The Disease

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 in cats is less frequent than in dogs. Cats are susceptible, but appear to be more resistant than dogs. Cats may exhibit non-specific clinical signs, such as vomiting, gagging, difficulty or rapid breathing, lethargy and weight loss.

The average time from when the cats are infected through the bite of a mosquito until the presence of adult worms is about 8 months. Outdoor cats are at the greatest risk on infection; however, a high percentage of indoor cats (as defined by their owners) have been infected. The distribution of feline heartworm infection in the US is geographically similar to canine infection, but it occurs in fewer numbers than it does in dogs.

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.