Filaroides hirthi: hyperinfective lungworm infection in immunosuppressed dogs.

Filaroides hirthi is a pulmonary parasite found primarily in beagle dogs. The modes of transmission and the life cycle of this parasite have been described recently, and the possibility of an autoinfectious cycle has been suggested. In this paper, we report two cases of experimentally immunosuppressed beagles (4 mg/day of prednisolone for over four months) in which massive disseminated F. hirthi infections developed. Large numbers of F. hirthi adults were present in the lungs, and larvae were found in lungs, mesenteric nodes, intestinal walls, and liver. While only minimal cellular responses were associated with the adult worms, a conspicuous mononuclear cell reaction was found around the larvae in the tissues. Since our dogs were caged to minimize contact with their own feces, we suggest that autoinfection took place in these dogs. Heavy worm burdens probably were reached because the reduced immune responses against the larva facilitated their successful penetration of the intestinal wall and their recirculation within the host.