Dogs infected with heartworm will show shortness of breath, coughing, poor exercise ability, and in severe cases, ascites, an enlarged and sagging abdomen, symmetrical distension on both sides, and a watery sound when touching the abdomen. Dogs infected with heartworm disease can also cause listlessness, loss of appetite, weight loss, anemia, and in severe cases, the inability to stand, and they may lie on the ground until their bodies fail.