Dirofilaria immitis detection test kit by using qPCR. Dirofilaria immitis (also known as heartworm or dog heartworm) is a parasitic roundworm, a type of filarial 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, ferrets, bears, seals, sea lions, and, under rare circumstances, humans.