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Healthy Dogs

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Heartworms in Dogs: Facts and Myths

WebMD separates the facts from fiction about canine heartworms.


Q: Why do I have to keep my dog quiet during the several months he’s being treated for heartworms?

A: After treatment, the worms begin to die. And as they die, they break up into pieces, which can cause a blockage of the pulmonary vessels and cause death. That’s why dogs have to be kept quiet during the treatment and then for several months afterward. Studies have shown that most of the dogs that die after heartworm treatment do so because the owners let them exercise. It’s not due to the drug itself.

Q: If my dog is diagnosed with heartworms, can I just give him his monthly preventative instead of having him go through treatment? Won’t that kill his heartworms?

A: Studies have shown that if you use ivermectin, the common preventative, on a monthly basis in a dog with heartworm disease, after about two years you’ll kill off most of the dog’s young heartworms. The problem is, in the meantime, all of those heartworms are doing permanent damage to the heart and blood vessels.

But if there’s no way someone can afford the actual treatment, at least using the preventative on a monthly basis could be a lesser alternative.

Q: Can I skip giving my dog his preventative during colder months, when there aren’t any mosquitoes?

A: The American Heartworm Society recommends year-round heartworm prevention. One reason is, there’s already a serious problem with people forgetting to give their dogs the heartworm preventatives. It’s a universal problem. Now if you use it year-round, and you miss a month, your dog will probably still be protected. But if you miss more than one or two months your dog could become infected.

The other reason not to stop is that many of the preventatives today also include an intestinal parasite control for roundworms, whipworms, or tapeworms. You want your dog to be protected against those at all times.

Q: If I don’t treat my dog with heartworms, will he “outgrow” his heartworms?

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