The life cycle of D. immitis has two stages, one in the dog and one in the mosquito. This organism has an indirect life cycle, needing both stages to perpetuate it. Adult female heartworms grow to about 11 inches long, and males grow to about 8 inches. Babies are known as microfilaria they are microscopic organisms that live in the bloodstream for one to three years.
Adults live for three to five years.
When a mosquito bites a dog with heartworm disease, it ingests the microscopic microfilaria in the dog's blood. Once inside the mosquito, the microfilarias develop into infective larvae. The larvae migrate from the mosquito's stomach to its mouth. When the mosquito feeds on an animal, these infective larvae known as third-stage are deposited into the animal's skin through the bite migrating through the dog's body to the heart developing into adult heartworms over the next two to three months. If both sexes of the worm are present microfilaria will be produced and released by the female worm into the dog's bloodstream.