When my dog poos there is blood in it, what's wrong with him, only noticed today

What to do: Take the dog to a veterinarian to diagnose and treat.

There are many causes including gastritis, gastroenteritis, and colitis due to bacterial infections (Salmonella, E. coli, etc.); viral infections (parvovirus, coronavirus); parasites (hookworms, roundworms, whipworms); gastric ulcers; inflammatory bowel disease; tick-borne diseases (ehrlichiosis, Rocky Mountain spotted fever); liver disease; coagulation disorders (congenital and disease-related); rectal polyps; and gastrointestinal neoplasia.

Vetinary Locator (http://vetlocator.petplace.com/default.aspx?search=0)

Hope this helps.