ursi is the most accurate method of diagnosis. ursi infections can be diagnosed either by blood smears (microfilaria) or by finding the adults in the preferred subcutaneous and visceral locations.īecause microfilaria of other filarids may be present, identifying the adult worms as D. ursi microfilaria, subadults, and adults do not appear to be pathogenic to the black bear. A 7-month period is required for the female worms to produce microfilaria and thereby complete the life cycle. The larvae migrate to the preferred sites mentioned above, mature, and mate. When the fly feeds again on a bear, the larvae leave the fly's mouthparts and enter the bear's body. Following a 2-week developmental period in the fly, D. They do not develop further until ingested by a blackfly. ursi adult females produce motile microfilaria which make their way to the blood stream. Though the actual rate may be higher, only an occasional bear is reported with a D. Eighty-nine blood smears from bears in the Houghton Lake area were negative for D. The rate of infection in bears ranges from nearly 100% in Minnesota and Ontario to nearly 0 % in Alberta and the southeastern United States. Adult bears and cubs, at least 10 months of age, can harbor adult or subadult D. Since 1941, the worm has been identified in black bears in a small number of states, including Michigan. 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. 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 bear filarial worm ( Dirofilaria ursi) was first described in an Asiatic black bear in Japan in 1941.