Dirofilaria ursi is a long slender white roundworm, females and males being 11-22 and 5-9 centimetres in length respectively.  This parasite is known only from bears.  Forty-seven percent of 4,579 adult D. ursi recovered from 56 bears of central Ontario were found along the trachea in the neck and thoracic cavity, 30% were found in connective tissues surrounding and posterior to the kidneys in the abdominal cavity, 7.5% were present in subcutaneous tissues of the groin region, 8.6% were found between muscles overlying the ribs and the remaining worms were distributed amongst other parts of the bears.  During the same study, 140 of 142 bears greater than one year of age, were infected with D. ursi.

D. ursi is similar to other filarial worms in that the mature female worms produce motile embryos called microfilariae.  The microfilariae are present in the circulatory system of the bear but will not develop within the bear.  They will only develop when ingested with blood by particular species of blackflies.  The parasite is not capable of developing in mosquitoes, deerflies or horseflies.  Development of the larvae in blackflies to a stage where they are capable of infecting bears takes approximately two weeks.  When infected blackflies feed again the larvae leave the mouth parts of the fly and penetrate into bears.  They migrate to the sites occupied by adult worms where they mature and mate.  Approximately seven months after entering the bear the new female worms produce microfilariae and the life cycle is completed.

D. ursi does not appear to be a serious pathogen of black bears.  Presence of either adult worms or microfilariae in bears does not affect the edibility of the meat.  It is unlikely that there would be any adverse reaction if humans received larvae of D. ursi when bitten by blackflies.

Selected Reference:

Addison, E.M. 1977.  Dirofilaria ursi of black bears in central Ontario.  Unpublished data.  Research Section, Wildlife Branch, Ministry of Natural Resources, Maple, Ontario.