Testing for heartworm disease in dogs.

Heartworm disease is caused by a parasite called Dirofilaria immitis, better known as heartworm. The parasites are long, hair-like worms that live in the right ventricle of the heart and the pulmonary artery, which is the large blood vessel that carries blood from the heart to the lungs. Dogs become infected when they are bitten by an infected mosquito that is carrying immature heartworms called stage 3 larvae (L3). The larvae pass from the mosquito into the dog, and move through the tissues of the body, eventually entering the bloodstream and migrating to the right ventricle of the heart. Within the heart, the worms mature into adults, mate, and produce immature worms called microfilaria, which circulate in the bloodstream. When the infected dog is bitten by another mosquito, the microfilariae are taken up by the mosquito. Inside the mosquito, the microfilariae develop through three stages of maturation to become infective L3 larvae. The larvae move into the mouth parts of the mosquito where they wait until the next time the mosquito bites a dog. It takes about 5 61⁄2 months for the heartworm life cycle to be completed.