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This article is the second of a two-part series. Last month we talked about what heartworms are, what heartworm disease is, and what damage heartworm disease can cause. This month we’ll discuss how problems resulting from heartworm disease are treated, and how heartworm disease can be prevented.

If a dog is diagnosed with heartworm disease, the normal procedure is to treat the disease – that is, to kill the worms. The usual drug for this purpose is called melarsamine. Melarsamine is a manufactured as a powder that is reconstituted with sterile water or saline immediately prior to use. Melarsamine is administered by an injection deep in the lumbar muscles, and is normally given two times over a 2-day interval. It is an arsenical, which means the drug contains arsenic. The arsenic gets into the dog’s bloodstream and kills the worms. The young female heartworms are the hardest to kill and melarsamine does a good job of wiping them out.

Despite the fact that a drug with arsenic sounds dangerous to a layperson’s ears, toxicity problems with melarsamine are rare. The most common problem is some swelling and pain at the site of the injection. In addition, when the worms die from the treatment, the dog generally does not have a severe reaction.

After the melarsamine treatment, the dog is checked after four months to see if re-treatment is needed. The ELISA test, which looks for antigens to the heartworms (as described last month), is used. This test is generally performed in the veterinarian’s office and the results known in about 10 minutes. Fortunately, the melarsamine treatment is very effective; rarely is a second course needed. If the test shows the dog is antigen negative, the next thing to do is to put the dog on the prevention program.

Rather than deal with the problems of heartworm disease, it is better to prevent it. All dogs are candidates for heartworm disease prevention, especially those living in regions with high populations of mosquitoes. These regions include the Southeastern United States, areas along the Mississippi River and its tributaries, as well as other locations around the world. Dogs can be started on the prevention program as puppies (when they get their immunizations) and should remain on the prevention program for their entire life.

Before a dog is placed on heartworm disease prevention, it is tested with the ELISA test to make sure it does not already have heartworm disease. It is important to make sure the dog does not have heartworm disease already, because the prevention medication will not cure the disease. Moreover, the dog could have an adverse reaction to the medication if heartworm disease is present. And, it is so easy to detect the disease in dogs, that it makes sense to perform the test and treat the disease if it is found. (This is different from the situation with cats. Because heartworm disease is difficult to detect in cats and is not treated, and because a cat with heartworm disease can be safely put on the prevention program, cats are not tested before starting the program.)

As described last month, dogs are infected by being bitten by a mosquito that carries the heartworm larvae. The larvae develop inside the dog’s body into adult worms. Prevention involves killing the larvae with macrolyde antibiotics – ivermectin or milbemycin. These antibiotics are given once a month in pill form. The usual medications are Heartgard ® for dogs, which contains ivermectin, or Interceptor ® Flavor Tabs ®, which contains milbemycin. These pills are disguised as tasty treats to make sure your dog eats them. It is important to follow your veterinarian’s directions regarding when to stop and start the treatment during the mosquito season. Normally, the pills may be temporarily stopped one month after the first hard freeze and re-started one month before last hard freeze. If you live in a location with mild winters, your veterinarian will probably recommend that your dog stay on the program year-round. If you forget to give your dog the prevention medication for a month or two during mosquito season, you should contact your vet and see if you need to re-test your dog before restarting the program. In general, you will need to have your dog re-tested for heartworm disease before restarting the prevention program if you skip too many months during the time of year when your dog could get bitten by a mosquito. The reason is because during the time off the medication your dog may have become infected and have developed heartworm disease. As we mentioned above, you cannot put your dog on the prevention program without making sure it is free of heartworm disease first.

The preventative medication is not a vaccine. That is, it will not prevent future larval infections. If a mosquito carrying the heartworm larvae bites a dog, the dog has a chance of getting infected with the larvae. But, the antibiotic in the preventative medication does kill the larvae after the infection and prevents them from developing into adult worms. The heartworm prevention medication kills any larvae that were introduced in the past 30 days. The prevention medication does not kill adult worms; consequently the medication must be taken while the heartworms are still in their larval stage. This means the medicine must be given consistently once a month.

In addition, the prevention medication has only been studied and approved by the FDA for regular, monthly usage. It is possible that the protection would extend a little longer than that. If you forget to give your dog its medicine one month, as we said, contact your vet. Another reason for the monthly dosage is to make it easier for the owner to remember to give the medication. If you always give your dog its heartworm preventative on the 1^st of the month, it will be much easier to remember. And, because heartworm disease can make your dog very sick, it is important that you don’t forget it.

"Heartworm Disease," by Clarence A. Rawlings and Clay A. Calvert, Veterinary Internal Medicine, Eds. Ettinger and Feldman, 1995, pp. 1046-1067.

® Heartgard is a registered trademark of Merial Limited, Iselin, New Jersey.

® Interceptor Flavor Tabs are registered trademarks of Novartis.

Special thanks also to Dr. Oltman of Countryside Veterinary Clinic in Ellicott City, MD (410-461-0517) for her helpful information. Any errors in this article are the responsibility of the author.