Heartworms are parasites that infect both dogs and cats. If left untreated, these parasites cause major health issues for pets and even result in death. When a mosquito bites an animal, it can transmit heartworm larvae into a pet’s bloodstream through capillaries in the skin. While blood flows through the heart, the larvae settle there and become adult heartworms. What Happens When a Pet Gets Heartworms? Adult heartworms cause havoc in a pet’s body. When heartworms live in a pet’s heart, the heart becomes filled with worms. These worms reproduce and release larvae into the bloodstream, continuing the parasites’ lifecycle.