Bear Filarial Worm
Causative Agent
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Parasitic disease of bears caused by infection with the long, hair-like nematode or filarial worm, Dirofilaria ursi.
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| Ontario Ministry of Natural Resources photo |
Distribution
Geographic:
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Bear filarial worms have been recorded black bears in both Canada and the United States.
Seasonality:
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Throughout the year.
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Adult D. ursi worms tend to occur beneath the skin in the area of the neck and groin, and in the connective tissues around the aorta, kidneys, and rectum.
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Adult female worms produce motile larvae called microfilaria measuring 0.19 to 0.29 mm in length that enter the circulatory system of the bear where they remain until ingested by a blackfly.
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Following a 2-week period within the blackfly, larvae become infective to bears. Larvae enter the new host as the fly begins taking a blood meal.
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Larvae migrate to preferred locations within bears where they mature and eventually mate.
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A 7-month period of time is required for the female worms to produce microfilaria and complete the life cycle.
Signs and Symptoms
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Adult D. ursi worms are white and slender and can range in size from 5 to 22 cm long.
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D. ursi does not appear to cause disease in bears or other species.
Meat Edible?
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Infection with D. ursi does not affect the quality of the meat.
Human Health Concerns and Risk Reduction
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There are no reports of any adverse reactions in humans bitten by blackflies containing microfilaria.
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While rare, infection with D. ursi, resulting in the formation of small, subcutaneous nodules, has been reported in humans.
Samples for Diagnosis
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D. ursi infections can be diagnosed either by examining blood smears for microfilariae or by finding the adults in preferred locations beneath the skin or surrounding internal organs of bears.
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Because microfilaria of other types of filarid worms may be present, identifying the adult worms as D. ursi is the most accurate method of diagnosis.
Further Reading
Michigan Department of Natural Resources - bear filarial worm information
Ontario Ministry of Natural Resources - bear filarial worm information
Elkin, B, and R. L. Zamke. 2001. Common wildlife diseases and parasites in Alaska. Alaska Department of Fish and Game. Anchorage, AK.
