Dirofilaria immitis: experimental infection of rabbits with immature fifth-stage worms.

To establish an animal model for human pulmonary dirofilariasis, we experimentally infected nine rabbits with immature fifth-stage worms of Dirofilaria immitis. The rabbits were infected by subcutaneous transplantation with various numbers of immature worms collected from 110- and 120-day-old infections of dogs. Four of seven rabbits infected with up to four larvae possessed encapsulated worms in the lungs at 196 or 308 days post-transplantation. Two rabbits transplanted with eight worms died of pulmonary hemorrhagic infarction 18 and 28 days post-transplantation. Marked histopathologic changes were observed in the lungs at the site of degenerating worms, which were encapsulated by a fibrous wall. Severe to mild infiltrations with eosinophils, heterophils, lymphocytes, plasma cells, and histiocytes were found in granulomas and their surrounding areas. The findings in these rabbits resemble those reported for human cases of pulmonary dirofilariasis.