**1. Introduction**

178 A Bird's-Eye View of Veterinary Medicine

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Congenital deformities of the anorectum are rarely encountered in small animals with atresia ani being the most common one. Atresia ani is a congenital defect of the anorectum, resulting in anal canal closure and /or abnormal routing of feces (Bright & Bauer, 1994). Among large animals congenital atresia ani most often occurs in pigs and calves and is considered hereditary. In these species atresia may be seen as a separate entity or in conjunction with other malformations of the distal vertebral column, urogenital tract and sometimes with intestinal atresia or colonic agenesis (Kilic & Sariepler, 2004; Maxie, 2007). In pigs atresia is associated with a high mortality rate but selective breeding decreased its occurrence (Partlow et al., 1993; Viana &Tobias, 2005).

#### **2. Epidemiology**

Atresia ani is uncommonly described in dogs and it is even less frequently reported in cats; the true incidence of this abnormality is difficult to determine and may be greater than reported because many newborn puppies and kittens are euthanatized before being evaluated based on the hypothesis that surgical correction is unsuccessful; additionally unpublished data may hide true prevalence as complications associated with surgical correction of these deformities are common (Prassinos et al.,. 2003; Mahler & Williams, 2005; Viana & Tobias, 2005). According to a review of Veterinary Medical Database atresia ani in dogs accounts for 0.0007 with females more likely to be affected than males (female/male =1.79/1) [Viana & Tobias, 2005]. Canine breeds overpresented include Finish spitz, Boston teriers, Maltese, chow chow, German shorthair pointer, toy poodle and miniature schnauzer (Viana & Tobias, 2005). In cats females are more commonly affected than males (Suess et al., 1992; van de Broek et al., 1988; Tsioli et al., 2009, Tomsa et al., 2011).

#### **3. Normal and abnormal embryologic development**

The embryologic development of the canine and feline anorectum resembles that of the human development (Greiner, 1972; Amand, 1974; Suess et al., 1992; Sadler, 1995; Viana &

Atresia Ani in Dogs and Cats 181

Four anatomic types of atresia ani have been described in dogs and cats (Aronson, 2003;

External anal sphincter and anal sacs are usually develop normally in type II anomalies (Seim, 1986; Ellison & Papazoglou, 2011), while agenesis of the external anal sphincter, anal sacs or tail are reported in type III anomalies (Rawlings & Capps, 1971; Knecht & Westerfield, 1971; Loug & van Schouwenburg, 1982; Ellison & Papazoglou, 2011). Occasionally dogs and more rarely cats with type II and more uncommonly with type III atresia ani may be associated with rectovaginal, rectovestibular or urethrorectal fistulas (Holt, 1985; van den Broek et al., 1988; Chandler & MacPhail, 2001; Aronson, 2003; Ellison & Papazoglou, 2011). Animals with type III atresia ani associated with rectovaginal fistula are also reported as having an ectopic anus (Prassinos et al., 2003). However, it is unclear if type

IV atresia ani has ever been reported in dogs and cats (Ellison & Papazoglou, 2011).

Type I Congenital stenosis of a patent anus Type II Persistence of a complete anal membrane

Type III Presence of an imperforate anus with the

Type IV Normal ending of the terminal rectum and

alone or a combination of an anal membrane with the rectum ending as a blind pouch cranial to the membrane.

rectum terminating further cranially.

a blind pouch within the pelvis.

anus while the cranial rectum terminates as

Viana &Tobias, 2005; Ellison & Papazoglou, 2011) [Figures 3-7].

Atresia Type Anomaly

Table 1. Anatomic types of atresia ani in dogs and cats

Fig. 3. Schematic representation of type I atresia ani.

**4. Classification** 

Tobias, 2005). The cloaca is a common route for gastrointestinal and urogenital tracts in the canine and feline embryo. Retention of the cloaca occurs in vertebrate animals other than placental mammals; in higher mammals the cloaca separates during embryologic development. By the seventh to eighth week of development the urorectal fold, which located between the allantois and the hindgut openings in the cloaca, initially divides the cloaca into the dorsal part called the rectum and the ventral part called the urogenital sinus (Figures 1 &2). The urogenital sinus is further differentiated into the urethra and the urinary bladder. The terminal end of the hindgut forms the cranial anal canal and the anus is formed at a later time by ectodermal ingrowth of the perineum. A breakdown in physiological embryologic differentiation of the cloacal region may lead to a variety of congenital malformations of the anorectum. Failure of the urorectal fold to divide the cloaca completely or failure of the anal membrane to rupture after anal creation results in atresia ani.

Fig. 1. Urorectal fold (arrows) located between the alantois and hindgut by 7th to 8th week of development.

Fig. 2. The cloaca is divided into urorectal sinus and rectum.
