You may have heard that heartworm disease isn’t common in cats, especially if they live indoors. You might also think it’s easily treatable. But that’s just not the case. The truth is, heartworm disease in cats can be very serious — even deadly. Unfortunately, it’s often misdiagnosed as a respiratory issue because it can affect the lungs and many other systems in the body. While the good news is you can avoid heartworm disease altogether by using a heartworm preventative, it’s still important to understand the differences between the disease and other respiratory problems.

Symptoms of Heartworm Disease in Cats

The hard part with heartworm disease is that signs can range from very subtle to dramatic and are often non-specific. Some cats show no symptoms at all, or the first sign may be acute respiratory collapse and death. In other cases, you may see:

  • Coughing
  • Difficulty breathing
  • Asthma-like attacks
  • Occasional vomiting
  • Poor appetite
  • Weight loss
  • Difficulty walking
  • Possible fainting
  • Seizures

Based on the above list, it is easy to see why feline heartworm disease and other respiratory diseases, such as asthma and bronchitis, cannot always be distinguished by clinical signs alone. 

How is Feline Heartworm Disease Diagnosed?

Unlike dogs, cats with adult heartworms usually have only two to three adult worms present. A small number of cats can also spontaneously get rid of the adult worms on their own, but there is often damage left to the heart, lungs and other organs. Plus, many cats affected by heartworms have no adult worms. Instead, they have many immature worms invading their lungs causing severe inflammation and lung damage. Meanwhile, since many heartworm tests only test for adult worms, false-negative test results are common in cats with heartworm disease, and infected cats often go undiagnosed even when testing is done.

The antigen test is available at most veterinary clinics. However, while this test gives quick results, within 10 minutes, it is much less accurate for an infection if there are few adult worms present or only immature worms present. 

So how can you tell the difference between feline heartworm disease, asthma, and other respiratory conditions if the test comes back negative, but your cat has symptoms? By combining multiple tests, such as both heartworm antigen and antibody testing (requiring your veterinarian send a blood sample to a reference laboratory), chest radiographs, heart ultrasound, eosinophilic counts, and clinical signs, veterinarians can often differentiate between heartworm disease and other respiratory illnesses. Again, no test is 100% reliable, and it can still be challenging to distinguish between the two.

The Truth About Heartworm Disease in Cats
Fleas, Ticks, & Heartworms
The Truth About Heartworm Disease in Cats
Take the Bite out of Mosquitoes and Heartworms
Fleas, Ticks, & Heartworms
Take the Bite out of Mosquitoes and Heartworms

Why Heartworm Disease Often Looks Like a Respiratory Issue

Immature heartworms can cause a condition known as heartworm-associated respiratory disease (HARD), causing damage to the lungs and circulatory system. This damage occurs well before clinical symptoms are evident[1]. And even when symptoms appear, they may not point directly to heartworms as the cause.

In cats, heartworms have also been known to cause blood clots and inflammation in the lungs when the adult worms die. Since cats, more commonly than other animals, can keep inflammatory cells in their lungs, they have an exaggerated reaction to pulmonary (a.k.a. lung) parasites[2]. This kind of allergic reaction is similar to what occurs with asthma. When cats suffer from asthma, a flare-up results in coughing and difficulty breathing. 

The cough associated with both asthma and HARD can be constant or cyclic, and often appears to look like vomiting a hairball. Cats can also have difficulty breathing. Respiratory symptoms such as these can also be the sign of other disease processes, complicating diagnosis.

Person putting preventative solution on cat

How to Help Prevent Feline Heartworm Disease

Keeping your cat inside does not mean they’re protected against heartworms. One study of cats diagnosed with heartworm disease showed that 27% were indoor-only[1]. The American Heartworm Society has estimated the nationwide average incidence of heartworm in cats to be about 12%[2]

Luckily, it’s simple to keep your cat safe from feline heartworm disease and potential misdiagnosis. The best thing to do is give your cat a monthly heartworm disease preventative. With the potential for heartworm disease off the table, if your cat develops respiratory symptoms, treatment can be more effectively directed to the true respiratory cause. You can make it easy. Buy 12 months of prevention and get your cat tested every 12 months too. It’s extra easy when your heartworm preventive is part of a parasite prevention treatment that also covers fleas, ticks, and gastrointestinal parasites. Make this a priority in your next conversation with your veterinarian. Prevention is simple, but the lack of prevention can be costly in many ways.


Kristine Smith, DVM, DACZM

Dr. Kristine Smith received her DVM from Tufts University, School of Veterinary Medicine and is a board-certified specialist in zoological medicine. Dr. Smith has 20 years of global experience in research of veterinary and zoonotic infectious disease, working with government, non-profit, and private sectors, in addition to years spent in veterinary clinical practice. Dr. Smith currently serves as Zoetis Petcare HQ Medical Lead (Preventatives). In her spare time, Dr. Smith loves to travel and spend time with her family, 3 cats, and Australian Shepherd mix.

IMPORTANT SAFETY INFORMATION: See Prescribing Information. The safe use of Revolution Plus has not been established in kittens less than 8 weeks old or in breeding, pregnant or lactating cats. Reported side effects in clinical trials included lethargy and anorexia. Use with caution in cats with a history of neurologic disorders. Revolution Plus contains sarolaner, a member of the isoxazoline class, which has been associated with neurologic adverse reactions, such as tremors, ataxia, and seizures in cats with or without a history of neurologic disorders. In humans, Revolution Plus may be irritating to skin and eyes.

Revolution Plus is a simple-to-apply, quick-drying, small-volume, monthly topical solution that protects against fleas (Ctenocephalides felis), ticks (lone star tick [Amblyomma americanum], black-legged or deer tick [Ixodes scapularis], Gulf Coast tick [Amblyomma maculatum] and American dog tick [Dermacentor variabilis]), ear mites (Otodectes cynotis), roundworms (Toxocara cati), hookworms (Ancylostoma tubaeforme), and heartworms (Dirofilaria immitis) for cats and kittens as young as 8 weeks of age and weighing 2.8 pounds or greater.

  1. Myths About Feline Heartworm Disease. Vetstreet Accessed April 9, 2021
  2. Feline Heartworm Disease: Fact or Fiction? MAY/JUNE 2017, PARASITOLOGY. Accessed April 9, 2021