fistula (fĭsˈchŏlə) [key], abnormal, usually ulcerous channellike formation between two internal organs or between an internal organ and the skin. It may follow a surgical procedure with improper healing, or it may be caused by injury, abscess, or infection with penetration deep enough to reach another organ or the skin. When open at only one end it is called an incomplete fistula or sinus. The most common sites of fistula are the rectum and the urinary organs, but almost any part of the body may be affected. Rectal fistulas are often associated with colitis, cancer, sexually transmitted diseases, and other disorders. Usually a fistula requires surgery. In horses an abscess on the withers from chafing and infection is termed a fistula.