Heartworm Disease in Cats

Heartworm disease, so named because the adult worms live in the right side of the heart, is common in dogs, less so in cats. In fact, cats may be accidental hosts only, and certainly they are less perfect hosts for this parasite than dogs are.

Heartworm Life Cycle

A knowledge of the life cycle of this parasite (Dirofilaria immitis) is needed to understand how to prevent and treat it. Infection begins when L3 infective larvae in the mouthparts of a mosquito enter the cat's skin at the site of a bite. The larvae burrow beneath the skin and undergo two molts that eventually lead to the development of small, immature worms. The first molt (L3 to L4) occurs 1 to 12 days after the cat is bitten by the mosquito. The larvae remain in the L4 stage for 50 to 68 days, and then molt into the L5 stage (immature worms).

Immature worms make their way into a peripheral vein and are carried to the right ventricle and the pulmonary arteries. In cats, the larvae may become disoriented and migrate into body cavities and the central nervous system. Approximately six months after entering the cat's body, they mature into adults. Adults can grow from 4 to 12 inches (10 to 30 cm) long and live up to two to three years.

In dogs, mature heartworms produce larvae, called microfilaria, that circulate in the bloodstream. This is much less common in cats, possibly because the cat's immune system removes the microfilaria or because low numbers of adult worms or same-sex worms actually prevent the production of microfilaria to begin with.

Heartworm Disease

Because of the small size of the cat's heart, one or two worms may be enough to cause serious heart trouble or even sudden death. Signs of heartworm infestation include a cough made worse by exercise, lethargy, loss of weight and coat condition, and bloody sputum. At this point, it may appear that the cat has asthma or allergic bronchitis. The cat's pulmonary artery response to heartworms is much more severe than is the dog's.

Cats who pass through this phase of infection may be relatively fine until the adult heartworms start to die in two to three years. Labored breathing and mild, low-grade, chronic respiratory signs may go on for a while. Congestive heart failure, along with heart murmurs, loss of condition and appetite, and intermittent vomiting may all appear late in the disease. Worms may be discovered at autopsy following sudden, unexplained death.

Diagnosis is generally done by blood tests looking for the heartworm antigens or antibodies produced to fight them. Both types of tests are valuable before starting treatment for a suspected infection. X-rays of the chest and the use of echocardiography can be especially helpful in diagnosing heartworms in cats.

Treatment: Treatment is complex and potentially dangerous. If the cat seems reasonably healthy, monitoring his condition and following the lifespan of the heartworms may be the best option. Medical support may be needed for any respiratory or cardiac signs. Corticosteroids may be useful in reducing reactions to the worms. Ivermectin has been used to treat heartworm infections in cats, but the drug is still considered experimental as a treatment. Surgery can also be done to physically remove any heartworms, but it is not common.

Prevention: Heartworms are spread by mosquitoes, and areas along coastal regions with swamps or other brackish water provide ideal conditions for mosquitoes to breed. Areas with warm temperatures most of the year have a longer mosquito season, and any nearby areas of standing water can provide a mosquito habitat. In theory, the best way to prevent heartworms is to keep your cat from being bitten by a mosquito.

Even indoor-only cats can become infected, however, because mosquitoes often get through screens or open doors and windows, or come in on other pets. Preventive drugs for cats include ivermectin, selamectin, and milbemycin oxime, all of which guard against some internal parasites as well. A heartworm test (preferably both antigen and antibody) is recommended but is not absolutely necessary before starting your cat on a preventive regimen. Many practitioners now advocate year-round prevention, although theoretically cats need not be protected in the winter months in cold areas, because there are no mosquitoes alive outside.

This article is excerpted from “Cat Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.