An anal fistula is an abnormal passage or opening between the rectum and another organ, or the skin near the anus. It may go completely around the anal opening (external fistula) or form a connection between the rectum and another organ, such as the bladder (internal fistula).
Anal fistulas often develop after an infection or abscess in the area around the anus. Anal sex can also cause an anal fistula. Symptoms of an anal fistula include drainage from the opening, pain, swelling, and fever.