Heartworms are parasites that live in the arteries of the lungs of dogs and wild canines and also some cats. Heartworms cause elevated pressure in the pulmonary arteries, which increases the workload of the right side of the heart. Heartworms can live 5-7 years. Heartworms also cause inflammation in the lungs. They’re found in most parts of the world and in every state of the United States. Heartworms are spread from an infected animal to other animals by the bite of a mosquito. Heartworms mold in the mosquitoes to their infective form and are then passed onto the dog.

Once the dog is bitten by an infected mosquito, the heartworm organisms molt and travel to the lungs, where they develop into adults. It takes about six months from the time the dog is bitten until the heartworms are mature in the pulmonary arteries. Adult female heartworms produce a substance that irritates the arteries of the lungs. The physical presence of heartworms also injure the linings of the arteries, which cause the arteries to thicken. As adult heartworms die, they become lodged in the smaller pulmonary arteries, which contributes to severe pulmonary disease.

Most dogs with heartworm disease have no signs and the disease is detected by routine heartworm blood tests. The more athletic and active the dog, the earlier the signs are seen. The first signs are often lethargy, decreased activity, and coughing. As the disease worsens, breathing rate and effort increase. Some dogs have fainting episodes when stressed. With advanced disease and right heart failure, fluid may build up in the abdomen and weight loss may occur. If a large number of adult heartworms obstruct blood flow in the heart, lethargy, weakness, loss of appetite, and fever may occur. The membranes in the mouth may be pale or yellow from red blood cell destruction and anemia.

Blood tests can diagnose heartworm disease in your dog. If the dog is positive for heartworms, chest x-rays are done to look for lung and heart changes. If other problems, such as liver or kidney disease are found, they are treated first.

Adult heartworms are killed by a drug called melarsomine. Immature heartworm or microfilaria are killed later using a drug called Ivermectin. There can be many potential side effects of heartworm disease and the treatment of heartworm disease. Treatment must be supervised very carefully by your veterinarian.


Dr. Karsten Fostvedt is a veterinarian at St. Francis Pet Clinic in Ketchum.

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