How Can I Prevent Ringworm from Spreading? Although the name suggests otherwise, ringworm is not caused by a worm at all-but a fungus. This highly contagious infection can lead to patchy areas of hair loss on a dog, and can spread to other animals-and to humans, too. What Are the General Symptoms of Ringworm? Classic symptoms of ringworm include lesions that typically appear on a dog’s head, ears, paws and forelimbs.