Dirofilaria immitis, the heartworm or dog heartworm, is a parasitic roundworm that is spread from host to host through the bites of mosquitoes. The heartworm is a type of filaria, a small thread-like worm, that causes filariasis. The definitive host is the dog, but it can also infect cats, wolves, coyotes, foxes and other animals, such as ferrets, sea lions and even, under very rare circumstances, humans. The parasite is commonly called "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 infections. Heartworm infection may result in serious disease for the host, with death typically as the result of congestive heart failure.