An anal fistula is a small infected tract that has an internal opening in the anal canal and an external opening in the skin near the anus. Most anal fistulas are the result of an infection that starts in an anal gland. This infection results in an abscess. The abscess may be drained surgically through the skin next to the anus or it may drain spontaneously. The fistula then forms a tunnel under the skin and connects with the infected gland that has not healed completely. It occurs especially when there have been recurring anal abscesses.
It is important to treat the fistula to reduce the risk of infection and relieve symptoms. Surgery is usually needed to treat an anal fistula.