Heartworms are worms that look very similar to roundworms, but live in the heart. Their microscopic larvae circulate in the blood and are spread by mosquitoes. Heartworms are common in dogs. Without treatment, heartworm infection causes damage to the heart and lungs and is often fatal. Therefore, prevention is crucial. A variety of convenient preventive medications are available from your veterinarian. Routine blood tests are recommended annually or more often if preventive treatment has been interrupted. Once infected, dogs can be successfully treated for heartworms, but the treatment is much more involved than that for intestinal worms.