Each year thousands of dogs become disabled or die from lung, heart or circulatory problems caused by heartworm disease. Heartworm disease in dogs and related canines is caused by a filarial nematode (a large thread-like round worm), Dirofilaria immitis . The adult worms live in the right side of the heart (right ventricle) and adjacent blood vessels (pulmonary arteries), and because of their location, are commonly called "dog heartworms."
Heartworm disease occurs worldwide in most tropical and subtropical regions, with increasing frequency in temperate climates. Until the late sixties, the disease was restricted to southern and eastern coastal regions of the United States. Now, however, cases have been reported in all 50 states and in several provinces of Canada. For most of North America, the danger of infection is greatest during the summer when temperatures are favorable for mosquito breeding. In the southern U. S., especially the Gulf Coast and Florida, where mosquitoes are present year-round, the threat of heartworm disease is constant.
Visible signs of heartworm disease may not appear until a full year after bitten by infected mosquitoes. In fact, the disease may be well advanced before the dog shows any symptoms. Dogs with typical heartworm disease fatigue easily, cough, and appear rough and not thriving. Blood and worms from ruptured vessels may be coughed up. Blockage of major blood vessels can cause the animal to collapse suddenly and die within a few days.
Heartworm is also an occasional parasite of humans. The parasite is usually found in the lung (pulmonary dirofilariasis), and less often in the heart. Although the worm forms "coin lesion" in the lung, which may be confused with other diseases on x-rays, such as carcinoma, its clinical significance in man has not been fully determined. During the last 30 years about 100 cases of human pulmonary dirofilariasis have been reported from Florida.