[PDF] Understanding Heartworm Disease & Prevention By Educational Resources, University of Georgia. Heartworm disease is a common, potentially life-threatening condition in dogs that is caused by the adult stages of Dirofilaria immitis. The D. immitis larvae that are transmitted to dogs from infected mosquitoes mature and then cause damage to the heart and pulmonary arteries. This results in a variety of clinical signs, including a persistent cough, fatigue during exercise, fainting and weight loss. Using a variety of interactive images and animations, this book depicts how heartworm disease develops, as well as its treatment and prevention.