Heartworm Disease – It’s Not Just for Dogs Anymore

Kia Benson, DVM
Associate Veterinarian, Clinical Toxicology

Heartworm disease has been a topic of discussion between veterinarians and dog owners around the country for decades. Heartworm disease has been diagnosed in all 50 states, and is a very serious, progressive and potentially fatal disease. Heartgard, Trifexis, Tri-Heart, Revolution, Interceptor and other heartworm prevention products are specifically designed to protect dogs from heartworm disease.

But when was the last time that your veterinarian discussed Heartworm disease as a risk for your CAT?

Transmission of Heartworm Disease

Dogs (and other canids like wolves, coyotes, and foxes) are considered to be a natural host for heartworms. The cycle begins when an uninfected mosquito bites an infected dog and takes a blood meal. The blood from the infected dog contains microfilaria (or “baby worms”) that are circulating thorough the infected dog’s blood. The microfilaria mature to larvae (or “immature worms”) inside the mosquito. When another, uninfected dog is bitten by that same infected mosquito, larvae are transmitted to the dog through the mosquito bite wound itself. The larvae invade the host, and in time mature and become very long and thin adult heartworms. They live in the heart, lungs and associated blood vessels. Adult heartworms mate and produce offspring, which can then be picked up by an uninfected mosquito biting the host dog to continue the cycle.

Of course, infected mosquitos don’t deliberately select only dogs to bite. And we all know how mosquitos do not respect the boundaries of doors and windows that separate our indoor pets from the outside! They are opportunistic pests and will bite cats as well as dogs, whether the cats are outdoors (supervised or not) or indoors. It just takes 1 bite from an infected mosquito to infect an animal, be a dog or a cat.

Cats and Heartworm Disease

Heartworm disease in cats is very different from that seen in dogs, which leads to the disease being vastly underdiagnosed in cats.

Cats are atypical hosts for heartworms. Since a cat is not an ideal host, many but not all immature larvae die prior to attaining adulthood. However, even if no adult worms are present, cats can be affected by heartworm disease. Both immature larvae and adult worms cause a great deal of lasting damage to a cat’s organs and immune system as they travel through the cat’s body.

Signs of Heartworm Disease in Cats

The primary damage in a cat occurs in the lungs as well as the heart. Damage to the lungs – called Heartworm Associated Respiratory Disease (HARD) – can result in coughing plus asthma-like symptoms of wheezing and difficulty breathing. HARD can be a permanent consequence of heartworm infection.

In addition to lung damage, intermittent vomiting, lack of appetite, weight loss, difficulty walking, seizures, or fainting episodes can be seen in heartworm affected cats. Heartworms in cats may even migrate to the brain, eye and spinal cord, causing organ damage as they travel. Sometimes the first and only sign seen is the sudden collapse or sudden death of an infected cat.

Coughing and asthma-like attacks are NOT normal in a cat. Contrary to popular opinion, cats do NOT regularly cough secondary to hairballs. Hairballs are vomited up from the gut, not coughed up from the lungs. Unless your cat is a long-haired cat or a cat who grooms another cat in the home in addition to itself, hairballs are actually uncommon.

Coughing in a cat could indicate heartworm disease or asthma, another underdiagnosed illness in cats. It can be difficult to distinguish between a cat who is trying to vomit up a hairball and one who is experiencing a coughing episode. If it looks like the cat is trying to produce a hairball and nothing comes up – it’s quite likely that the cat is really coughing instead.

Testing Cats for Heartworm Disease

Heartworm infection in cats is harder to detect than in dogs. The test in dogs detects the presence of heartworm proteins (antigens). Because cats are much less likely than dogs to have adult heartworms, just checking for the presence of these proteins is not enough. Both an antigen test and an antibody test – which detects exposure to heartworm larvae – are needed. Additionally, chest x-rays can check for lung damage caused by heartworms, and an ultrasound may be able to visually reveal the presence of heartworms in a cat’s heart or pulmonary blood vessels.

Treating Cats for Heartworm Disease

There is no medication that can be safely used to treat heartworm disease in cats. The medication used to treat heartworm disease in dogs can be fatal to a cat. Nevertheless, cats with heartworm disease can be helped. Mild cases can be treated with steroids to decreased inflammation caused by the presence of heartworms, and the affected cats can also be monitored with chest x-rays every 6-12 months. Cats suffering from more severe cases of heartworm disease may need treatment with cardiovascular drugs, intravenous fluids, antibiotics and oxygen.


Like with many diseases, the key is year-round prevention for both indoor and outdoor cats – even in regions of the US where winters are cold. Differences in the duration of the mosquito season from the north of a state and to the south of that state (micro-climates) can affect the exposure risk for a cat.  Plus, mosquito species are constantly changing and adapting to cold climates. Some species of mosquito are now even successfully overwintering indoors.

Remember those heartworm prevention products for dogs that I mentioned earlier, like Heartgard, Revolution, etc.? Versions of those same heartworm prevention products also exist now for cats, and can prevent cats from suffering permanent organ damage and possible death secondary to heartworm disease.

Only 1 in 10 cats in the US is currently on Heartworm Disease prevention. Isn’t your cat worth protecting?


  1. American Heartworm Society; https://www.heartwormsociety.org/heartworms-in-cats
  2. Cornell Feline Health Center; http://www.vet.cornell.edu/FHC/health_information/Heartworm.cfm
  3. Feline Nutrition Foundation; http://feline-nutrition.org/health/another-furball-it-might-be-feline-asthma