Gastroenterology Consultants,SC: Diseases of the GI tract ANAL FISSURE, ABSCESS, AND FISTULA [<<back | list all>>] An anal fissure is a small tear in the lining of the anus frequently caused by constipation. An abscess is a localized pocket of pus caused by infection from bacteria. When bacteria seep into the underlying tissues in the anal canal, an abscess may develop. A fistula is a tiny channel or tract that develops in the presence of inflammation and infection.