An anal fistula — also called fistula-in-ano — is a tunnel that develops between the inside of the anus and the outside skin around the anus. The anus is the muscular opening at the end of the digestive tract where stool exits the body.

Most anal fistulas are the result of an infection that starts in an anal gland. The infection causes an abscess that drains on its own or is drained surgically through the skin next to the anus. This drainage tunnel remains open and connects the infected anal gland or the anal canal to a hole in the outside skin around the anus.

Surgery is usually needed to treat an anal fistula. Sometimes nonsurgical treatments may be an option.