Heartworm is a parasitic roundworm (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, under very rare circumstances, humans. The parasite is commonly called 'heartworm' because the adult reproductive stage of its life cycle resides primarily in the right ventricle of its host where it can live for many years. Heartworm infection may result in serious disease for the host. Heartworms go through several life stages before they become adults infecting the heart of the host animal. The worms require the mosquito as an intermediate stage in order to complete their life cycle. Development in the mosquito is temperature dependent, requiring approximately two weeks of temperature at or above 27°C. Below a threshold temperature of 14°C, development cannot occur, and the cycle will be halted. As a result, transmission is limited to warm months, and duration of the transmission season varies geographically. The period between the initial infection when the dog is bitten by a mosquito and the maturation of the worms into adults living in the heart takes 6 to 7 months in dogs and is known as the 'prepatent period'.