The immature adult heartworms migrate to the right side of the dog's heart within a few days of their final molt, presumably carried by the venous circulation. Final maturation and mating occur in the pulmonary arteries. The adult worms live in the right side of the heart and pulmonary arteries, where they may survive for up to seven years. Production of microfilaria by fertilized female worms begins approximately six and a half months after infection. Microfilaria are then released into the circulation, for a mosquito to ingest during a subsequent blood meal.
Several diseases can result from heartworm infection.
Of paramount concern is cardiopulmonary disease, where one finds narrowing and blocking of the pulmonary arteries. The distribution and severity of the damage depends on both the number and location of adult worms. If there is a high worm burden, worms may be found in the right atrium and ventricles, and the vena cava may be involved as well. Individuals with cardiopulmonary disease may show exercise intolerance, and this can lead to right-side heart failure. Heart failure in these individuals can be acute or gradual.