|A German Shepherd dog heart infested with Dirofilaria immitis|
Dirofilaria immitis, also known as 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, but it can also infect cats, wolves, coyotes, jackals, foxes, and other animals, such as ferrets, bears, seals, sea lions and, under rare circumstances, humans.
Dirofilaria immitis is commonly called "heartworm". Contrary to their name however, adult heartworms often reside in the pulmonary arterial system (lung arteries) as well as the heart, and a major effect on the health for the infected animal host is a manifestation of damage to its lung vessels and tissues. In cases involving advanced worm infestation, adult heartworms may migrate to the right heart and the pulmonary artery. Heartworm infection may result in serious complications for the infected host if left untreated, eventually leading to death, most often as a result of secondary congestive heart failure.