Most people think of heartworm disease as a problem that affects only dogs, but even though cats are more resistant hosts to heartworms, and they typically have fewer and smaller worms than dogs with a shorter lifespan, it is considered a more serious threat in cats and can lead to significant pulmonary damage and even sudden death.

What causes heartworm disease?

Heartworm disease is caused by a parasite that is carried by mosquitoes, and cats become infected when a mosquito bites into a cat for a blood meal and deposits heartworm larvae into the cat’s bloodstream.  These larvae migrate and mature through several lifestages into adult worms.  At about 3-4 months, they usually settle into the arteries and blood vessels of the cat’s lungs, where they continue to mature into adult worms for another 4-5 months.  Worms do not have to develop into adults to cause symptoms.

Which cats can be affected?

While outdoor cats are more susceptible, even indoor cats can be affected (all it takes is one mosquito bite).  Studies have shown infection rates as high as 10-14% in endemic areas.

What are the clinical signs of heartworm infection?

Symptoms can be non-specific and are often similar to those of other feline diseases.  Affected cats may exhibit general signs of illness such as intermittent vomiting, lack of appetite, coughing, and asthma-like signs such as difficulty breathing or wheezing.  Some cats may show acute symptoms, often related to the organs where the adult worms are thriving.  Cats with an acute onset of symptoms may die quickly without allowing sufficient time for diagnosis or treatment.

How is heartworm disease diagnosed?

Heartworm disease in cats is much harder to diagnose than in dogs, once again proving the old adage that cats are not small dogs.  Physical examination will often be non-specific.  Further diagnostics may include x-rays, echocardiogram, and blood testing.   Diagnostics have limitations, and sometimes, even a negative test cannot rule out infection.

How is heartworm disease treated?

Currently, there are no medications approved in the United States for treatment of feline heartworm disease.  Cats who don’t show any clinical signs will often simply be monitored periodically and given time for a spontaneous cure.  Monitoring through x-rays every 6-12 months may be all that is needed.  If there is evidence of the disease in the lungs or blood vessels, treatment is generally focused on supportive care, sometimes using gradually decreasing doses of prednisone, a steroid.  Cats with severe manifestation of infection may require additional supportive care such as intravenous fluids, oxygen therapy, antibiotics, cardiovascular drugs, and restricted activity through cage confinement.

Can heartworm disease be prevented?

Currently, there are four heartworm preventive products approved for use in cats:  Heartguard for Cats (Merial), Revolution (Pfizer), and Advantage Multi for Cats (Bayer).  Heartguard is taken orally, Revolution and Advantage are topical products.  All of these products come with known side-effects, and deciding whether to use them for your cat will require an informed risk assessment in conjunction with your veterinarian.   It is recommended that cats are tested for antibodies and antigens prior to beginning use of these preventatives.  Never give heartworm or any other parasite prevention product for dogs to cats.

As with all parasites, it is believed that a healthy immune system makes cats more resistant to them.  A healthy diet is key to a healthy immune system.  Feeding a species-appropriate grain-free canned or raw diet may help prevent heartworms and other parasites.

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11 Comments on Feline Heartworm Disease

  1. Esme, it might be worth doing the antibody/antigen test for Penelope – it’s a simple blood test. Even though it won’t change what you’re doing as far as treatment, it might be good information to have for future monitoring.

  2. Thanks for this-we are still trying to figure out Penelope’s asthma and this week the doctor suggested it may be heartworm-given the treatment is the same I am not sure if it is worth subjecting her to treatment.

  3. Thank you for the clarification on testing, Dr. Lopez – it can be very confusing when it comes to cats, it’s a lot more straightforward in dogs.

  4. Teri, interesting you mention the tests run on your cat. One BIG difference between cats and dogs regarding heartworms and their testing is that dogs usually are infected by many heartworms vs cats usually only have 1 or 2 worms. What does this mean for the test? Well an antigen test may be negative in a cat that has heartworms as 1 or 2 worms is not enough to produce a positive. The antibody test is better in cats as 1 or 2 worms are enough for the body to make antibodies and therefore these will be detected. Therefore, it would be no surprise to have a cat infected with heart worm test negative on the antigen test and test positive on the antibody test! There are a few other reasons as well that I will send to Ingrid and maybe she can include in a future newsletter. Yes, cats ARE NOT small dogs! And as Ingrid mentioned always weight the risks vs benefits for your cat with the guidance and expertise of your veterinarian. That is what we are here for.

  5. Thanks for sharing your decision process, Terri – that’s exactly what I’m talking about when I say it’s an individual decision based on weighing risks and benefits.

  6. I am a veterinary assistant at an all feline practice and have 5 indoor only cats, live in Northern Virginia, and on a lake and occasionally I will find 3 or 4 dead mosquitos on my stovetop as I use the light as a nightlight. I would always test my cats once a year but never put them on preventative. But this last year, one of my cats tested HWAb positive and he had a slight cough–hairballs? not typical in a Cornish Rex but that is what I though. Nope, he has some lung changes and while his HWAg was negative, the changes in him were enough reason for me to put all my cats on Revolution. It was a worry to me to watch him for other signs that I didn’t want to go through that with any of my cats. That’s how I came to my decision…

  7. Ingrid I have enormous respect for your opinion and expertise and I thank you for verifying what my instincts were telling me. I chose to not use it after only using it for one month and at this point your response is all of the confirmation that I need for that decision!
    Once again thanks sooo much for posting this informative piece and for your timely, caring and informative response!

  8. Marg, I’m glad you found the article interesting.

    Caren, as with most decisions involving whether to use a medication or other products for our pets, it becomes a case of weighing risk versus benefit. I gave Heartguard to Feebee for a brief period of time in the late 90s, when Heartguard for Cats first came out, and discontinued it when he started to undergo chemotherapy for lymphoma (no causal link between the Heartguard and his cancer, I just didn’t want to bombard his immune system with anything he didn’t absolutely need at that time). I’ve chosen not to use any of the products on my cats since. For me, the potential side effects of the chemicals, especially in the topicals, are not worth the protection they provide. But it is a decision each individual has to make for their cats – just because I’ve chosen not to use the products doesn’t mean it’s the right decision for someone else. Talking it over with your vet can help you make the decision that’s right for you. I hope this helps!

  9. Ingrid thanks so much for this most important posting.
    I had my Bobo for 18 years (he was predominantly an indoor cat but occasionally went outside on his leash to eat grass) and I never gave him heartworm meds.
    When we adopted Cody (who is an indoor cat) our vet suggestion we use the topical treatment which I have only used once and then discontinued it.
    We do have a dog as well (we live on the 2nd floor of a condo) and yes I do worry about “one” mosquito infecting my cat.
    Should I revisit the heartworm issue with my vet once he finishes the antibiotics he is on for the “rash” on his head?
    Thanks Ingrid!

  10. That is very interesting about the heart worms in Cats. At one time, I was giving some of my cats the Revolution since it took care of the fleas and the heart worms. Thanks for this good information.

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