What causes a fistula in ano?
An anal fistula usually develops after an anal abscess (a collection of pus) bursts, or when an abscess has not been completely treated. After an abscess has been drained, a tunnel may persist connecting the anal gland from which the abscess arose to the skin. If this occurs, persistent drainage from the outside opening may indicate the persistence of this tunnel. If the outside opening of the tunnel heals, recurrent abscess may develop.
A fistula can also be caused by conditions that affect the intestines, such as Crohn's disease or ulcerative colitis. As many as 50% of people with Crohn’s disease get an anal fistula. Up to 30% of people with HIV (a virus that attacks the body's immune system) will also develop an anal fistula.
Anal fistulae are more common in men, mostly between the ages of 20 and 40. A study published in 2007 that looked at four different European countries, including England, found that there are between one and three cases of anal fistulae for every 10,000 people.