The life cycle of the heartworm begins when an infected dog, carrying tiny immature heartworms (microfilariae) circulating in its blood, is bitten by a mosquito. The mosquito takes in microfilariae (larvae) when it feeds.
During the next two-three weeks, the larvae develop within the mosquito into the infective state.
When the mosquito feeds again, it can transmit infective larvae to
the healthy dog. The larvae penetrate the dog's skin and migrate through
the tissues and develop over the next few months, eventually reaching
the dog's heart.
Once in the dog's heart, the worms can grow to as long as 14 inches and cause significant damage to the heart, lungs and other vital organs. If left untreated, heartworm disease can result in death.