All About Heartworms In Cats

Heartworms in cats can be deadly, so find out what you can do to keep your cat safe.

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Heartworms are more difficult to detect in cats than in dogs. Sushaaa/iStock/Thinkstock
Dr. Jerry Murray

Heartworm disease is a serious and potentially fatal disease for cats, dogs and ferrets. It has been reported in all 50 states and in numerous countries all over the world, according to the American Heartworm Society. Heartworms are actual worms (Dirofilaria immitis) that can live in the lungs and heart of cats. Adult heartworms can grow to be around 8 inches long and cause damage to the lungs and heart of cats. Cats can get heartworms from mosquito bites just like dogs do. Mosquitoes are not picky and will bite cats or dogs. Mosquitoes can easily bite outdoor cats. Indoor cats can also be bitten by a mosquito that gets inside your home or if your cat manages to sneak outside. Thus indoor cats are at risk just like outdoor cats.

The Spread Of Heartworm In Cats
Heartworms are transmitted directly by mosquitoes. Mosquitoes pick up microfilaria (“baby” heartworms) when they bite a dog or a wild canine like a fox, coyote or wolf that already has heartworms. The microfilaria mature inside the mosquito to the infective larvae stage (“juvenile” heartworms) in roughly two weeks. When the mosquito bites a cat, it will deposit the infective larvae. The larvae will travel to the heart and blood vessels of the lungs in just two to three months. It will take six to seven months for the larvae to mature into adult heartworms. Mature adults usually do not produce microfilaria in cats, but they can live for two to four years. They will cause damage to the cat’s lungs, blood vessels and heart.

An infected cat cannot pass heartworms directly to another cat or dog. This is because the microfilaria, which cats rarely produce, require a mosquito to develop into the infective larvae stage.

Cat Heartworm Symptoms
Cats with heartworms can have problems at two different times. When the larvae stage enters the cat’s heart and lungs, or when the larvae die, it can produce a severe inflammatory reaction in the lungs and blood vessels in the lungs. This can produce heartworm associated respiratory disease (HARD). Signs at this time can include coughing, vomiting, lethargy, difficulty breathing, wheezing, weight loss and sometimes sudden death. This is often misdiagnosed as feline asthma or allergic bronchitis. Clinical signs usually improve as the larval stage matures to the adult heartworm stage, but the damage to the lungs remains.

When the mature adult heartworms start to die, the dead heartworm fragments in the lungs can produce severe inflammation again. This will cause clots in the lungs and can be fatal. Clinical signs at this time include coughing, vomiting (which frequently has blood in it), difficulty in breathing, anorexia, weight loss, collapse and sudden death.

Diagnosing Heartworms In Cats
Diagnosis of heartworms in cats is more difficult than it is in dogs. Cats typically have less than six adult heartworms and frequently have infections with just male heartworms. This makes it more difficult for the common antigen test, which detects protein from only the adult female heartworms, to generate a positive result. In addition, cats can have clinical signs from the larvae stage, but the antigen test will not turn positive during the larvae stage. The antigen test has to have adult female heartworms, and adults are not yet present when the larvae are causing the initial problem in the lungs.

The second type of test is an antibody test. This will detect an antibody that the cat’s immune system produces in response to the larvae and adult heartworms. Unfortunately, the antibody test is not very accurate. Roughly 20 to 25 percent of the cats with adult heartworms will have a negative antibody test. On the other hand, there can be antibody produced even if there are only male heartworms present and early in the disease when there are just larvae present. Antibodies can still be present for a few months after all the adult heartworms die.

Neither the antigen or antibody test is very sensitive in cats, so radiographs of the chest and ultrasound of the heart are often used to help diagnose heartworms in cats. Radiographs may provide strong supportive evidence of heartworms, showing enlargement of the blood vessels in the lungs and inflammation in the caudal lung lobes. Ultrasound examination of the right side of the heart and the arteries of the lungs by an experienced sonographer can often visualize the heartworms.

Heartworm Treatment For Cats
There is no approved medication for the treatment of adult heartworms in cats. Melarsomine is approved for use in dogs, but it is actually toxic to cats and thus not recommended. The goals of treatment are to reduce the inflammation in the lungs and to treat the clinical signs that develop. Prednisolone (cortisone) is recommended to reduce the inflammation in the lungs. The antibiotic doxycycline is often used to help eliminate Wolbachia, a bacterium inside the heartworm that can add to the inflammation in the lungs. In addition to prednisolone and doxycycline, cats with severe signs of respiratory distress may need oxygen therapy, bronchodilators and IV fluids.

Preventing Heartworms In Cats
Currently there is no safe medication to treat adult heartworms in cats, so it is vital to prevent heartworms. Several products are available to prevent heartworms in cats, so cat owners can choose from pills, topicals or injections. There are monthly flavored chewables and tablets for prevention of heartworms. Some cats can be rather difficult to give oral products to, so another option might be better. There are two liquid products that are applied directly to the cat’s skin. These are both applied monthly. In addition, these topical preventives also kill fleas, ear mites and the two common intestinal parasites of cats (hookworms and roundworms). Injections are given every six months. All of these products work well, so talk to your veterinarian about which heartworm preventive is best for your cat and keep your cat on it year-round.

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Article Categories:
Cats · Health and Care