**4. Classification**

180 A Bird's-Eye View of Veterinary Medicine

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

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

development.

Four anatomic types of atresia ani have been described in dogs and cats (Aronson, 2003; Viana &Tobias, 2005; Ellison & Papazoglou, 2011) [Figures 3-7].

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).


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

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

Atresia Ani in Dogs and Cats 183

Fig. 7. Schematic representation of type II atresia ani combined with rectovaginal fistula.

A history of absence of defecation, which sometimes may go unnoticed, is reported. Clinical signs of atresia ani are usually evident within a few weeks of birth and depend on the type of atresia (Aronson, 2003; Viana &Tobias, 2005; Ellison & Papazoglou, 2011). Animals with type I atresia ani may exhibit constipation and tenesmus soon after weaning (Figure 8). A stenosed anal opening is evident on digital rectal palpation. Puppies and kittens belonging to types II, III and IV are clinically normal for the first 2 to 4 weeks of birth. A history of anorexia, depression, absence of defecation, tenesmus and abdominal distention are reported to follow this time frame (Figure 9). Physical examination may also reveal a dimple at the site of the closed anal opening (Figures 10 &11), abdominal enlargement, discomfort on abdominal palpation and perineal swelling depending on the type of anomaly. Abdominal enlargement may be attributed to colonic distention with feces or gas, fecal impaction or even megacolon. Longstanding cases may develop vomiting and dehydration. Animals with rectovaginal or rectourethral communications often show passage of watery to formed feces through the vagina or urethra; these animals may be in better physical condition that others with no rectovaginal or rectourethral communications (Prassinos et al., 2003; Rahal et al., 2007) [Figure 12]. Multiple congenital anomalies such as umbilical hernias, cleft palates, open fontanels, hypospadias, tail agenesis and deafness in dogs and sacrocaudal dysgenesia and hydrocepahalus in kittens may accompany atresia ani and should not escape a thorough physical or other diagnostic examination (Suess et al., 1992; Aronson, 2003; Prassinos et al., 2003; Rahal et al., 2007; Ellison & Papazoglou, 2011) [Figure 13]. Rectocutaneous fistulas associated with type II atresia ani have been recently reported in a cat (Tsioli et

**5. History and clinical signs** 

al., 2009) [Figure 14].

Fig. 4. Schematic representation of type II atresia ani.

Fig. 5. Schematic representation of type III atresia ani. The blind rectal pouch is more than 1 cm away from the anal dimple.

Fig. 6. Schematic representation of type III atresia ani. It is not clear if this type of atresia has been reported in small animals.

Fig. 5. Schematic representation of type III atresia ani. The blind rectal pouch is more than 1

Fig. 6. Schematic representation of type III atresia ani. It is not clear if this type of atresia has

Fig. 4. Schematic representation of type II atresia ani.

cm away from the anal dimple.

been reported in small animals.

Fig. 7. Schematic representation of type II atresia ani combined with rectovaginal fistula.

#### **5. History and clinical signs**

A history of absence of defecation, which sometimes may go unnoticed, is reported. Clinical signs of atresia ani are usually evident within a few weeks of birth and depend on the type of atresia (Aronson, 2003; Viana &Tobias, 2005; Ellison & Papazoglou, 2011). Animals with type I atresia ani may exhibit constipation and tenesmus soon after weaning (Figure 8). A stenosed anal opening is evident on digital rectal palpation. Puppies and kittens belonging to types II, III and IV are clinically normal for the first 2 to 4 weeks of birth. A history of anorexia, depression, absence of defecation, tenesmus and abdominal distention are reported to follow this time frame (Figure 9). Physical examination may also reveal a dimple at the site of the closed anal opening (Figures 10 &11), abdominal enlargement, discomfort on abdominal palpation and perineal swelling depending on the type of anomaly. Abdominal enlargement may be attributed to colonic distention with feces or gas, fecal impaction or even megacolon. Longstanding cases may develop vomiting and dehydration. Animals with rectovaginal or rectourethral communications often show passage of watery to formed feces through the vagina or urethra; these animals may be in better physical condition that others with no rectovaginal or rectourethral communications (Prassinos et al., 2003; Rahal et al., 2007) [Figure 12]. Multiple congenital anomalies such as umbilical hernias, cleft palates, open fontanels, hypospadias, tail agenesis and deafness in dogs and sacrocaudal dysgenesia and hydrocepahalus in kittens may accompany atresia ani and should not escape a thorough physical or other diagnostic examination (Suess et al., 1992; Aronson, 2003; Prassinos et al., 2003; Rahal et al., 2007; Ellison & Papazoglou, 2011) [Figure 13]. Rectocutaneous fistulas associated with type II atresia ani have been recently reported in a cat (Tsioli et al., 2009) [Figure 14].

Atresia Ani in Dogs and Cats 185

Fig. 11. An anal dimple associated with type II atresia ani in a dog.

Fig. 12. Atresia ani type II associated with rectovaginal fistula in a puppy.

Fig. 13. Atresia ani type III associated with rectovaginal fistula and tail agenesis in a puppy.

Fig. 8. Atresia ani type I in a mixed breed puppy.

Fig. 9. A mixed breed puppy with abdominal distention associated with type II atresia ani.

Fig. 10. Atresia ani type II of the dog of figure 9.

Fig. 9. A mixed breed puppy with abdominal distention associated with type II atresia ani.

Fig. 8. Atresia ani type I in a mixed breed puppy.

Fig. 10. Atresia ani type II of the dog of figure 9.

Fig. 11. An anal dimple associated with type II atresia ani in a dog.

Fig. 12. Atresia ani type II associated with rectovaginal fistula in a puppy.

Fig. 13. Atresia ani type III associated with rectovaginal fistula and tail agenesis in a puppy.

Atresia Ani in Dogs and Cats 187

Fig. 15. Abdominal radiograph of a dog visualizing gas accumulation in the colon and rectum. The location of the terminal rectum is also shown (arrow) [Courtesy Dr. M.N.

Fig. 16. Abdominal radiograph showing megacolon associated with type II atresia ani in a

dog (Courtesy Dr. M.N. Patsikas, Aristotle University of Thessaloniki).

Patsikas, Aristotle University of Thessaloniki].

Fig. 14. Rectocutaneous fistulas ventral and lateral to the tail (arrows) in a cat with type II atresia ani
