Heartworm Disease in Dogs

Heartworm disease is a potentially fatal infection in dogs (also cats and ferrets). Although it is most common in the Midwest and Southeastern United States, heartworm has been diagnosed in every state. You can see the risk in your state and county on interactive maps at Pets & Parasites: http://www.petsandparasites.org/parasite-prevalence-maps/ .

Heartworm is a parasite—a worm—which is transmitted by mosquitoes. When a mosquito bites the dog, it injects microscopic larval (baby) worms from its saliva into the dog’s bloodstream. The larvae migrate through the dog’s body and grow for six months. Eventually they make their home in the pulmonary arteries (the blood vessels which take blood from the heart to the lungs). The adult worms living in the heart and lungs are over ten inches long! As you can imagine, the presence of worms in the heart causes a lot of problems.

Early in heartworm infection there are usually no symptoms. Later in the course of infection, once irreversible damage has been done to the heart and lungs, symptoms develop: coughing, weakness, reluctance to do normal activities, weight loss, and difficulty breathing. Severe cases can lead to heart failure, with fluid buildup in the belly, collapse, and sudden death.

Heartworm is diagnosed with a blood test that looks for a protein made by the adult worm. This test can show a false negative (meaning the test is negative but the dog is truly infected) in an early infection or if there are only a few worms. Other tests that may be recommended by your veterinarian include a full blood panel, chest x-rays, echocardiogram (ultrasound of the heart), and ECG (electrical rhythm of the heart).

Heartworm disease is treated with a drug called melarsomine, which is injected into the muscle of the dog’s back and kills the adult worms. This treatment can be dangerous, because as the worms die, fragments of dead worms can break off and “wash away” through the bloodstream into the lungs, where they can cause clots and sudden death. It’s very important to strictly limit the dog’s activity level for at least a month after the treatment to avoid this.

Because both heartworm infection and melarsomine treatment have potentially serious side effects, it’s much better to prevent the infection in the first place. There are several heartworm preventative medications available: monthly chewable pills, monthly topical applications, and a six-month injection. Many also prevent other parasite infections, including intestinal worms or fleas, depending on the product. Some people only recommend preventative medications during the mosquito season, but for my patients I recommend them year-round because although heartworms aren’t a problem in cold winter, intestinal parasites still are. Also, for my own pets, it is easier for me to remember to give the medication every month rather than to remember to stop in the fall and restart in the spring. Talk to your veterinarian about the best prevention (brand and schedule)


Leave a Comment