Humans can occasionally serve as a dead-end host for Dirofilaria immitis, the dog heart worm. Larvae are introduced into the bloodstream through the bite of an infected mosquito and travel to the heart, the normal site of residence in the canine host. In humans, the larvae are unable to survive in the heart and are embolized to the lung where they typically occlude the blood vessel and result in tissue infarction and nodule formation. When excised, the worm is seen within the necrotic lesion and can be identified by its large size and presence of internal cuticular ridges (arrows) on cross-section. Of note, humans can become infected with other Dirofilaria species, such as D. repens, D. tenuis, and D. ursi. These larvae cause nodules within subcutaneous tissue rather than in the lung.

You can view a whole slide image from a case of dirofilariasis HERE and another case HERE.