Bear Filarial Worm Zoonotic
Causative Agent
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Bear Filiarial Worm
  • Bear filarial worms have been reported in black bears (Ursus americanus) in both Canada and the United States.
  • Throughout the year.
Hosts, Transmission and Life Cycle
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Bear Filarial Worm Life Cycle 
During a blood meal, an infected black fly (genus Simulium) introduces third-stage filarial larvae of Dirofilaria ursi onto the skin of the ursine definitive host (although humans may also serve as hosts), where they penetrate into the bite wound . In subcutaneous tissues, the larvae develop into adults, which commonly reside in subcutaneous connective tissues . Adult females are usually 115-225 mm long by 0.46-0.70 mm wide; males are usually 50-90 mm long by 0.33-0.48 mm wide. Adults can live for 5 - 10 years. In the subcutaneous tissues, the female worms are capable of producing microfilariae over their lifespan. The microfilariae are found in peripheral blood . A black fly ingests the microfilariae during a blood meal . After ingestion, the microfilariae migrate from the black fly's midgut through the hemocoel (cavities containing blood) to tubules in the lower regions of the gut (Malpighian tubules) . There the microfilariae develop into first-stage larvae  and subsequently to third-stage infective larvae . The third-stage infective larvae migrate to the black fly's mouthparts (proboscis)  and can infect another definitive host when the fly takes a blood meal . Humans are not common hosts, but may become infected after being fed upon by infected black flies .
Transmission and Life Cycle:
  • See above figure from the US Centers for Disease Control for a complete description of the life cycle of D. ursi.
  • 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.
  • 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 black fly.
  • Following a 2-week period within the black fly, larvae become infective to bears. Larvae enter the new host as the fly begins taking a blood meal.
  • Larvae migrate to preferred locations within bears where they mature and eventually mate.
  • A 7-month period of time is required for the female worms to produce microfilaria and complete the life cycle.
Signs and Symptoms
  • Adult D. ursi worms are white and slender and can range in size from 5 to 22 cm long.
  • D. ursi does not appear to cause disease in bears or other species.
Meat Edible?
  • Infection with D. ursi does not affect the quality of the meat.
Human Health Concerns and Risk Reduction
  • There are no reports of any adverse reactions in humans bitten by black flies containing microfilaria.
  • While rare, infection with D. ursi has resulted in the formation of small, subcutaneous nodules, in humans.
Samples for Diagnosis
  • D. ursi infections can be diagnosed either by examining blood smears for microfilariae or by finding the adult worms in preferred locations beneath the skin or surrounding internal organs of bears.
  • 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
Return to Manual Home Page Disease List - Body Region Affected Disease List - Causative Agent or Risk Factor Disease Surveillance Form Glossary Contact Information
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