Meconium impaction in foals: clinical signs, diagnosis and 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.
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 by sepsis.
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.
Unlike mature horses, abdominal radiography can provide some useful information in a neonatal foal with colic. Grid, rare-earth screens and sufficient mAs (5-28) and kVp (80-120) should be used.
Radiographs may be taken with the foal either standing or in lateral recumbency; however, keep in mind that fluid-gas interfaces are easier to detect in the standing foal, when the radiographic beam is horizontal or perpendicular to the dorsoventral plane of the gas/fluid interface in the abdomen.
Right and left lateral views should be obtained because some structures will be more obvious, depending on which side of the abdomen is closer to the film. Some degree of gas is normally visible in the stomach, small intestine, cecum and small colon. However, severe generalized gas distension of the large intestine is more commonly associated with mechanical obstruction than with inflammatory lesions of the large colon (Photo 2, p. 4E).
Meconium frequently appears as granular contents in the ascending or descending colon. Lower intestinal contrast studies (i.e., barium enema) have been reported to have 100 percent sensitivity and 100 percent specificity for identifying mechanical obstruction (meconium impaction, atresia coli) of the transverse colon or small colon in foals less than 30 days of age.
The foal should be restrained or lightly sedated and placed in lateral recumbency. A 24-french Foley catheter is placed into the rectum and the bulb gradually inflated. By gravity flow, administer up to 20 ml/kg of 30% wt/vol barium.
Transabdominal ultrasonography with linear array 4 to 7 MHz transducers is sufficient for visualization of most intra-abdominal structures in the neonatal foal; however, a curvi-linear transducer will optimize image quality.
The foal may be scanned either in lateral recumbency or standing, but remember that a heavily impacted small colon may descend to the dependent portion of the abdomen and could be overlooked if only the nondependent side of a recumbent foal is scanned. Furthermore, gas in nondependent bowel, especially gas in the large colon, may preclude examination of deeper structures in laterally recumbent foals.
Meconium may appear hyperechoic, hypoechoic or as a mixture of echogenicities in the hypomotile intestine, and fluid or gas-distended intestine may be present proximal to the obstruction (Photo 3, p. 4E).
Retained or impacted meconium in the small colon often appears as a row of "balls" surrounded by a thin layer of hyperechoic gas in sacculated intestine and is most easily identified in the dependent portion of the left caudal abdomen in the standing foal. It may be traceable dorsal to the urinary bladder.
Meconium in the large colon typically is more amorphous. Extensive gas in the large colon frequently hinders visualization of other intra-abdominal structures.
Foals with an uncomplicated meconium impaction should have a normal complete blood count and serum biochemical profile, though changes consistent with stress (leukocytosis characterized by mature neutrophilia and lymphopenia and hyperglycemia) and dehydration (azotemia, hemoconcentration and hyperproteinemia) can be present.
As with any neonatal foal, testing for adequate transfer of passive immunity is advisable.
Medical therapy for meconium impaction includes use of analgesics, intravenous polyionic, isotonic fluids, oral laxative therapy and enemas.
Foals with meconium impactions are expected to exhibit some degree of pain. Judicious use of analgesics is required to balance the necessity to provide relief from pain with the ability to appropriately assess the patient's progress.
Low doses of flunixin meglumine (0.25 to 0.5 mg/kg IV q 12 hours) are frequently sufficient. Higher doses of flunixin meglumine (1 mg/kg IV) may be necessary, but repeat dosing should be accompanied by careful reassessment of the patient to assure that medical management is sufficient and that the diagnosis of meconium impaction is accurate and uncomplicated.
Other shorter-acting sedatives or analgesics that can be used in the foal include diazepam (0.05-0.2 mg/kg IV) and butorphanol (0.01-0.04 mg/kg IV). Mineral oil (4 to 8 ounces given via a nasogastric tube) is used for its lubricating effect. Milk of magnesia (1 to 2 ounces per os or nasogstric tube) provides an osmotic laxative effect but should be used sparingly because it may be dehydrating.
The detergent dioctyl sodium sulfo-succinate can be quite irritating and should be avoided in both oral and rectal therapy. Castor-oil therapy has been described, but can provoke violent abdominal pain in the foal.
Enemas are a mainstay of treatment for small-colon meconium impactions. Warm-water liquid-detergent enemas (1/2 teaspoon liquid detergent to 500 ml water) are purportedly gentle to the rectal mucosa and are effective.
Commercial phosphate enemas also can be used, but repeated administration (> 2 enemas) may increase the risk of phosphate toxicity.
Recently, acetylcysteine retention enemas have been reported to be a highly successful treatment. It is believed that the acetylcysteine cleaves disulphide bonds in the mucoprotein molecules in meconium, decreasing its overall tenacity. A 4 percent acetylcysteine solution, pH 7.6, is made by adding 20 grams of baking soda and 8 grams of acetylcysteine to 200 ml of water.
A 30 french Foley catheter with a 30 ml bulb is inserted approximately 2.5 to 5 cm into the rectum and the bulb is slowly inflated to occlude the rectum. One hundred to 200 ml of the 4 percent acetylcysteine solution is administered by gravity flow and retained by clamping the catheter for 30 to 45 minutes, at which point the catheter cuff is deflated and the tubing removed.
At the University of California-Davis, from 1987 until 2002, 41 of 44 foals (93 percent) with meconium impactions were successfully treated medically with acetylcysteine enemas. About 40 percent of these former cases required more than one acetylcysteine enema (up to three were given at about 12-hour intervals), and in about 20 percent of the cases it took more than 12 hours for the impaction to resolve.
An acetylcysteine retention enema kit is available commercially (EZ-Pass Foal Enema Kit, Animal Reproduction Systems, Chino, Calif.). Occasionally, repeated use of an enema generates significant rectal and small-colon mucosal irritation that leads to continued straining despite effective removal of the impaction.
Although the majority of foals with meconium impactions are treated medically, surgery is indicated in some cases. It should be considered in foals with progressive abdominal distension, persistent tachycardia (especially if > 120 beats/min), and signs of persistent and progressive pain.
Post-operative intra-abdominal adhesion formation is the most common anticipated complication; thus surgery often is delayed for fear of poor long-term survival. The prognosis for foals with a meconium impaction following either medical or surgical treatment is generally considered to be good to excellent with 80 percent to 94 percent surviving long term.
•Pusterla N, Magdesian K, Maleski K, et al.: Retrospective evaluation of the use of acetylcysteine enemas in the treatment of meconium retention in foals: 44 cases (1987-2002), Equine Vet Educ June 170, 2004.
• Fischer A Yarbrough T: Retrograde contrast radiography of the distal portions of the intestinal tract in foals, J Am Vet Med Assoc 207 (6): 734, 1995.
• Hughes F, Moll H, Slone D: Outcome of surgical correction of meconium impactions in 8 foals, J Equine Vet Sci 16 (4): 172, 1996
Michelle Henry Barton is the Josiah Meigs Distinguished Teaching Professor at the University of Georgia's College of Veterinary Medicine, where she holds the Fuller E. Calloway Professorial Chair and is a large-animal internist in academic practice. She received her DVM from the University of Illinois in 1985, her PhD in physiology at the University of Georgia in 1990 and became an ACVIM diplomate in 1990.