Heartworm Disease in Cats: Testing

What is heartworm disease?

Heartworm disease typically affects dogs but occasionally occurs in cats. The disease is caused by a parasite called Dirofilaria immitis, better known as heartworm. Heartworms live in the right side of the heart and the pulmonary artery (the large blood vessel that carries blood from the heart to the lungs). Cats become infected when they are bitten by an infected mosquito carrying infective heartworm larvae, and the larvae pass from the mosquito into the cat. Heartworm infection is much less common in cats than in dogs. However, heartworm infection in cats is difficult to diagnose, so the true rate of infection may be higher than it appears.

Does heartworm disease affect cats differently than dogs?

Yes, heartworm disease affects cats differently than dogs because cats are not a normal host for heartworms.

• In dogs, heartworms complete their life cycle and mature into adult worms that live in the heart, then mate and reproduce, producing microfilariae (immature worms).
• In cats, heartworms usually cannot complete their life cycle. They either die in the tissues, especially the lungs, or get “lost” and end up in abnormal places like the brain, skin, or body cavities. A few worms may mature in the heart, but usually too few to mate and produce microfilariae. On rare occasions, microfilariae are produced but they typically stay in the bloodstream for only a brief time.

Why does my cat need heartworm testing?

The American Heartworm Society recommends yearly heartworm testing for all cats in the United States. Testing is important because:

• Heartworm disease may not cause obvious signs until it is advanced.
• Many infected cats show no signs of illness or show non-specific signs
• Regular testing can provide knowledge about heartworm exposure history and risk, identifying gaps in parasite prevention and need for increased monitoring for potential clinical signs

How is heartworm disease diagnosed in cats?

Heartworm disease in cats is difficult to diagnose, and a variety of testing methods are usually needed to confirm infection.  A combination of clinical signs and supportive test results may be needed. These include blood tests, X-rays (radiographs), and echocardiography (ultrasound of the heart).

Routine blood tests such as a complete blood count and serum biochemistry profile may show non-specific changes, such as a high eosinophil count (a type of white blood cell that responds to parasitic infections) or a high globulin level (due to increased inflammatory proteins).

The heartworm antibody test determines if the cat's immune system has been exposed to heartworms. This test is relatively sensitive, so it is used first. If it is positive, the next test is performed. However, a positive antibody test can indicate several possibilities:

• The cat has an active heartworm infection
• The cat had heartworms in the past (antibodies remain for an unknown time, even if the worms have died)
• The cat has mature larvae that are not yet adults
• The cat has adult heartworms in places other than the heart

The heartworm antigen test detects the presence of adult female heartworms. A positive test indicates that heartworms are present, but a negative test does not mean that they are absent. The cat must have at least two adult female worms present to have a positive test result, so a negative test may mean that the cat has only a small number of worms or that all the worms are male. Infected cats often have a low number of heartworms, so it is common for cats to test false-negative on antigen tests.

A blood sample can be tested for the presence of microfilariae; however, many cats with heartworms do not have microfilariae in their blood, and if they do, the microfilariae are transient and only present for one to four weeks. A positive test indicates heartworms are present, but a negative test does not mean that they are absent.

Chest X-rays view the size and shape of the heart and assess for any lung changes, such as enlarged pulmonary arteries. However, many cats with heartworms have no abnormal findings on their X-rays, especially early in the infection.

Cardiac ultrasound (echocardiography) allows the internal structures of the heart and surrounding vessels to be directly viewed and heart function to be assessed. Adult heartworms may be seen, but most infected cats have a low number of worms, so this does not always occur.

The heartworm parasite often has bacteria called Wolbachia living inside it. PCR tests are being studied to determine their usefulness in finding these bacteria, as this may provide a way to confirm that heartworms are present with the cat.

How is a heartworm diagnosis confirmed in cats? 

A diagnosis of feline heartworm infection is confirmed when both the antibody and antigen tests are positive, but many infected cats will not test positive on both tests as neither test is 100% reliable. The two tests may give contradictory results or be inconsistent with the signs the cat is showing. The challenges with diagnosing heartworm disease in cats unfortunately mean that many will go undiagnosed.

What is the treatment for heartworm disease in cats?

Since adult heartworms are relatively short-lived in the cat, infection is often self-limiting and frequently resolves spontaneously. Treatment is usually limited to drugs that control the inflammation triggered by the presence of worms in the lungs, and other medications to make it easier for the cat to breathe (e.g., bronchodilators).

In serious infections, surgical removal of adult worms from the heart may be the best option. Critically ill cats may need to be hospitalized for further care. Cats are usually not given drugs to kill adult heartworms, unlike dogs, because this treatment can cause serious and potentially fatal complications.

How can I prevent heartworm disease in my cat?

The American Heartworm Society recommends year-round heartworm preventives for every cat throughout the United States. In Canada, your veterinarian can advise you on the risk for your cat and whether a preventive program is appropriate.