Whipworms are intestinal parasites that are about 1/4 inch (6 mm) long. They live in the intestinal tract of dogs where they cause severe irritation. Whipworm infection results in watery, bloody diarrhea, weight loss, and general debilitation. Any dog with chronic large bowel diarrhea should be suspected to have whipworms, even if the stool sample was negative. Whipworms are far less common today than in previous years, because of widespread use of modern heartworm prevention products.