Heartworm Symptoms Canine heartworm disease symptoms include: - Difficulty breathing - Coughing - Fatigue, a dog that tires easily - Listlessness - Weight loss - Rough hair coat Your veterinarian can perform a simple blood test that can verify the presence of heartworms.     Tri-Heart is a registered trademark of Schering-Plough Animal Health Corporation. Copyright © 2008 Schering-Plough Animal Health Corporation. All rights reserved. Privacy Policy