It's one of the most common causes of colic in the newborn foal.
Photo 1: This is the typical stance of a foal that is straining to defecate. The piece of white tape on this foal's back was
used as a reference point for serially measuring abdominal circumference during treatment.
Meconium impaction implies failure to evacuate sufficient quantities of meconium — the sticky, caramelized feces of the foal
that is composed of intestinal secretions, swallowed amniotic fluid and cellular debris — with subsequent development of signs
of colonic obstruction.
In one study of 30 foals, it was reported that the total weight of meconium was equal to 1 percent of the foal's body weight.
Most foals will start to evacuate meconium within one to two hours after birth, shortly after the ingestion of colostrum that
acts both as a laxative and stimulator of the gastrocolonic reflex. Most is evacuated within 12 hours after birth and is replaced
by "milk" feces that are pasty and yellow in appearance.
But evacuation of meconium may be delayed (meconium retention) as the result of ileus secondary to another primary disease,
such as septicemia or neonatal encephalopathy.
Photo 2: These abdominal radiographs were taken on a foal with a meconium impaction. The excessive gas-distended large colon
is characteristic for distal mechanical obstruction.
In these cases, although passage of meconium may be slower than expected, clinical signs of obstruction are not present. Meconium
can be impacted or retained in the small colon (low impaction) or within the ascending colon, particularly in the transverse
or right dorsal colon (high impaction). Passage of "milk feces" does not necessarily indicate that all of the meconium has
been removed from the colon.
Photo 3: Transabdominal ultrasonographic appearance of four "balls" of meconium impacted in the small colon that is displaced
dependently in the left caudoventral abdomen of a foal.
History and clinical signs
It has been suggested that meconium impaction is more likely to occur in colts and in foals of more than 340 days gestational
age. Early signs in neonatal foals may manifest only as reduced frequency of nursing and prolonged recumbency. Other subtle,
but significant, signs of abdominal pain in foals include general restlessness, especially while in recumbency, such as stretching
of limbs, twisting of the head or neck, rolling onto the dorsum and frequent straining to defecate and/or urinate.
In the standing foal, signs classically associated with straining to defecate include tail swishing, a "water spout" tail
and a "camped under" leg stance with a dorsiflexed back (Photo 1).
In contrast, a flat or ventroflexed back with the hind legs stretched backward and the tail held up is associated with urination.
Other signs of abdominal pain include lip curling, flank biting or watching, pawing at the ground and kicking at the abdomen.
As the impaction intensifies, progressive abdominal distension develops.
Typically, the rectal temperature of affected foals remains within normal limits, unless the meconium impaction is accompanied
Tachycardia and tachypnea are expected. Intestinal borborygmi usually are present, but are not a reliable sign of obstruction.
Digital rectal examination may identify the impaction within the pelvic inlet. Deep transabdominal palpation may reveal firm
ingesta in the colon proximal to the pelvic inlet.
Gas distension of the large intestine or cecum proximal to the obstruction may elicit an audible high-pitched "ping" with
percussion. Repeated measurement of the abdominal circumference is helpful to objectively determine the course of progression
of abdominal distension.
The urachus can become patent in foals that strain excessively from meconium impaction; thus the umbilicus should be carefully
examined. With intense abdominal pressure from obstruction, rolling and straining, the urachus or the urinary bladder may
rupture and confound the diagnosis.
Extensive mural damage from an extensive meconium impaction can lead to bacterial translocation and secondary septicemia and,
in such cases, fever, injected sclerae or mucous membranes and aural petechial hemorrhages may be concurrently present.