Histopathological diagnosis of subcutaneous Dirofilaria repens infection in humans

The host tissue response in humans, based on a study of biopsies of 14 subcutaneous nodules caused by Dirofilaria repens is described. The response was characterized by accumulation of eosinophilic material or eosinophilic pus adjacent to the parasite with surrounding granulomatous inflammation associated with extensive eosinophil infiltration of the surrounding tissue. Unlike with fungal lesions, fat necrosis was conspicuously absent. Nodules were all less than 3 cm in size. Fifteen percent of the smaller nodules (less than 1 cm) and 30% of the larger nodules required extensive examination of tissue to visualize the parasite. It is concluded that when typical tissue reactions are seen, extended histological processing is indicated for accurate diagnosis of dirofilariasis, by demonstrating the parasite.