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Are Heartworm Medicines Beginning To Fail?

As a dog owner, now you have yet another reason to hate mosquitoes – they are the carriers and transmitters of heartworms in dogs. Heartworms, as their name suggests, are worms that live in the heart and surrounding circulatory system of a dog. Once a dog contracts heartworms, you, your dog and your veterinarian are in a race against time to kill the worms before they kill your pet.

What Prevents Heartworms?
There are two treatment types currently in mainstream use: one is preventative and one is curative. The preventative treatment is the most common, as its goal is to ensure the heartworm larva (called “microfilaria”) do not survive, even if a carrier mosquito bites your dog.

The curative treatment is used only for dogs that are currently infected. The drug used is called melarsamine. It is not without risks (one of the base ingredients is arsenic) but most vets feel the drug risks are trumped by the risks caused by the heartworms themselves.

Why Current Treatments are Starting to Be Less Effective
Unfortunately, in some areas in the south United States, specifically around the Mississippi Basin, vets are reporting that preventative heartworm treatments are becoming less effective.

According to experts in parasite research, this may be due to overexposure in the heartworm population. If you have ever discovered that an antibiotic that used to work well for you no longer works, the principle is very similar for heartworms. The heartworm population has developed a tolerance for, or resistance to, preventative heartworm medications, so they no longer work as well in some areas.

How to Protect Your Dog
There is no ready replacement for current preventative medications, but melarsamine can be used if your dog does become infected. Careful monitoring during vet check-ups will be crucial to be sure any infestation is not allowed to take root and spread.