Light and electron microscopic pulmonary lesions associated with retained microfilariae in canine occult dirofilariasis.

Lungs from dogs infected with adult, gravid Dirofilaria immitis but without microfilaria circulating in systemic vessels were studied by light and transmission electron microscopy. Microfilariae were seen within the pulmonary microvasculature and in interstitial areas of interalveolar septa. An acute inflammatory reaction characterized by aggregates of eosinophils, neutrophils, macrophages and lymphocytes was associated with intact microfilariae. Necrotic and fragmented microfilariae were surrounded by macrophages and epithelioid cells. Other lesions in interalveolar septa of dogs with chronic occult dirofilariasis included endothelial degeneration and hypertrophy, perivascular aggregates of plasma cells, hypertrophy and hyperplasia of smooth muscle cells, and interstitial fibrosis. The findings suggest that a cell-mediated hypersensitivity reaction is at least partially responsible for retention of microfilariae in pulmonary parenchyma of dogs with occult dirofilariasis.