The life cycle of D. immitis has two stages, one primarily in the dog and one in the mosquito. This parasitic organism has an indirect life cycle, meaning that it needs both life cycle stages to perpetuate itself. Other affected species usually act as end hosts, and they can experience clinical problems from the presence of the adult worms in their bloodstream. Adult female heartworms can grow to about 11 inches long, while the males can grow to about 8 inches long. They can occasionally migrate to unusual areas like the brain, eye, and liver.
Adults can live for three to five years.
The mosquito perpetuates heartworm disease by hosting a part of the life cycle. When a mosquito bites a dog with heartworm disease, it can ingest the microscopic microfilaria in the dog's blood. Once inside the mosquito, the microfilaria develop into infective larvae. The larvae migrate from the mosquito's stomach to its mouth in 8 to 28 days, depending on the ambient temperature. When the mosquito feeds on an animal, the infective third-stage larvae are deposited into the animal's skin through the bite wound.