Heartworm in Cats: Why Prevention Matters More Than Treatment

By Tailwisp Editorial TeamUpdated October 20, 20264 min read

Heartworm has a reputation as a dog disease, yet cats can get it too, and in cats it can be harder to detect and impossible to cure. A single infected mosquito bite is enough. For this reason, vets in many regions recommend prevention for all cats, including those that never go outside.

How cats get heartworm

Heartworm disease is caused by Dirofilaria immitis, a parasite spread by mosquitoes. A mosquito that bites an infected animal picks up immature worms, and passes them to the next animal it bites. In cats, most larvae die, but a few mature, and move into the blood vessels of the lungs, and sometimes the heart. Cats are not natural hosts, so they usually have only a few worms, which live a shorter time, around two to three years, than in dogs.

Why it is different in cats

In dogs, heartworm is mainly a disease of the heart. In cats, the main target is the lungs. Even immature worms can cause an inflammatory reaction, called heartworm-associated respiratory disease, or HARD, which can look like asthma. When adult worms die, they may cause sudden, severe lung damage, or sudden death. See feline asthma.

Signs

Many cats show no signs, or only mild ones. Signs may include:

• Coughing, wheezing, rapid or labored breathing.
• Vomiting, not related to eating.
• Reduced appetite and weight loss.
• Lethargy.
• Fainting, or collapse.
• Seizures, or neurological signs in rare cases.
• Sudden death, sometimes without prior warning.

See cat emergency warning signs.

Diagnosis

Diagnosis is harder than in dogs. Because cats have few worms, standard antigen tests can miss infections. Vets often use a combination of antigen and antibody tests, along with chest X-rays, ultrasound of the heart and bloodwork. A positive antibody test shows exposure, not necessarily active disease.

Treatment

There is no approved treatment to kill adult heartworms in cats. The drugs used in dogs can be dangerous for cats. Management is directed at the signs:

• Steroids, to reduce lung inflammation.
• Bronchodilators and oxygen, for breathing difficulty.
• Supportive care, including fluids and anti-nausea medication.
• Regular monitoring with X-rays or ultrasound.
• Surgery, in rare cases, to remove worms from the heart.
• Prevention, to stop new infections.

Many cats overcome infection, as worms die naturally within a few years, but the lungs may be permanently affected.

Prevention

Preventive medication is the safest option. Monthly products, applied to the skin or given by mouth, are approved for cats, and many also protect against fleas, ear mites and some intestinal worms. Ask your vet which is best for your cat and region. Year-round use is commonly advised, because mosquitoes can survive in homes, and climates change. Reduce mosquito exposure by using screens, avoiding standing water and keeping cats away from mosquito-heavy areas.

See fleas on cats and worms in cats.

Indoor cats

Studies show that a notable share of cats with heartworm lived indoors. Mosquitoes enter through doors and windows. Do not assume your cat is safe. Discuss prevention with your vet.

Kittens

Preventives can usually be started early, depending on the product. See kitten first vet visit.

Why prevention beats treatment

For dogs, there are approved treatments. For cats, no approved drug kills adult heartworms, and the available options carry risk. The infection can also cause lung disease even when the worms are not mature. Monthly preventives are safe, effective and often cover other parasites. The cost of prevention is small compared with the cost of treating complications, or the risk of sudden death.

Testing before and during prevention

Your vet may recommend a heartworm test before starting prevention, particularly in older cats or those that have lived in high-risk areas. Annual testing is advised by some experts, because no test is perfect. Discuss what is right for your cat and region.

Mosquito control at home

Reduce standing water, use window screens, and repair gaps around doors. Mosquitoes breed in small amounts of water, such as flowerpot saucers and gutters. Keep indoor cats away from open doors at dusk and dawn. These steps help, but they do not replace preventive medicine.

Living with a cat that has heartworm

Cats with heartworm need rechecks every few months, with chest X-rays to monitor lung changes. Keep your cat calm and indoors. Give steroids as prescribed, and watch for breathing changes. Call your vet if breathing worsens, since this can happen suddenly. Many cats recover, but ongoing monitoring is needed.

Travel and moving

If you move or travel with your cat to a region with more mosquitoes, ask your vet whether prevention should change. Carry records of the last dose. If you rescue a stray or adopt a cat from a shelter in a warm region, ask whether it has been tested or treated. Keeping prevention up year-round means your cat stays covered even if you forget a month or the weather changes.

When to see a vet

See a vet if your cat coughs, wheezes, vomits repeatedly or has trouble breathing. Use the US vet directory to find a clinic.

This article is general educational information, not veterinary advice. Every cat is different. If your cat is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.