Solitary lung nodule due to Dirofilaria immitis (dog "heartworm").

The filarial parasite Dirofilaria immitis (dog "heartworm") is the cause of a progressive zoonosis among canine species in the United States; it is rarely transmitted to man, in whom it can present as a solitary lung nodule. There are currently no reliable clinical or radiologic features to distinguish it from other solitary necrotizing granulomas, and thoracotomy is currently the only means of establishing the diagnosis. D immitis can be identified and properly classified by routine light microscopy despite being immature and partially necrotic. Canine Dirofilariasis may well assume even greater medical importance because of its expanding geographic range and the concomitant increase in the number of cases observed among humans.