Heartworm is a parasitic roundworm (Dirofilaria immitis) that lives in the heart and pulmonary arteries of an infected animal. It is generally associated with dogs, but it can also infect humans. The parasite is transmitted by the bite of a mosquito. Heartworms enter the bloodstream as tiny, invisible larvae and travel through the bloodstream, harming arteries and vital organs as they go. Ultimately they complete their journey to the vessels of the lung and the heart chamber about six months after the initial infection.

In rare cases, migrating heartworm larvae get "lost" and end up in unusual sites such as the eye, brain, or an artery in the leg, which results in unusual symptoms such as blindness, seizures and lameness. Typically until the larvae mature and congregate inside the heart, they produce no symptoms or signs of illness.

Although the parasite is commonly called "heartworm"; that is a misnomer because the adult actually resides in the pulmonary arterial system (lung arteries) for the most part. The primary effect on the health of the animal is a manifestation of damage to the lung vessels and tissue. Occasionally, adult heartworms migrate to the right heart and even the great veins in heavy infections. Several hundred worms can live in one (dog) for five to seven years and can reach lengths of more than twelve inches at maturity.

Many (dogs) will show little or no sign of infection even after the worms become adults. These animals usually have only a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections may show the classic signs of heartworm disease. Early signs include a cough, especially on exercise and early exhaustion upon exercise. In the most advanced cases where many adult worms have built up in the heart without treatment, signs progress to severe weight loss, fainting, coughing up blood and, finally, congestive heart failure.