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Heartworm Disease

Heartworm is a serious, life-threatening disease of dogs. It is due to the presence of the adult stage of the parasite, Dirofilaria immitis, in the pulmonary arteries and right ventricle of the dog's heart. Until the early 1970s, the occurrence of heartworm in the United States was primarily confined to the southeastern part of the country. Today, it is found almost everywhere in the continental United States and is a major threat to the dog population of Canada.

How Are Animals Infected?

Transmission of heartworm depends upon the mosquito population of an area. About 70 species of mosquitoes are capable of transmitting the disease. The more mosquitoes in an area, the greater the chance of heartworm transmission.

Heartworm disease occurs most commonly in dogs. It has recently been shown that heartworm is a major cause of heart disease in cats. Heartworm also infects wild animals. Coyotes, wolves and foxes are carriers of the disease in the wild. In a particular area, when the wild animals are infected, the disease is permanent

The adult heartworm is 6-14 inches in length. It is threadlike, white in color, and primarily found in the pulmonary arteries and right ventricle of the dog's heart. When adult male and female heartworms are present, mating occurs. The female releases large amounts of small, microscopic "microfilariae" into the bloodstream.

After the microfilariae have gone through their development, they are ready to infect a new victim. During a blood meal (mosquito bite), the mosquito injects the microfilariae into a new dog. These small, microscopic worms migrate under the skin and eventually enter the dog's blood stream. About 6 months after the initial mosquito bite, the microfilariae arrive at the heart. The final maturation and the mating of the heartworm occur in the pulmonary arteries. The adult worms live in the pulmonary arteries and right side of the heart, where they can survive for several years.

What Are the Signs of Heartworm Disease?

For both dogs and cats, clinical signs of heartworm disease may not be recognized in the early stages, as the number of heartworms in an animal tends to accumulate gradually over a period of months and sometimes years.

Recently infected dogs may exhibit no signs of the disease, while heavily infected dogs may eventually show clinical signs, including a mild, persistent cough, reluctance to move or exercise, fatigue after only moderate exercise, reduced appetite and weight loss.

Cats may exhibit clinical signs that are very non-specific. Chronic clinical signs include vomiting, gagging, difficulty or rapid breathing, lethargy and weight loss.

Not only is heartworm dangerous, but the treatment for heartworm disease is dangerous as well.

Administration of preventive medication is the best method for keeping a dog free from heartworm disease.