Heartworm: Dirofilaria immitis. A common parasite of dogs in many parts of the world that occurs in other animals, including cats, but rarely infects humans. The adult worms, which are up to 10 inches (25 cm) in length, typically live in the dog's pulmonary artery and right side of the heart. Inflammation and thickening of the heart may result in heart failure, respiratory insufficiency, chronic cough, and vomiting. The disease can be fatal.
The female worms produce microfilariae that are found in the dog's blood. The microfilariae are ingested by a mosquito when it feeds on a dog, the microfilariae mature into infective juveniles in the vector, and the infection is transmitted to a new host when the mosquito feeds. Many species of mosquitoes serve as vectors for the heartworm.
ProHeart 6, a twice-a-year time-released heartworm medication, was recalled in 2004 at the request of the FDA after thousands of dogs suffered adverse reactions, including liver and bleeding abnormalities followed in some cases by death. The active ingredient in ProHeart 6 is moxidectin. Other agents are available to treat heartworm.