Skip to main content

Bear Filarial Worm

(Dirofilaria ursi)

photo of bear filarial worm


The bear filarial worm (Dirofilaria ursi) was first described in an Asiatic black bear in Japan in 1941. The North American black bear harbors two developmental forms; the microfilaria stage which circulate in the blood stream and are extremely small, measuring 185 to 292 microns in length, and the long, white, slender adults which reside in the subcutaneous space of the neck and groin, and in the connective tissues around the aorta, kidneys, and rectum. The adult worms differ in length according to their sex, with males measuring 51 to 86 mm and the females 117 to 224 mm in length.


Since 1941, the worm has been identified in black bears in a small number of states, including Michigan. Adult bears and cubs, at least 10 months of age, can harbor adult or subadult D. ursi. The rate of infection in bears ranges from nearly 100% in Minnesota and Ontario to nearly 0 % in Alberta and the southeastern United States. Eighty-nine blood smears from bears in the Houghton Lake area were negative for D. ursi microfilaria in a 1978-1980 study. Though the actual rate may be higher, only an occasional bear is reported with a D. ursi infection in Michigan.

Transmission and Development

D. ursi adult females produce motile microfilaria which make their way to the blood stream. They do not develop further until ingested by a blackfly. Following a 2-week developmental period in the fly, D. ursi larvae are infective to the bear. When the fly feeds again on a bear, the larvae leave the fly's mouthparts and enter the bear's body. The larvae migrate to the preferred sites mentioned above, mature, and mate. A 7-month period is required for the female worms to produce microfilaria and thereby complete the life cycle.


D. ursi microfilaria, subadults, and adults do not appear to be pathogenic to the black bear.


D. ursi infections can be diagnosed either by blood smears (microfilaria) or by finding the adults in the preferred subcutaneous and visceral locations.

Because microfilaria of other filarids may be present, identifying the adult worms as D. ursi is the most accurate method of diagnosis.

Treatment and Control

There are no current treatment or control measures for this parasite and, as it is quite insignificant in black bears in Michigan, these measures are not necessary.


The primary significance of this parasite is its being found by the hunter during the skinning of a bear. The microfilarial stage and the subadult and adult stages do not affect the edibility of the meat nor are the worms of public health significance. It is unlikely that the bite of a blackfly containing microfilaria would cause any adverse reaction in a human.

Return to Index

For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.