Heartworm is a parasitic rounworm (Dirofilaria immitis) that is spread from host to host through the bites of mosquitoes. The heartworm is a type of filaria, a small thread-like worm. 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 humans. The parasite is commonly called "heartworm" because the adult reproductive stage of its life cycle resides primarily in the pulmonary artery of its host where it can live for many years. Heartworm infection may result in serious disease for the host; infected dogs that are untreated may die and treatment also has risks. The best defense against heartworm disease is the use of prophylactic treatment given monthly.