A perianal fistula is a collection of pus under the skin next to the anal canal (back passage). It is usually caused by an infection of the glands of the anal canal. This presents as a painful lump in the area and may be associated with a fever. The treatment involves drainage of the pus usually by operation. Some of these infections go on to form a longstanding fistula (a track running from the skin near the anus to the inside of the anal canal). These intermittently swell and discharge pus and blood. The treatment involves surgery to either open the track to the skin or close the inside opening.

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