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Feline Heartworm Disease

Since April is heartworm awareness month, I decided to come back and finish heartworm disease, and we will talk about the disease in our feline friends. Heartworm disease is very different in cats than in dogs – would you expect anything any different?

Even though cats are susceptible to heartworm disease, they are more resistant to infection than dogs. Cats typically have far fewer worms when they are affected, with most having 6 or less adult heartworms. Because of cats’ small size, even that low number of worms can cause life-threatening disease.

There are several differences in the life cycle of heartworm in cats versus dogs. Heartworms often take 7 – 8 months to reach maturity in cats, and this is slightly longer than in dogs. Cats rarely have circulating microfilariae (baby heartworms) in their blood stream. Adult heartworms have an average life span of 2 – 4 years in cats, which is significantly shorter than in dogs.

Stages of heartworm disease in Cats

There are two stages of the heartworm life cycle when cats show signs of disease. The first stage occurs at 3 – 4 months post-infection when immature adult worms are reaching the blood vessels in the lungs. The heartworms cause inflammation in the lungs, and the cats exhibit signs of respiratory disease (rapid breathing and coughing). These cats are often misdiagnosed with feline asthma or bronchitis. The second stage occurs as the heartworms die. They then cause significant inflammation, blood clots, and in some instances death.

In addition to respiratory disease and death, cats with heartworm disease can exhibit intermittent vomiting. Some cats will develop anorexia and weight loss. Uncommon clinical signs include having free fluid in the chest or abdomen, seizures, fainting, or coughing blood.

As expected with our feline friends, making a diagnosis of heartworm disease is much more difficult than in dogs. If you remember, the gold standard for testing in dogs is the antigen test, which looks for a protein secreted by the female worm. This test works well in cats, too, as long as at least one mature female worm is present, but detectable levels of antigen are not seen until 5.5 – 8 months after infection.

Antibody tests look for the cat’s response to heartworms, and this test is much more sensitive for detecting disease. A positive result can be seen as early as 2 months post infection. However, the test can remain positive long after the heartworms are gone.

Thoracic radiographs (chest x-rays) can help provide evidence for heartworm disease. Unfortunately, approximately 50% of cats have no changes, and even if the cats have changes, they can disappear completely during infection, and the cats will have normal heart, lungs, and blood vessels.

Heartworms in a cat

Cardiac ultrasound (echocardiography) can be useful to diagnose heartworm disease if the adult heartworm can be imaged in the pulmonary artery as it comes out of the heart.

Cardiac ultrasound on a cat

So, to make a diagnosis of heartworm disease in cats, you generally need both a positive antigen and antibody test. Thoracic radiographs should be taken to look for typical heartworm changes, and cardiac ultrasound can be performed to look for worms and help determine the number of worms infecting the cat. These same tests should be repeated at 6 – 12 month intervals to monitor infection status in cats.

Once the diagnosis is made, we face the dilemma of treatment. Melarsomine, the drug used to treat dogs, is not recommended in cats. The heartworms can be removed surgically. The worms have to be removed carefully, though. If the worms break during removal, they can cause acute collapse of the circulatory system and death. Most frequently, cats are treated with prednisone to decrease inflammation associated with the lung disease induced by heartworms, and as mentioned previously, the cats are monitored closely until the infection has resolved.

Just as with dogs, prevention is of primary importance. Cats can be given oral or topical preventives just like dogs, and some of the heartworm preventives also protect against other parasites such as fleas and ticks.

To wrap up feline heartworm disease, cats are very different from dogs, and cats can die suddenly from disease or be left with significant persistent respiratory disease. Diagnosing heartworm disease is difficult, and multiple tests are required to make a definitive diagnosis. Once the diagnosis is confirmed, good treatment options are not available, and usually, the best option is supportive care (prednisone) with continued monitoring until the infection has resolved. To paraphrase one of our founding fathers, an ounce of monthly prevention is truly worth a pound of cure. Please talk to your veterinarian about starting your feline friend on heartworm preventive, whether they are housed indoor, outdoor, or both.

Black & White cats make heart

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