Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and occasionally in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms) and are filarids, one of many species of roundworms. Dogs and cats of any age or breed are susceptible to infection.
Filarids rely on different insect species to be transported from animal to animal. The specific filarid causing heartworm in dogs and cats is known as Dirofilaria immitis.
Dogs or other animals harboring adult worms are the recognized reservoir of heartworm infection. The disease is spread by mosquitoes that become infected with microfilariae while taking a blood meal from an infected dog. Within the mosquito, the microfilariae mature into the infective larval stage. When the mosquito then bites another dog, cat, or susceptible animal, the larvae are deposited on the skin and actively migrate into the new host. For about 2 months the larvae migrate through the connective tissue, under the skin, then pass into the animal’s venous blood stream and are quickly transported to the arteries of the lung. It takes a total of approximately six months for the infective larvae to mature into adult worms that begin producing offspring, microfilariae. Adult heartworms can live for five to seven years in the dog.
In the dog, the larvae progress in their development to an adult form of the worm, and live in the pulmonary vessels, where they continue the life cycle and cause extensive injury. The period of time when heartworms are reproductively capable is referred to as patency. In cats, it takes seven to eight months before adult worms potentially reach patency in the pulmonary vessels, and this is referred to as transient patency, as reproductive capability in the cat is usually very short (months) compared to that of dogs (years). In most cases the cat is not an effective reservoir host, since microfilaria are produced in less than 20% of the cats.
In the cat, the larvae molt as well, but fewer worms survive to adulthood. While dogs may suffer from severe heart and lung damage from heartworm infection, cats typically exhibit minimal changes in the heart. The cat’s primary response to the presence of heartworms occurs in the lungs.