Cats and Heartworm Disease


By Rachel Walton, DVM

Heartworm disease is best known for infecting dogs, but it can be a problem for cats too. As in dogs, heartworms are transmitted by feeding mosquitoes. Heartworm larvae mature in the pet’s body and migrate to the right side of the heart and the large vessels of the lungs. While there are some similarities between dogs and cats, there are also many important differences.

All About Feline Heartworms

Infection rates in cats are lower than in dogs, but the disease may be more common than previously estimated. Studies have shown that up to 10 – 14 percent of shelter cats are infected. Because mosquitoes transmit the disease, being an indoor only cat does not prevent a cat from getting infected. For cats, the likelihood of infection is directly related to the number of infected dogs in the area.

Cats are not a natural host for heartworm, which means that very few worms are able to develop to adults. A dog can have 50 or more worms when infected; cats typically have less than 6 worms. But because the feline heart is so small, even a single worm can cause major problems. While it is the same parasite in both species, feline heartworms are smaller in size and have a shorter life span than canine heartworms. In cats, heartworm disease is a result of the inflammation caused by the worms. For this reason, cats can have the disease in the absence of adult heartworms. Even the small larvae can create enough inflammation to wreak havoc for a cat, while in dogs, the disease is mostly a result of the physical obstruction of blood flow through the heart and lungs by the presence of the worms.

What are the symptoms?

Cat playing

Cat playing

Heartworm disease in cats is primarily a respiratory disease. Symptoms include coughing, wheezing, difficulty breathing, lethargy, weight loss, or vomiting. The disease is often misdiagnosed as feline asthma, because the symptoms can be similar. Some cats have few or mild symptoms, while others have severe signs or even sudden death.

How is heartworm disease diagnosed?

Diagnosing heartworm in cats can be challenging. In dogs, a blood sample is tested for proteins that can only be found in the body of the adult female heartworm. In cats, adult female worms are not always present, so this test has limited value. A combination of a physical exam, blood tests, x-rays, and an ultrasound of the heart may all be needed to determine whether or not a cat has been infected with heartworms.

Is feline heartworm treatable?

Unfortunately, there is no approved drug therapy for eliminating adult heartworms from infected cats. The medication used to treat the disease in dogs is not safe for cats. Nevertheless, cats with heartworm disease can still be helped with good veterinary care. The symptoms of the disease can be managed using anti-inflammatory medications. Severe cases may need supportive care in the hospital. Some infections can resolve on their own as the cat’s immune system kills the worms; however, these cats can still be left with lasting respiratory system damage requiring management. Because there is not a safe or effective treatment, prevention is the best strategy.

Multiple types of heartworm preventatives are available for cats. These medications are safe and effective and come in both spot-on and pill form. Preventatives work by eliminating the larval stages of the heartworm parasite and are given once a month. It is important not to give your cat the same medicine your dog gets, as their needs are different. Talk with your veterinarian about your cat’s risk of heartworm disease and other parasites and the type of prevention that is best for your situation. Many heartworm medications have the added benefit of preventing other parasites, including fleas, roundworms, hookworms, and ear mites.


Rachel Walton is a small animal veterinarian at University Veterinary Hospital and Diagnostic Center in Salt Lake City. She received her Doctor of Veterinary Medicine degree from Colorado State University in 2007. She is the owner of 3 cats and 1 dog, 2 human children, and 1 husband.