Heartworm disease in cats

Everyone knows how important it is to keep dogs on monthly heartworm preventative. But can cats get heartworm? Let’s see…..

For starters, heartworm is transmitted the same way in dogs and cats. A mosquito flies around, sucks on an animal with heartworms, ingests the larvae from that pet’s blood, and then processes it into a more advanced larval stage. The mosquito then bites your dog or cat, introducing the infective larvae. If your pet is not on preventative, this larvae will become adult worms in 6 months.  These worms live in the heart and, primarily, in the pulmonary artery (the vessel that pumps blood from the heart into the lungs). On reaching adulthood, female worms begin to release a protein that is detected in our modern day heartworm tests.

Fortunately, the vast majority of dogs with heartworm disease are diagnosed before they develop symptoms.  Cats, however, tend to show symptoms much sooner than dogs. Why? Heartworms are the same size in all pets. They can reach 5-10 inches long! In a lab with a big heart, there is a lot of room for worms to move around. A cat with a tiny heart can be very sick with only one worm!

So, we all know dogs can get heartworm. We didn’t know cats could get heartworm until about 15 years ago. Good news for cats: heartworms for some reason do not like their bodies (cats never cooperate, do they?), so it’s harder for heartworms to establish and become adult worms in cats. Dogs, on the other hand, pretty much roll out the red carpet and welcome any and all heartworms!

With cats being mainly indoors, and with their bodies not being an ideal environment for heartworms, the risk of heartworm infection is relatively low. That’s the good news.

The bad news – if you’re cat is unlucky enough to get heartworms, there is no treatment, and it is fatal. Often the worms cause an inflammation throughout the cardiopulmonary (heart-lung) system. Some cats present with a cough, while others present with lethargy, heavy breathing, or even seizures! We can address this inflammation with treatment, but we cannot kill the worms like we can in dogs. Unfortunately, the drugs that kill heartworms also kill cats!

Prevention is the best option. I personally like Revolution, because it prevents heartworm, as well as fleas, ear mites, and intestinal parasites. And it’s a liquid applied to the skin, which makes it simpler, although I have some feline patients who think it’s war every time they hear the Revolution box open!

I recently started carrying a test for heartworm disease that is specific to cats. Remember how cats often have room for just one worm? And how the traditional heartworm test detects female worms? What if your cat has a boy worm? That test is only 50% accurate!

The new, cat-specific test detects antibodies (the body’s immune response) against heartworm. Like all antibody tests, it does not 100% mean your cat has adult heartworms. It means your cat was infected with heartworm larvae significant enough for the immune system to mount a major response. If a cat has a cough or other suspicious signs, I like to do the test to rule heartworm in or out. However, we have to put all the pieces together to get a diagnosis, not just one test (although it is helpful!)

Again, while we cannot kill the heartworms in cats, we can manage the inflammation and irritation that it causes, bringing some relief to symptoms. Therefore, in a sick cat, it’s a good idea to know his or her heartworm status.


Posted in Feline specific, General health, Parasites.

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