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, and which causes dirofilariasis. It is spread from host to host through the bites of mosquitoes. Four genera of mosquitoes transmit dirofilariasis, Aedes, Culex, Anopheles, and Mansonia. 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.

Adult heartworms often reside in the pulmonary arterial system (lung arteries) as well as the heart, and a major health effect in the infected animal host is 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.

Although at one time confined to the southern United States, heartworm has now spread to nearly all locations where its mosquito vector is found. In the southeast region of the United States, veterinary clinics saw an average of more than 100 cases of heartworm each in 2016. Transmission of the parasite occurs in all of the United States (cases have even been reported in Alaska), and the warmer regions of Canada. The highest infection rates are found within 150 miles (240 km) of the coast from Texas to New Jersey, and along the Mississippi River and its major tributaries. It has also been found in South America, southern Europe, Southeast Asia, the Middle East, Australia, Korea, and Japan.

Heartworms go through several life stages before they become adults infecting the pulmonary artery of the host animal. The worms require the mosquito as an intermediate host to complete their lifecycles. The rate of development in the mosquito is temperature-dependent, requiring about two weeks of temperature at or above 27 °C (80 °F). Below a threshold temperature of 14 °C (57 °F), development cannot occur, and the cycle is halted. As a result, transmission is limited to warm weather, 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 pulmonary arteries takes six to seven months in dogs and is known as the "prepatent period".

The first larval stage (L₁) and second larval stage (L₂) of heartworm development occurs within the body of a mosquito. Once the larvae develop into the infective third larval stage (L₃), the mosquito locates and bites a host, depositing the larvae under the skin at the site of the bite. After a week or two of further growth, they molt into the fourth larval stage (L₄) . Then, they migrate to the muscles of the chest and abdomen, and 45 to 60 days after infection, molt to the fifth stage (L₅, immature adult). Between 75 and 120 days after infection, these immature heartworms then enter the bloodstream and are carried through the heart to reside in the pulmonary artery. Over the next three to four months, they increase greatly in size. The female adult worm is about 30 cm in length, and the male is about 23 cm, with a coiled tail. By seven months after infection, the adult worms have mated and the females begin giving birth to live young, called microfilariae. Heartworms can live for 5 to 7 years in a dog.

The microfilariae circulate in the bloodstream for as long as two years, and are ingested by bloodsucking mosquitos, where development occurs and the cycle repeats.^{[citation needed]}

Hosts of Dirofilaria immitis include:

Reservoir hosts for D. immitis are coyotes and stray dogs.