Dirofilaria immitis, the heartworm or dog heartworm, is a parasitic roundworm that is a type of filarial worm, a small thread-like worm, that causes dirofilariasis. It is spread from host to host through the bites of mosquitoes. The definitive host is the dog.
Dirofilaria immitis is commonly called the "heartworm"; however, adults often reside in the pulmonary arterial system (lung arteries), as well as the heart, and a major effect on the health for the animal is a manifestation of damage to the lung vessels and tissues.
Occasionally, adult heartworms migrate to the right heart and even the great veins in heavy infestations. Heartworm infection may result in serious disease for the host, with death typically as the result of congestive heart failure.