Benedict, Barboza, et al., 2023 ). The sores leave the moose exposed to secondary infections and the cost of tissue repair and immune response.…”