Heartworm is a parasite that lives in the pulmonary vessels of the heart. While within the vessels, they can increase in numbers and grow up to 12 inches in length.
As they take up space within the heart vessels, they will cause high blood pressure, difficulty breathing, and eventually cause death. Heartworms are spread form dog to dog by mosquitoes. All it takes is a bite from an infected mosquito to infect your dog.