Dogs aquire this infection through mosquito bites as mosquitoes readily pick up larval heartworms from infected dogs and carry them to new dogs.
Some geographic areas have severe heartworm problems while other areas have none.
The adult heartworm is fairly large, several inches in length, and it prefers to live, not in the heart, but in the pulmonary arteries. It swims into a cozy tubular artery, where it is massaged and nourished by the blood coursing past it. In the pulmonary arteries of an infected dog, the worm's presence generates a strong inflammatory response and a tendency for blood to inappropriately clot. If enough worms are present, the heart must work extra hard to pump blood through the plugged up arteries. The worms actually take up a significant amount of space within the heart, leading to less blood being pumped.