Anal fistulas or fistula-in-ano generally cause discomfort due to pus or watery discharges and sometimes pain. This is a condition where a nodular swelling appears in the area surrounding the anal opening. Intermittent pus discharge is noticed from this nodular swelling. Instead of a nodular swelling; there might be simply an opening in this region. An anal abscess is one of the most common causes of this condition.