| Bear
Filarial Worm |
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| Causative Agent |
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Parasitic disease of bears (Ursidae) caused by infection with the long,
hair-like
nematode or
filarial worm, Dirofilaria ursi.
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| Images |
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Click on
image to enlarge. |
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| Distribution |
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Geographic: |
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Bear
filarial worms have been reported in black bears (Ursus
americanus) in both Canada and the United States.
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Seasonality: |
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| Hosts, Transmission and Life
Cycle |
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Click on image to enlarge. |
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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
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| Transmission and Life Cycle: |
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See above figure from the US Centers for Disease Control for a complete
description of the life cycle of
D. ursi.
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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 black fly.
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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.
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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.
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| 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.
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| Meat Edible? |
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Infection with D. ursi does not affect the quality of the meat.
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| Human Health Concerns and
Risk Reduction |
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There are no reports of any adverse reactions in humans bitten by black
flies containing
microfilaria.
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While rare, infection with
D. ursi has
resulted in the formation of small,
subcutaneous
nodules, in humans.
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| Samples for Diagnosis |
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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.
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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.
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| Further Reading |
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