Wednesday, April 1, 2009

Heartworm Infections in Dogs and Cats

The diagram to the left (still working on images on my posts but I seem to be getting better at it) shows the life cycle of the heartworm parasite in the dog. There is an important distinction between dogs and cats but we'll get to that later.

Remember posts ago, I said that I like to have my pets on year round parasite prevention. It's not for the heartworm, because as you can see, the vector/intermediate host is the mosquito and there are none around in the cooler months here in NY. Those of you in the south need to talk to your vets about heartworm season. We keep our pets on parasite control all year round because of intestinal parasites.

The infected mosquito bites our pet, our us (although people are rarely infected as our immune systems take care of the problem) and the parasite goes through development and matures to an adult. Males and females live in the heart and the large vessels of the lungs. They mate and baby worms (microfilaria) are released into the blood stream to be picked up by the next mosquito and the cycle continues. The time from infection to microfilaria in the blood is about 7 months, so when your pet is tested this spring (and the American Heartworm Society recommends testing all dogs yearly even if they are on preventatives) we are really looking for last year's infections. In cats, the worms do not really mature as well and we often see single sex infections or no adults at all, but in a moment we'll discuss why this disease is still important in our cats.

In dogs, the most common clinical signs that we see are those associated with congestive heart failure. Large masses of worms are in the heart and obstruct blood flow. I thought I could post a photo of a heart with worms, but It seems that if I try to upload more than one image per post it just stacks them at the top of the post and it doesn't look right.
In cats, we see respiratory disease or sometimes just sudden death. The syndrome was recently identified and named Heartworm Associated Respiratory Disease or HARD. Many cats with chronic coughs have had previous infections with heartworms.
In both species it is easier to prevent disease than to treat them. We have already reviewed different medications and your veterinarian can recommend one that he or she feels is best for your pet.
There is no treatment for cats. We have to wait for the parasites to die and hope that they don't kill the cat first. Seems like a good reason to put them on prevention. We did a massive screening test on our feline patients about 5 years ago, sponsored by Pfizer (the manufacturers of Revolution) and found several cats that had heartworm exposure and one that had active adult infection. Interestingly enough, indoor and outdoor pets were equally affected. I guess those screens do have some holes in them.
In dogs the treatment involves hospitalization and treatment with several injections of an arsenic compound. Some dogs cannot withstand the treatment and some are left with permanent damage to their hearts and lungs. If caught early, most will respond OK and go on to live happy lives. But it seems given all we have talked about concerning our options, that prevention is the way to go.
I'm going to try to post some interesting cases that I've seen in the next couple of weeks. However, you all know that I can get distracted. Until then, follow me on twitter at and check out our web sites.
Keith Niesenbaum, VMD

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