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The heart of the matter: Heartworm disease in dogs

Sharon M. Albright, DVM, CCRT, American Kennel Club on

Published in Cats & Dogs News

More than 100,000 dogs test positive for heartworm infection every year in the United States. It has been diagnosed in all 50 states and American pet owners spend approximately $75 million annually on heartworm prevention and treatment.

It’s safe to say that heartworm infection is the most important parasitic disease facing our canine companions. Find out what dog owners need to know about this parasite and what scientists are doing to help prevent and treat infections.

What are heartworms? Heartworms are parasitic worms that live inside the heart, lungs, and surrounding blood vessels in dogs, cats and ferrets, plus other mammals like wolves, coyotes, foxes, and sea lions. Immature heartworms enter through the skin when an infected mosquito bites the dog. These immature worms grow to adulthood, mate and produce offspring inside the dog, disturbing normal blood flow and heart function. This can cause lung disease, congestive heart failure and damage to other organs. Clinical signs of heartworm infection include persistent cough, getting tired easily, decreased appetite and weight loss. The infection eventually leads to congestive heart failure and collapse of the cardiovascular system.

Diagnosis and Treatment: Heartworm infection is diagnosed by detecting adult heartworm antigens (immune stimulating molecules) in the blood and/or visualizing immature worms in a blood smear under the microscope. Unfortunately, false positive and false negative results can occur.

The treatment for heartworm disease in dogs relies on a single approved drug. It kills most adult worms within one to three months. However, this process can strain the dog’s cardiovascular system, so dogs must be kept relatively calm and quiet during treatment to avoid complications. The antibiotic doxycycline is also recommended to kill the bacterium known as Wolbachia, which lives inside heartworms. Killing Wolbachia makes the heartworms more susceptible to treatment. Heartworm preventatives are also given during treatment to stop additional worms from maturing in the dog’s heart.


Heartworm Prevention: Preventing heartworm infections is easier and cheaper than treating them. All dogs need heartworm prevention. There are numerous preventative medications approved for dogs, but they all work by killing immature stages of the parasite in the dog’s bloodstream. While some reports suggest heartworm resistance in recent years, there is no widespread ineffectiveness, and proper prevention is still the best approach. The American Heartworm Society ( reports that every preventative on the market has been less than perfect in at least one scientific study. However, it still advises annual testing and regular use of preventatives.

Learning to stay ahead of heartworms: If heartworm disease sounds frightening – it is. Thankfully, the AKC Canine Health Foundation (CHF,, a nonprofit organization dedicated to advancing the health of all dogs, continues to fund research studies that will help dog owners and veterinarians stay ahead of heartworm infections. For example, CHF funded research to develop a test that rapidly identifies preventative-resistant heartworm infections in dogs and to help us understand which mosquito species spread the parasite. In one ongoing study, investigators are searching for new treatment options using drugs that inhibit the activity of an enzyme unique to heartworms. Finally, CHF-funded investigators are looking to improve diagnostic methods by determining if testing for Wolbachia DNA is a more accurate method to diagnose heartworm infection than current options.

Prevention is Key: The American Heartworm Society emphasizes the importance of annual testing and regular use of heartworm prevention for all dogs. By adhering to veterinarian recommendations and staying informed about heartworm prevention, dog owners play a vital role in safeguarding their pets’ health. With ongoing support from organizations like CHF, dedicated to advancing research in this field, we can collectively strive towards a future where heartworm infections are effectively managed. Remember, prevention is not just a recommendation; it’s a powerful tool in protecting dogs from this dangerous parasite.

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