Heartworm prevention for dogs

Updated February 21, 2017

Once considered to be a problem only in southern environments, heartworms are now a worldwide threat to a number of species including dogs. Even with the widespread concern over heartworms, many owners fail to properly protect their canine companions. A severe heartworm infestation can be deadly, so preventing this invasive pest should be a part of your pet-care routine.


Heartworms were originally documented more than a century ago. First identified in dogs in "The Western Journal of Surgery and Medicine" in 1847, heartworms were originally found only in warm southern climates. As people migrated north and west, heartworms spread to all 50 states in the United States, as well as Mexico and southern regions of Canada. The first commercially available heartworm prevention medication was an ivermectin-based pill made in 1987.


Dogs with severe heartworm infestations often appear to be in the early stages of heart failure. Coughing, shortness of breath and lack of energy are common in dogs with heartworms. The worm itself begins life as a minuscule larvae passed onto the dog from the bite of an infected mosquito. Adult heartworms are long, thin worms that look much like strands of spaghetti. They migrate through the bloodstream to the heart.


Preventing heartworm infestation is essential to a dog's health and well-being. A prevention plan costs under £13 a month and will stop heartworms from infecting your dog. A full-blown infestation reduces the normal function of the heart, weakening the muscle and impeding blood flow. Treating heartworms can cost hundreds of dollars and puts the dog's life at risk as the dead worms must be forced from the heart and can clot in thin veins and capillaries.


Your veterinarian will provide heartworm tablets and explain how they should be administered. Typical preventive tablets are given once a month and should be started when your puppy is eight weeks old or before her first mosquito season. Older dogs will be given a blood test to make sure they are not heartworm-positive before treatment. Once on a prevention plan, the dog should be medicated year round to prevent an infestation.


Heartworm pills should never be given to an adult dog that has not been tested for heartworms. Giving pills to a heartworm-positive dog can cause a potentially deadly shock to the circulatory system. Herding breeds such as collies and sheepdogs should never be given ivermectin-based heartworm tablets because they have a lethal sensitivity to ivermectin. Heartworm tablets are very potent and no more than one pill should be given every 30 days to prevent a possible overdose.

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About the Author

Louise Lawson has been a published author and editor for more than 10 years. Lawson specializes in pet and food-related articles, utilizing her 15 years as a sous chef and as a dog breeder, handler and trainer to produce pieces for online and print publications.