That mosquito buzzing in your ear at night can drive you a little bit mad. But it can have far more serious consequences for your cat. Though cats are less likely than dogs to get heartworm disease, it does happen. Mosquitoes can carry heartworm larvae, and a bite from an infected insect could mean heartworm disease for your cat down the line. A cat might not show any signs of infection, or he or she might vomit, cough, or have a hard time breathing. Unlike for dogs, there is no treatment for heartworm disease in cats, and prevention through oral or topical drugs is the best bet.

Overview

Heartworm disease is a serious and potentially fatal condition that affects dogs, cats, and up to 30 other species of animals. It is caused by parasitic worms (heartworms) living in the major blood vessels of the lungs and, occasionally, in the heart. These worms are transmitted (as microscopic larvae) through the bite of an infected mosquito. The scientific name for the heartworm parasite is Dirofilaria immitis.

Despite the fact that heartworm disease is preventable, many cats are diagnosed with it each year. Although cats are considered resistant to heartworms and can often fight off an infection on their own, heartworm disease can still be a serious health problem for cats, resulting in significant illness and even death. Keeping a cat indoors does not prevent infection. Studies have shown that more than 25 percent of heartworm-infected cats live indoors exclusively.

The American Heartworm Society (AHS) estimates that one million dogs in the United States have heartworm disease today, and that this number may be rising. Wherever dogs are infected studies have shown that cats are likely to be infected, too, though at a reduced rate.

Signs and Identification

When infected, cats usually have fewer heartworms than dogs in their system. However, because of the vagaries of the feline immune system and anatomy, the presence of even a few worms can cause significant damage, primarily to the lungs. Hence, respiratory abnormalities are the most common sign of heartworm disease in cats.

Some cats with heartworm disease never show any signs. When present, the signs of heartworm infection in cats can be confused with those of many other diseases, including feline asthma. Affected cats may vomit, cough, and have difficulty breathing. This condition is called heartworm-associated respiratory disease (HARD).

Sometimes, the only sign of heartworm infection in cats is sudden death.

Heartworms are spread through the bite of a mosquito, and dogs serve as the source of infection for other dogs and for cats. When a mosquito bites an infected dog it draws blood that contains immature heartworms (called microfilariae [pronounced micro-fill-air-ee-ay]). These microfilariae mature inside the mosquito to become infective larvae. When the mosquito eventually bites another dog or a cat, the larvae enter the new host. In dogs, these larvae often mature to become adult heartworms, which produce more microfilariae and continue the heartworm’s life cycle.

The life cycle of heartworms in cats is slightly different from the life cycle in dogs. For example, many heartworms die during development in a cat, so they don’t live long enough to produce microfilariae. Additionally, the immune system of some cats can eliminate the heartworm infection before the worms reach adulthood. For these reasons, heartworm testing for cats is more complicated than for dogs.

Diagnosis of feline heartworm disease may involve other types of diagnostic tests besides bloodwork. Sometimes, evidence of heartworms can be seen on ultrasound images or radiographs (“X-rays”) of the heart and lungs. Unfortunately, these tests can also be inconclusive. Diagnosis, therefore, is a necessarily complex process.

Affected Breeds

Dogs and cats of any breed can get heartworm disease.

Treatment

In cats, there is no real treatment for the heartworm infection itself. Veterinarians usually recommend monitoring cats for signs of associated disease (such as HARD and feline asthma). Because of the dangers of definitive treatment, managing these signs of disease is usually the mainstay of treatment in feline patients.

Infrequently, surgical removal of the worms may be recommended. However, this surgery is costly and carries significant risks. A cardiologist and/or surgeon is the ideal specialist should a surgical approach be considered.

Prevention

Safe, easy-to-give, effective medications are available to prevent feline heartworm disease and should be used. These are available as both topical and oral monthly medications. Ask your veterinarian about the risk of heartworm disease for your cat.

This article has been reviewed by a Veterinarian.

Reviewed October 2017