An anal fistula is usually a side effect of an anal abscess, an infected wound that drains pus from your anus. The draining abscess can create a tunnel through your anus to the skin outside. Anal pain, swelling and redness are the primary symptoms. fistula is an abnormal passageway that develops from inside your anus to the skin outside. It usually develops in the upper part of your anus (butthole), where your anal glands are. When these glands become infected, drainage from the infection can create a fistula. This infection is called a perianal abscess. (Sometimes an anal fistula is also called a perianal fistula. “Perianal” means in the region of your anus.)