Heartworm disease is caused by a parasite called Dirofilaria immitis. They’re called heartworms because the worms are typically found in infected animals’ heart, lungs, and blood vessels. Commonly found in dogs and cats, our pets serve as hosts for the worms to mature, breed, and multiply. When mature, the worms look like long strands of spaghetti. Female worms can be 10 to 12 inches in length, and male worms are a little shorter, averaging four to six inches long. Heartworms can cause lung disease, heart failure, and organ damage in dogs.