Symptoms in dogs appear when more than 25 worms are present. Up to 60 worms cause circulatory difficulty. With 100 or more worms, there is blockage of the pulmonary artery and right side of the heart, accompanied by interference with the heart valves. The right side of the heart becomes dilated and enlarged. Blood backs up in the liver and other parts of the body, causing general congestion and degeneration. Dogs fatigue easily, cough, and appear unthrifty