Heartworm disease is a potentially fatal, but preventable, infection caused by a worm parasite called Dirofilaria immitis. The worms are spread through the bite of a mosquito, and the dog is the definitive host, meaning that the worms mature into adults, mate, and produce offspring while living inside a dog. Heartworm disease affects dogs, cats, and ferrets, but heartworms also live in other mammal species, including wolves, coyotes, foxes, sea lions, and humans. The worms live in the heart, lungs, and associated blood vessels of affected pets, causing severe lung disease, heart failure, and damage to other organs in the body. The severity of the infection and the signs depend on the number of worms present, the immune response of the infected dog, the duration of infection, and the activity level of the animal. Most dogs are highly susceptible to heartworm infection, and the majority of infective larvae develop into adult worms. Signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen. Dogs with large numbers of heartworms can develop a sudden blockage of blood flow within the heart leading to a life-threatening form of cardiovascular collapse.

To diagnose heartworm disease, veterinarians may perform a serological test for antigens to adult heartworms, chest X-rays, and other tests. The earlier the disease is detected, the better the chances the pet will recover. There are few, if any, early signs of disease when a dog, cat, or ferret is infected with heartworms, so regular check-ups with a veterinarian are important. Heartworm disease is a serious, progressive disease, and if left untreated, it can cause lasting damage to the heart, lungs, and arteries, and can affect the dog’s health and quality of life long after the parasites are gone.