Heartworm Disease in Cats

What causes heartworm disease?

Heartworms are blood-borne parasites called Dirofilaria immitis that live in the heart or nearby large blood vessels of infected animals. Female worms are 6-14" long (15-36 cm) and 1/8" wide (3 mm). Males are about half the size of the females.

Heartworm disease is much more common in dogs than in cats. However, recent studies of cats with heart and breathing diseases have found an incidence of heartworms that is far greater than previously thought.

Cats are relatively resistant to adult 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 four. Many pet owners are surprised to learn that up to 1/3 of infected cats live indoors only.

How are heartworms transmitted to a cat?

The life cycle of the heartworm is complex and requires two host animals to complete it. Heartworms require the mosquito as an intermediate host; as many as 30 species of mosquitoes can act as host and transmit heartworms.

Mosquitoes ingest immature heartworm larvae, called microfilariae, by feeding on an infected cat or, more commonly, an infected dog. The microfilariae develop further for 10–14 days in the mosquito's gut and then enter its mouthparts. When an infected mosquito bites a cat, infective larvae move into the cat. The larvae migrate through the cat’s tissues as they mature and enter the bloodstream. The larvae end up in the pulmonary arteries (blood vessels that carry blood from the heart to the lungs) and right side of the heart. Many immature worms die at this point, as cats are not natural hosts for heartworm. Those that survive mature further into adult heartworms capable of reproduction within about 7–8 months.

If there is a male and female worm, they will 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. Unlike in dogs, adult heartworms only live for 2-4 years in cats. Because of this life cycle, a cat must be bitten by an infected mosquito to become infected with heartworms. Heartworms are not transmitted directly from one cat to another or from a dog directly to a cat. The risk of infection is greatest when mosquitoes are actively feeding. This typically requires temperatures over 50°F (10°C).

What happens to a cat with a heartworm infection?

As most heartworm infections in cats don’t develop to the adult stage, the more common disease they get is called Heartworm Associated Respiratory Disease (HARD). This disease results from inflammation caused by dying immature worms in pulmonary vessels, small airways, and lung tissue. Clinical signs are easily mistaken for asthma: coughing, wheezing and difficulty breathing. Some cats vomit or have a decreased appetite.

Adult heartworms cause heartworm disease through their effect on the heart and blood vessels that lead from the heart to the lungs (pulmonary arteries). They cause chronic inflammation that leads to scarring and narrowing of the pulmonary arteries and thickening (fibrosis) of the surrounding lung tissues.

These irreversible changes cause pulmonary hypertension (increased blood pressure in the vessels of the lungs), which means the heart must work harder to pump blood to the lungs for oxygen. Worms can also interfere with heart valves, further worsening the heart's workload and ultimately leading to heart failure. Adult heartworms have a more severe effect in cats than dogs, because cats’ blood vessels are so much smaller. Even one adult heartworm can cause fatal lung disease.

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 difficult parts of diagnosing feline heartworm disease is that there are no specific clinical signs. The most common signs are a sudden onset of coughing and rapid breathing - signs that can also be caused by several other diseases.

Other common, non-specific clinical signs include weight loss and vomiting. On occasion, an apparently normal cat may be found dead, or may develop sudden overwhelming respiratory failure, and heartworm disease is diagnosed in an examination after death (postmortem). Sudden death is thought to be due to a reaction within the lungs to the young heartworms, or to a reaction to heartworms entering the pulmonary arteries and obstructing the flow of blood to the lungs.

B) Blood Tests

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

1.The heartworm antibody test determines if the cat's immune system has been exposed to heartworms. A positive test may indicate that an active infection is present. However, cats who have had heartworms but whose heartworms have died will also have antibodies for an unknown time. Cats with mature larvae that are not yet adults, and cats with adult heartworms in places other than the heart, may also test positive with the antibody test. This test is relatively sensitive, so it is used first. If it is positive, the next test is performed.

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 are male. The low number of worms often seen in infected cats results in a high number of cats testing false-negative on antigen tests.

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.

3. A blood sample can be tested for the presence of microfilariae; however, only a small number of cats with heartworms have microfilariae in their blood, and microfilariae are only present for one to four weeks. Although a positive test is diagnostic, a negative test means little. As a result, this test is often not very useful.

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 larger 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 (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 many cats, adult heartworms can be seen; this finding confirms the presence of heartworms. However, because most infected cats have a low number of worms, this does not always occur.

The heartworm parasite often has a bacteria called Wolbachia pipientis that lives within it. Work is being done to study the usefulness of PCR tests to look for the presence of this bacteria. This may be a way of potentially confirming heartworms are present with the cat.

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: if a cat is diagnosed with heartworm but is not showing clinical signs, all that may be needed is to monitor antibody and antigen testing, along with regular chest radiographs, while the heartworm infection is naturally eliminated.

If there are changes on radiographs, prednisolone may be recommended to reduce inflammation and scarring, even if your cat has no signs of disease. If the cat is sick, however, there are several options:

1. Treat with the drug designed for dogs. This drug can have serious side effects in cats, though, including acute lung failure and death. For this reason, this approach is not recommended.

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 (prednisolone, dexamethasone) to relieve the reaction occurring in the pulmonary arteries and lungs. If needed, they are given 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. In addition, some clinicians recommend treating with an antibiotic called Doxycycline, as this can kill the Wolbachia bacteria mentioned earlier. Studies to determine if this is beneficial are still ongoing.

3. A slow-kill method is sometimes used to reduce worm burdens, using a monthly dose of ivermectin. This treatment slowly kills the worms, but an inflammatory response to the death of the worms can still happen. This may cause serious consequences or even death.

4. Surgical removal of the heartworms can be attempted 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 prevention. There are excellent heartworm preventives now available for cats, making prevention safe and easy. These include NexGard® Combo, Revolution®, Revolution® Plus, Advantage Multi®, Interceptor®, Heartgard®, Bravecto Plus®, and Centragard®.

The reasons that heartworm prevention is now recommended for all cats are:

1. Diagnostic Difficulty: Diagnosing heartworms is not as easy in cats as in dogs.

2. Unknown Incidence: Heartworms are not as common in cats as they are in dogs. However, they are probably more common than we realize. As we look more aggressively for heartworms in cats, with better and better tests, we expect to find that the incidence is greater than previously thought. Studies have shown that up to 15% of all cats in certain locations, regardless of whether they are indoor or outdoor cats, have been exposed to heartworms.

3. No Good Treatment: There is simply no good treatment for heartworm-infected cats. Effective drugs are not available and cats that seem to be doing well may die suddenly. Treating heartworm infections in cats is risky at best, and not treating these cats is just as risky. It will take about two years for the parasitic infection to be eliminated in the cat, and serious clinical signs can suddenly appear at any time during this period.

4. Prevention Is Safe and Easy: Cats given heartworm prevention drugs have not shown signs of toxicity. There is a wide margin of safety, even in kittens as young as six weeks of age.

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.