Wednesday, March 24, 2010 2:43 PM

Can Heartworm Cause Heart Problems in Cats?

Heartworm is a disease that is mostly associated with dogs. Actually, it is an infestation rather than an actual disease. If the heartworms (yes, they are actually roundworms!) proliferate to the point that they begin to clog heart blood vessels, symptoms of heart disease occur. Since the parasite is transmitted by mosquitoes, the worms' young, or microfilaria, circulate in the animal's bloodstream in order to be picked up by the next biting mosquito.

I remember learning about heartworm in my college parasitology course. The text showed a necropsied dog heart, opened up to show all those gross worms. They even managed to find a cat heart to photograph in this manner, as well. While it is entirely possible for cats to contract heartworm, in reality, it is seldom seen. Why is this?

Well, for one thing, animal doctors aren't looking for it. When I worked for veterinarians, this time of year was a real money maker for them. We actually had a name for it: Heartworm Season. For a set fee, clients would bring in their dog, have us do a quick blood test to look for microfilaria, and, if all was well, leave with a large supply of heartworm preventative tablets. If the blood test was positive, it entailed intravenous treatments of an arsenic-based drug to kill the worms before the preventative treatment would be prescribed.

I saw a handful of dogs with heartworm in the years I worked for vet clinics, and they were usually large dogs. Smaller dogs, with smaller hearts, seemed more resistant. Cats were never tested, and I never heard of one that was diagnosed with heartworm disease. The dogs we treated did not exhibit any symptoms, either; we found the infestation upon examining the blood. Very possibly, they may never have exhibited symptoms, or their immune systems may have taken care of the problem before the heart was even involved.

Once again, economics was very probably the impetus behind heartworm testing and prevention, rather than any real health threat. If a cat actually did have heartworm, I would imagine it to be secondary to an existing heart problem. Heart disease would not only depress the animal's immune system so as to allow these parasites to gain a foothold, but would present an enlarged heart with slower pumping action. Since these worms can grow very large, a small animal just doesn't make the best host. There would have to be extenuating circumstances for this scenario to exist.

By the way, unless you live near wetlands and have large dogs that are outside most of the time, you might want to reconsider preventative heartworm medication. The idea behind it is to keep a very low level of a drug lethal to microfilaria circulating in your dog's blood for 6-8 months of the year. Talk to your vet, but also do a little research. The drug may not be doing your dog's circulatory system any good, either.
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Amanda has worked with animals for many years and has always had cats in her life. She lives in Massachusetts with her husband and two excellent cats.
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