Knottenbelt and Pascoe's Color Atlas of Diseases and Disorders of the Horse

Part 5: The gastrointestinal tract

Congenital/developmental disorders

Intestinal atresia: atresia ani (Fig. 1.86) / atresia coli (Fig. 1.87)

Both disorders are uncommon congenital abnormalities. Atresia coli, although rare, is the most common type of intestinal atresia and may be related to vascular accidents during intestinal development; it usually occurs in the region of the pelvic flexure, that is between the left ventral and left dorsal colon, but may affect any site in the large or small colon. Foals affected by either condition are born normally and appear to be normal for the first 24 hours of life. As feces and gas accumulate proximal to the blind-ended anus or section of colon, progressive signs of abdominal pain and distension develop. Atresia coli is associated with more severe and earlier onset signs than atresia ani. In some fillies with atresia ani, development of rectovaginal fistula will be associated with some relief of pain, depending on the amount of feces and gas that can be passed. In cases of atresia coli in which intestinal rupture occurs, there may be a transient period of improvement, based on lessening of intraintestinal pressure, followed by rapid deterioration within hours as septic peritonitis ensues.

Figure 1.86 Atresia ani. Fortunately in this foal it was only the external opening of the anus that was absent and surgical creation of an opening yielded a good result.
Differential diagnosis: other causes of progressive abdominal distension with failure to pass feces:

• Meconium impaction

• Lethal white syndrome.

Figure 1.87 Atresia coli. The large colon in this case ends in a blind sac with subsequent enlargement in size due to failure of passage of contents.

Diagnosis:

• Atresia ani. The perineum should be examined for evidence of a normal anus. In some cases, the anus is absent. In others, an external anus is visible but not patent. If feces are present in or around the vulva, vaginal examination for a rectovaginal fistula should be performed. If an external anus is present, a blind-ended structure may be palpable within the rectum. Proctoscopy can be used to confirm that a palpable obstruction is indeed atresia ani. Foals should be examined for other congenital abnormalities. Rarely, deformities (or absence) of the tail or vagina may also be present.

• Atresia coli. Diagnosis is more difficult because the lesion is not evident grossly; meconium impaction is often suspected initially because of the failure to pass feces and progressive abdominal pain and distension. Digital rectal examination may be highly suggestive as there will be no fecal ‘staining’ of the glove and only mucus evident in the rectum. A blind-ended lesion may be evident with proctoscopy/colonoscopy. Distension of the proximal intestine may be evident radiographically. Contrast radiographs (barium enema, barium series) can confirm an obstruction but not necessarily atresia coli. Exploratory laparotomy may be required for definitive diagnosis. Thorough examination for other congenital problems that may accompany atresia coli (i.e. renal aplasia or hypoplasia, hydrocephalus, cerebellar dysplasia) should be performed.

Treatment and prognosis

• Atresia ani. Prompt surgical correction is required, and has been successful in some cases. The prognosis with surgery is dependent, in part, on the amount of intestine involved and whether concurrent abnormalities are present. The prognosis is better when the anal sphincter is normal and the obstruction is only a thin, bulging layer of tissue. Affected horses should not be bred because of the possibility of a genetic basis.

• Atresia coli. This condition should be regarded as fatal. Intestinal motility problems or failure of the anastomosis have resulted in poor outcomes in cases in which surgical correction has been attempted.

Inguinal hernia (see also p. 48) (Figs1.88 & 1.89)

Most congenital inguinal hernias are indirect, with the intestines passing through an intact vaginal ring and contained within the parietal layer of the vaginal tunic. Indirect hernias are usually reducible, are not life-threatening and usually resolve with manual reduction within a few days. A direct hernia occurs when the parietal vaginal tunic or the peritoneum in the vaginal ring region tears and the intestines become positioned under the skin. Direct hernias are normally irreducible, contain large amounts of intestine, are life-threatening and regarded as a surgical emergency.

Figure 1.88 Congenital indirect inguinal hernia. Reproduced from McAuliffe SB, Slovis NM (eds), Color Atlas of Diseases and Disorders of the Foal (2008), with permission from Elsevier.
Figure 1.89 Congenital direct inguinal hernia. This foal had its entire colon herniated through the inguinal ring and was lying in a subcutaneous position.

An uncomplicated hernia is any type of hernia which either does not contain intestine or contains non-incarcerated intestine. A complicated hernia is any type of hernia which contains incarcerated or strangulated intestine.

Diagnosis and treatment

• Hernias are usually apparent visually or detected by palpation. Individual loops of small intestine may be apparent under the skin in foals with direct inguinal hernias. The inguinal skin is thin and associated stretching may result in tearing of the skin or disruption of dermal circulation.

• Signs of colic are often but not always observed when strangulation has occurred but will develop as intestinal compromise progresses and intestinal distension develops.

• Direct inguinal hernias and complicated hernias are regarded as a surgical emergency. Indirect hernias depending on their size may be managed conservatively with frequent reduction or bandaging to facilitate reduction although bandage sores are a frequent complication.

Lethal white syndrome (Figs. 1.90 & 1.91)

Also known as ileocolonic aganglionosis, lethal white syndrome is an inherited, congenital condition that occurs in foals that are homozygous for the lethal white gene. It is an autosomal recessive condition that means that if two carriers (heterozygotes) are bred approximately 1 in 4 of their offspring will be homozygous or affected. Heterozygotes usually have the Overo color pattern, which is most common in American Paint horses but may also be found in Quarter Horses, Pintos and Saddlebreds. Most affected foals are entirely white with white irises, but some may have small areas of pigmentation on the forelock and tail. Within 4–24 hours of life, abdominal distension and pain develop. These signs are progressive and become very severe. Minimal feces are passed.

Figure 1.90 Lethal white foal showing early signs of colic.
Differential diagnosis: other causes of progressive abdominal distension with passage of little or no feces:

• Atresia ani/coli

• Impending enteritis

• Peritonitis

• Meconium impaction.

Figure 1.91 Colonic hypoplasia. No histopathology was performed in this case but it was believed that the etiology may have been some form of ileocolonic aganglionosis (similar to that found in lethal white foals). This was a 2-day-old TB foal that presented with early colic signs (at 24?h). Serial ultrasound examinations showed progressive small intestinal distension with no intestinal motility detectable on ultrasound or auscultation.

Diagnosis and treatment

• A white foal born to an Overo–Overo mating with progressive abdominal distension and minimal fecal production is highly suggestive. This history and progression of clinical signs is typically used to make a diagnosis. It should not be confused with colic in white foals born to non-Overo parents.

• Diagnostic imaging (ultrasound, radiography) demonstrates gaseous intestinal distension.

• Genetic testing can determine whether the foal is homozygous for the condition but is impractical considering the time delay.

• There is currently no treatment; this is an invariably fatal condition and affected foals should be promptly euthanized.

Meconium impaction (Figs. 1.92–1.95)

Normally, meconium is passed within a few hours of birth....