Anorectal fistulas have been the subject of medical and lay literature for over 2,500 years. The term fistula is ascribed to John Arderne (1307–1392) whose classic work on anal fistula is still in print. However, it is important to note that in Ayurvedic medicine, Suhruta (b ~800 bc) described both fistulotomy and fistulectomy as well as the chemical seton using Kshara Sutra [1]. Hippocrates (b ~460 bc) described the use of horsehair (seta) in the treatment of anal fistulas. Fistulas have been written about in many languages and geographical locations throughout the years [2–4]. The true incidence of anal fistulas is unknown.

Most publications on anal fistula reflect the authors’ experience, some quite large, from a single institution [4–6]. This, however, does not address the incidence of the disease due to the lack of proper denominator. Also it is difficult, if not impossible, to accurately assess the incidence of anorectal abscess because so many drain spontaneously or are incised and drained in a surgeon’s office, emergency department or surgicenter. On the other hand, hospital discharges or formal operations in the operation rooms are usually recorded and are available for statistical evaluation. Thus among the 1,000 patients presented to the Surgical Section of the Diagnostic Clinic at the University of Virginia, 150 had anorectal pathology, 4 (0.4 %) had an abscess and 8 (0.8 %) had fistula. This is quite comparable to 532 fistulas treated in a population of 77,372 patients admitted to Brooklyn Hospital between 1930 and 1939 for an incidence of 0.69 % [7]. Also Buie reported an incidence of 5 % anal fistulas in patients with anorectal abscesses seen at the Mayo Clinic [8].

Using operating room data in Helsinki Finland (1969–1978), the incidence of fistulas was calculated to be 8.6 per 100,000 populations (males 12.3 %, females 5.6 %). Nelson in his meta-analysis equated this with 20,000–25,000 fistulas treated annually in the US [9]. Interestingly, the ambulatory case of the National Center for Health Statistics reported 24,000 patients with a primary diagnosis of fistula treated in US Hospitals in 1979. This number has decreased drastically to 3,800 in 1999 possibly due to more and more ambulatory approaches [9].

The incidence of anorectal fistula can be estimated from the number of anorectal abscesses. In a series reported by Ramanujam from a large inner city hospital, the incidence of fistula was 34 % [4]. This is almost identical to another US study [10] and a Canadian study [11] both reporting from single institutions. Calculating backward, this would translate to an estimated annual incidence of 68,000–96,000 cases in the US [9].

Fistulas in the overwhelming majority of cases arise from prior abscesses. Other causes such as hemorrhoidectomy, foreign body perforation, and trauma are of less frequency. Inflammatory bowel disease, more commonly Crohn’s disease, has been known to be associated with anorectal fistulas. Specific diseases such as tuberculosis and actinomycosis are much less frequent in the Western world. Tubercular fistulas are covered in a separate section. The origin of anal abscess and the relationship of abscesses to fistulas is covered in Chap.​ 3.

Data on age and sex can be extracted from single series. Most patients with anal fistula are between the ages of 20 and 60 with mean age of 40 in both genders. In the Sainio report, men were afflicted twice as frequently as women (12.3 % vs. 5.6 %) [5]. In two large series reported from Cook County Hospital in Chicago, the male to female ratio was also 2:1 [4, 12]. Hill reported treating 636 patients of whom nine were less than 9 years old and were all boys [13]. Similarly, Mazier reported 1,000 cases of fistula treated at the Ferguson Clinic of whom 25 were younger than 10 years of age and nine of the ten were boys [14]. Piazza and Radhakrishnan reported anorectal abscesses in the pediatric population. Of 40 patients, 33 were boys and seven girls. Twenty-one were younger than 2 years old, 20 were less than 9 months of age and all were boys [15].