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heartworm parasite

Description and Distribution

Heartworm disease is caused by a parasitic roundworm (Dirofilaria immitis). Usually only dogs are affected; however, cats and other mammals are susceptible to infection. Heartworm infections are common along the Atlantic and Gulf coasts and the Mississippi River Valley and its tributaries, and now have been reported in every state in America, including Alaska, and in Canada as well. In Michigan, it is most common in the Lower Peninsula, but it has also been diagnosed in the Upper Peninsula.

Infection may also occur in various wild mammals. In the United States, the worms have been reported from foxes, coyotes, wolves, raccoons, and muskrats. In Michigan, the parasite has been identified in a gray fox from Shiawassee County, red fox from Clinton, Jackson, Gratiot, and Saginaw Counties, coyotes in the Lower Peninsula, and wolves from the Upper Peninsula.

Transmission and Development

In the host, usually a dog, adult worms live in the heart and the large blood vessels entering and leaving the heart. Female worms are about 10 inches long; males are somewhat smaller. The female gives birth to microscopically small worms called microfilariae which circulate in the host's blood. The microfilariae must be ingested by a mosquito to continue their development. Inside the mosquito they develop to infective larvae. The larvae migrate to the mouthparts of the mosquito and remain there until the mosquito feeds again. They then leave the mosquito through the mouthparts and enter the new host through the skin. The young larvae begin to grow in the tissue under the skin and muscles. In about two months they enter the right side of the heart where they grow to maturity. The route they follow from the skin tissue and muscles to the heart is unknown, but likely involves the vascular system. 

Clinical Signs and Pathology

The first sign of infection is either a chronic cough which is aggravated by exercise, of tiring on exercise, or both. In advanced cases, heart failure with fainting and collapse may occur. Even light infections may cause irreversible damage to the heart, blood vessels, kidneys, and liver.

The most common pathological process seen is inflammation of the pulmonary artery. This condition is due to chronic irritation cause by adult worms. The result is a thickening and roughening of the interior artery wall. This can become severe enough to close off circulation to the lungs. The rough inner surface of the artery also causes small blood clots to form which are swept into the lungs where they block small blood vessels. All this results in a back pressure that causes enlargement of the right side of the heart and decreased lung efficiency. The liver and other organs depend on normal blood pressure to function properly; when back pressure occurs, these organs may be affected and eventually they may be permanently damaged.


The best method for diagnosing heartworm infection in the living animal is by finding microfilariae in the blood. Evidence of infection can often be detected by X-ray as well. In dead animals, the worms can be easily found in the right side of the heart at necropsy. Occasionally, they are also found in the pulmonary artery and the vena cava.

Treatment and Control

Dog owners who are concerned for the health of their dogs should contact a veterinarian for advice on treatment and prevention. Treatment of free-living wild animals is not feasible.

In dogs, drugs used to kill the adult worms are very toxic and must be administered carefully. When the worms die they are swept into the lungs where they can cause severe blockage of blood vessels. In the lungs, they decompose and may cause edema, blood clots and hemorrhage, or contribute to secondary infection resulting in pneumonia. Therefore, it is essential that the dog have complete rest for at least two weeks following treatment.

Unfortunately, no single compound is effective against both adult worms and microfilariae. Consequently, a dog has to be treated a second time using a different drug in order to kill the microfilariae.

Considering the problems associated with treatment, it is advisable to prevent the infection. This is best done by giving the dog preventable treatment monthly for the entire year.


Although the parasite is not considered to be of great public health significance, there have been authentic reports of the heartworm infecting humans. The usual infection is a single worm confined to a small nodule in the lung. It is rarely found in the heart. The nodule is usually discovered during a routine chest X-ray.

The significance of heartworm infection in humans is not its threat to the individual's life, but that the lung lesion must be differentiated from lung cancer, lung cysts and other infectious diseases.

Whenever a parasite of humans or domestic animals also infects wild animals, the question of a wildlife reservoir invariably arises. In view of our current knowledge on heartworm in Michigan wildlife, it is unlikely that foxes or other wild animals constitute a significant reservoir of the parasite in this state.

What effect this parasite has on foxes and fox populations is unknown. However, if infected foxes are affected in the same manner as dogs, they are probably not efficient predators and their life span may be shortened.

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For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.