

Heartworm Disease in Cats

By Catherine Barnette, DVM; Ernest Ward, DVM

Infectious Diseases, Medical Conditions, Parasites, Pet Services

What causes heartworm disease?

Heartworms are a blood-borne parasite called Dirofilaria immitis that reside in the heart or adjacent large blood vessels of infected animals. Female worm are 6 - 14" long (15 - 36 cm) and 1/8" wide (3 mm). Males are about half the size of the female.

Heartworm disease is much more common in dogs than in cats. However, recent studies of cats with heart and respiratory diseases have found an incidence of heartworms that is far greater than previously thought. Cats are relatively resistant to heartworm infection when compared to dogs, with the infection rate in cats reported to be 5-20% of the rate in dogs in the same geographic location; however, infection can still occur. Typically, cats have fewer adult worms than dogs, usually less than six. Many pet owners are surprised to learn that approximately 1/3 of infected cats live indoors only.

How are heartworms transmitted to a cat?cat-heartworm_updated-01



The larvae migrate into the bloodstream, ending up in the right side of the heart and the pulmonary arteries. There they mature into adult heartworms capable of reproduction within about 6 to 7 months. Shortly thereafter, at around eight months after infection, they begin to produce a new crop of microfilaria that will live in the cat's blood for about one month. Cats are resistant hosts, and few circulating microfilariae are generally found.




How are heartworms diagnosed?

There are several methods used in diagnosing heartworms; unfortunately, none are 100% reliable, so a combination of tests is often needed. The diagnostic sequence usually progresses as follows:

A) Clinical Signs

"One of the most challenging aspects of diagnosing feline heartworm disease is that there are no specific clinical signs."


Acute Clinical Signs Chronic Clinical Signs Collapse Coughing Shortness of breath Vomiting Convulsions Shortness of breath Diarrhea/Vomiting Lethargy Blindness Lack of appetite Rapid heart rate Weight Loss Fainting Chylothorax (accumulation of fluid around the lungs) Sudden Death

B) Blood Tests

Several blood tests are used for heartworm diagnosis, but the heartworm antibody test and the heartworm antigen test are proving to be most helpful in diagnosing heartworm disease in cats.


2.  The heartworm antigen test detects the presence of adult female heartworms. It is very specific, but not as sensitive as the antibody test. A positive test indicates that heartworms are present, but a negative test does not mean that they are absent. Because the cat must have at least two adult female worms present to have a positive test result, a negative test may mean that the cat has only a small number of worms or that all the worms present are male. The low worm burdens often seen in infected cats results in a high number of cats testing false-negative on antigen tests.

"A diagnosis of feline heartworm infection is confirmed when
both the antibody and antigen tests are positive."

In summary, a diagnosis of feline heartworm infection is confirmed when both the antibody and antigen tests are positive, but not all infected cats will test positive on both tests.


4.  An eosinophil count can be measured in cats suspected of harboring heartworms. Eosinophils are a type of white blood cell that occur in increased numbers when certain parasites are present. They are elevated in the presence of heartworms, but this elevation only occurs for a few months. This test is not specific, since cats with allergies or other parasites (e.g., intestinal worms, fleas) also commonly have increased eosinophil counts. An eosinophil count is often performed with a complete blood cell count (CBC) and serum chemistries during the initial diagnostic workup.

5.   Radiographs (X-rays) permit your veterinarian to view the size and shape of the heart. They also allow measurements on the diameter of the pulmonary arteries. Many cats with heartworms have an increased size of the pulmonary arteries, or the arteries may appear blunted (suddenly come to an apparent stop) on their way to the lungs, due to worms obstructing them. However, many other cats with heartworms have no abnormal findings on their radiographs, especially early in the infection.

6.   Cardiac ultrasound or echocardiography allows the internal structures of the heart and surrounding vessels to be directly viewed and the condition and function of the heart to be assessed. In some cats, adult heartworms can actually be seen; this finding confirms the presence of heartworms. However, because most infected cats have a low number of worms this does not occur often.

Can feline heartworm disease be treated?

There is no drug approved for treating heartworms in cats. One of the drugs used for treating dogs has been used in cats, but it causes significant side effects.

To complicate things further, when the adult heartworms die during this treatment, they pass through the pulmonary arteries to the lungs where the reaction to the dead and dying worms can cause sudden death. Thus, there is a dilemma when a cat is diagnosed with heartworms. One of three choices must be made:

1.  Treat with the drug designed for dogs. However, this drug can have serious side effects in cats including acute pulmonary (lung) failure and death. For this reason, this approach is not recommended.heartworm-3

2. Treat the symptoms of heartworm disease and hope the cat outlives the worms. Since heartworms live in a cat for about two to three years (as opposed to five to seven years in dogs), several months of treatment are needed. When cats are in a crisis, they are treated with oxygen and corticosteroids (cortisone) to relieve the reaction occurring in the pulmonary arteries and lungs, and, if needed, drugs to remove fluid from the lungs (diuretics). When they are stable, they are treated continuously or periodically with corticosteroids. In many cats, this treatment will reduce clinical signs and improve their quality of life. However, the threat of an acute crisis or sudden death always exists.

3.  Surgical removal of the heartworms is currently the recommended treatment for cats with severe signs of heartworm disease. This procedure must be performed by a specialist, often at a college of veterinary medicine. Studies have shown that up to 40% of cats may die during or after this procedure, so surgical heartworm removal is typically reserved for those cats who have severe disease and a poor prognosis without surgery.

Is there a way to prevent heartworms?

Veterinarians now strongly recommend that all cats receive year-round monthly heartworm preventives in areas where mosquitoes are active all year round.


Cats that live in colder areas, where mosquitoes are seasonal, should be on monthly preventative for at least 6 months of the year. There are excellent heartworm preventives now available for cats, making prevention of heartworm disease safe and easy.






5. Indoor Cats Get Heartworms Too. Exposure to mosquitoes is required for transmission. Cats do not have to be exposed to cats or dogs infected with heartworms. Obviously, cats that go outdoors are more likely to be exposed; however, an infected mosquito can easily get into the house and infect the cat.

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