Page 816 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Gastrointestinal system: 4.2 The lower gastrointestinal tr act 791

VetBooks.ir Pain is usually intermittent. Heart rate may be normal Aetiology/pathophysiology
Ileocolonic aganglionosis results when a foal inher-
or elevated consistent with the degree of pain, but rarely
exceeds 60 bpm. Borborygmi are usually increased.
Faeces may be normal or soft. Signs of severe pain are its two copies of the lethal white gene. Breeding of
two horses that are heterozygous for the lethal white
uncommon but can be observed transiently. gene will result in development of this condition in
approximately 25% of foals. Typically, horses carry-
Differential diagnosis ing the gene responsible for this condition display
A variety of causes of colic should be considered, par- the Overo colour pattern, but this is not always the
ticularly tympanic colic and large-colon impaction. case. The gene is most common in American Paint
horses, but may also be found in other breeds, par-
Diagnosis ticularly Quarter horses, Pintos and Saddlebreds.
Physical examination findings are suggestive but not This is an autosomal recessive condition, and
diagnostic. Usually, no abnormalities are palpable p/r. therefore the abnormal gene must be acquired from
Gastric reflux is not present. Borborygmi are increased both parents. This gene codes for endothelin recep-
and intestinal hypermotility may be evident ultraso- tor B; however, it is unclear how the alteration in
nographically. Peritoneal fluid and haematology are endothelin B receptor produces ileocolonic agan-
unremarkable. A presumptive diagnosis is often made glionosis. There is a complete absence of myenteric
on clinical presentation and response to therapy. ganglia from the terminal ileum to the small colon.

Management Clinical presentation
Horses with spasmodic colic may require no treat- Affected foals may be entirely white or have small
ment, although analgesics are commonly admin- areas of pigmentation, mainly on the forelock and
istered, and their use is logical. Administration of tail. Irises are white. Foals are normal at birth but
an analgesic/spasmolytic drug combination may develop signs of colic within the first 4–24 hours of
be useful (Table 4.2). Fluid therapy is rarely neces- life (Fig. 4.117). Abdominal pain and distension are
sary, although it may help restore normal intestinal progressive as intestinal distension develops. A small
motility and ensure adequate hydration. As such the volume of faeces can occasionally be passed follow-
administration of 10–20 ml/kg of water or balanced ing enema administration.
electrolyte solution via a NG tube should be consid-
ered. Feeding should be restricted until signs of colic
and the effects of the analgesics have abated. 4.117

Prognosis
The prognosis for full recovery is excellent. Most
affected horses respond to a single treatment course.
If pain is progressive or poorly responsive to analge-
sics, the diagnosis should be reconsidered. If there
is reoccurrence then an attempt should be made to
identify an underlying cause.

ILEOCOLONIC AGANGLIONOSIS
(OVERO LETHAL WHITE SYNDROME)

Definition/overview
This is an inherited (homozygous recessive), con-
genital disorder of white-patterned horses character-
ised by myenteric aganglionosis and fatal functional Fig. 4.117 Foal with ileocolonic aganglionosis
intestinal obstruction. displaying signs of colic.