Canine Heartworm Disease

Heartworm disease in dogs, caused by Dirofilaria immitis, used to be primarily thought of as a disease of the southeastern and Gulf coast areas of the United States, but in recent years, it has spread to nearly all parts of the country.

Heartworm is spread by mosquitoes, who ingest microfilariae (immature parasites/larvae) from the blood of an infected dog. The larvae then mature in the mosquito and become “infective,” which, after entering another dog through the bite of this same mosquito, can cause “heartworm disease.” First, the larva migrate to tissue, muscle, or fat, and approximately 110 days later and after two molts in the dog’s body, young adult heartworms have made their way to the infected dog’s heart.

Heartworm can be detected in few different ways. Your veterinarian can look for microfilariae in a dog’s blood; these microfilariae do not circulate in a dog’s blood, however, until 6-7 months after initial infection. Or, he or she can run a test that will detect heartworm antigens in a dog’s serum. Because up to 65% of dogs with heartworm disease may not have circulating microfilariae, serum testing is often the preferred method. Radiographs (“X-rays”) or ultrasounds may show signs of the disease as well. As for symptoms, dogs who have been recently infected may show none, while dogs who are heavily infected may have a mild, persistent cough; exercise-related fatigue; lethargy; reduced appetite; and weight loss.

Treatment is threefold: 1.) Kill the adult heartworms (adulticide) in the heart and blood vessels; 2.) Kill any circulating microfilaria (microfilaricide); and 3.) Provide preventative medication. Adulticide therapy is achieved with melarasomine dihydrochloride (Immiticide®), an arsenical compound, which, depending on the severity and progression of the disease, is given up to a handful of times intramuscularly in the epaxial muscles of the lumbar region (that is, in the dog’s back muscles), from between 24 hours to sometimes months apart. Adult heartworms will die slowly in the 2-3 weeks following each injection. Microfilaricide begins a few weeks after adulticide therapy. Ivermectin (for example, Heartgard®) is often the chosen treatment for this step.

Thromboembolism is a major concern when treating for heartworm disease; fever, coughing, difficulty breathing, and coughing up blood may be seen in dogs in whom the worms die and pass to their lungs, causing pulmonary embolism.  Aspirin therapy may be recommended for dogs with severe disease to help ward off this life-threatening complication. Also to help prevent this complication in all treated dogs, strict cage rest is advised for at least 4 weeks after each injection. 

Remember, preventative once a month, every month, is key to protecting your dog—and sparing yourself and your pet the often painful, always frustrating, and occasionally life-threatening treatment that follows a heartworm positive diagnosis.

For more information, visit the American Heartworm Society at

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