Heartworms are large worms that live in the hearts of dogs and cats. Dogs are the most common hosts for this parasite. Heartworms are also known as Dirofilaria Immitus. The adult heartworm lives in the right chamber of the heart and pulmonary artery which routes blood through the lungs where carbon dioxide is removed and oxygen is added to the red blood cells. Heartworms can cause significant damage and even death.