Introduction
Heartworm disease is caused by a parasite, Dirofilaria immitis, which lives in the arteries of the lungs and in the right side of the heart. Heartworm disease occurs primarily in dogs, but can occur in cats as well as other animals on rare occasions.
Clinical signs of heartworm disease are seen when heartworms accumulate in the heart and associated blood vessels. In dogs, clinical signs may include a mild persistent cough, reluctance to move or exercise, fatigue with moderate exercise, reduced appetite and weight loss.
The Heartworm Parasite
Infective larvae are transmitted into the host’s bloodstream by the bite of an infected mosquito. The mosquito transmits the larvae, the larvae grow, develop and migrate in the body over a period of several months to become sexually mature male and female worms. These mature adult heartworms live in the arteries of the lungs and right side of the heart and can be up to 7-14 inches long. As mature adults, the worms mate and females release tiny immature worms called microfilariae into the blood stream.
Where is Heartworm Disease Found?
Heartworm disease occurs worldwide in tropical, subtropical, and some temperate regions. Until the late sixties, the disease was restricted to southern and eastern coastal regions of the United States. Now, however, cases have been reported from dogs native to all 50 states (Figure 1).
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.
Figure 1. Distribution of Dog Heartworm in the United States.
CREDITS: American Heartworm Society (www.heartwormsociety.org)