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Monthly Medication, Yearly Testing Prevent Heartworm Worries

Pet Column for the week of July 11, 2011

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Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Andrea Lin
Information Specialist

If you live where there are mosquitoes—and that includes pretty much all of the United States—you live where heartworm poses a threat to your pets. Current recommendations for preventing heartworm disease include giving pets preventive medication every month and having them tested for heartworm by their veterinarian every year.

According to Dr. Allan Paul, veterinary parasitologist at the University of Illinois College of Veterinary Medicine in Urbana, heartworm disease has been found in all 48 continental states and Hawaii. Even Alaska has conditions in which it can spread.

The heartworm is an actual worm that can grow to 12 inches long inside your pet. A pet becomes infected through the bite of a mosquito that is carrying the worms in an early stage of their life cycle. The worms mature and reproduce in the pet's bloodstream, eventually lodging near the heart. When a mosquito bites an infected pet, the cycle starts again.

If every dog received monthly heartworm preventatives, the heartworm population would decrease because they would have fewer and fewer places to live. The worms need a host to complete their life cycle. In effect, heartworm preventatives work to help the whole population of dogs.

Cats as well as dogs can get heartworm disease. Cats are much less susceptible than dogs are to heartworm. Unfortunately, heartworm causes much more serious problems in a cat than in a dog. It can take only a few adult worms to kill a cat, while that number would probably not cause any visible sickness in a dog. In cats, the main clinical sign of heartworm disease is sudden death. Your veterinarian can prescribe a heartworm preventative formulated for cats.

Preventive heartworm medications prescribed by veterinarians are effective and safe for pets. However, because these medications are preventatives and don't kill worms that are already present, your pet must be tested before the medication is started to ensure that he is heartworm negative.

Annual testing is important in case your pet accidentally becomes infected if a dose of medication was skipped one month. Early detection—catching it before your pet is noticeably sick—makes treating the disease much easier. While an effective treatment exists, it can be dangerous to the dog if there are too many worms already present. The earlier the problem is caught, the safer treatment can be.

Yearly testing is also important because a recently identified strain of heartworm—the Mississippi Delta strain—can be resistant to current heartworm preventatives.

"There have been reports of preventatives not working," says Dr. Paul. "At present, the Mississippi Delta strain makes up a very small segment of the heartworm population, but to be safe, I recommend testing your dog yearly for heartworm."

While heartworm is prevalent and a serious disease, it is also easy to prevent. The best practice is to keep your pet on a year-round preventative and to test for heartworm once a year. Extra testing may be necessary if you are switching from one brand of medication to another.

Consult your veterinarian if you have any questions about heartworm in dogs and cats.