Enterolith - Horse

Also known as: Intestinal calculi

Description

Enteroliths are mineralized concretions of magnesium ammonium phosphate (struvite) that develop in the right dorsal colon of the horse. The enteroliths typically form in a concentric pattern around a central nidus such as a silicone stone, metal object, or rarely a piece of fibrous material such as baling twine. The pathogenesis of the disease is not fully understood but long-term ingestion of a diet rich in protein, phosphorus and magnesium (such as alfalfa hay) is thought to be a major contributing factor. The relative hypomotility of the right dorsal colon is also thought to have a role in enterolith formation.

Signalment

All horses may be affected by enterolithiasis but certain breeds are predisposed including Arabians, Morgans and Miniature horses. Females are more likely to develop enteroliths than males; this is hypothesised to be due to the presence of prostaglandins in the serum leading to alterations in gut motility. Thoroughbreds appear to be underrepresented. Middle aged horses are most affected. Enterolithiasis in horses less than four years of age is rarely reported.

Clinical signs

Horses with enterolithiasis generally have a history of being fed a large quantity of alfalfa hay or pellets in their diet. Small enteroliths may pass through the gastrointestinal tract unnoticed. Horses with a partial obstruction of the right dorsal or transverse colon may display signs of chronic or recurrent colic, sometimes occurring over a period of weeks up to a year. Clinical signs in these horses may be non-specific including weight loss, anorexia, changes in demeanour and lethargy. Signs indicative of mild abdominal discomfort may also be present such as pawing, flank-watching, kicking or resentment of a girth.

If the enterolith is large enough to cause an obstruction (most commonly in the transverse or small colon), signs of acute, severe abdominal pain are usually seen. Other clinical signs such as tachycardia, tachypnoea, prolonged capillary refill time and bilateral abdominal tympany are frequently observed. If the integrity of the colonic wall has been compromised, signs reflecting endotoxaemia may develop.

Diagnosis

In most cases of enterolithiasis, passage of gas, faeces and lubricants such as mineral oil will still occur. In the case of obstruction, however, the passage of faeces will be blocked and defecation will no longer take place. Rectal examination rarely leads to the identification of an enterolith but may reveal distension of the large colon and tight mesenteric bands. Abdominal radiography may be useful in many horses and is ideally performed after fasting for 24 hours. It is more helpful for detecting enteroliths in the large colon and is generally most successful in small horses or ponies. Enteroliths are seen as sperical, homogeneously opaque bodies within the intestinal lumen. Transrectal or transabdominal ultrasonography may also be diagnostic.

For help with diagnosing colic see Colic Diagnosis in Horses.

Treatment

Treatment for all large obstructions is removal via surgical laparotomy (coeliotomy). The colon is evacuated via a colotomy at the pelvic flexure, after which the enterolith is gently manipulated to the incision site and removed.

Also see Medical Treatment of Colic in Horses.

Prognosis

The prognosis is excellent when surgery is performed early. If colonic necrosis or rupture has occurred the prognosis is grave. Horse in which severe cardiovascular compromise has occurred also have a poorer prognosis.

Prevention

It is recommended to ensure that horses that have been affected by this condition are fed roughage that consists of at least 50% hay or grass. Alfalfa should preferably be eliminated from the diet. Anecdotal reports suggest that cider vinegar added in to hard feed may help to prevent the formation of enteroliths. It is also suggested to counteract the alkalinising effects of alfalfa if no alternative diet is available.

Literature Search

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Enteroliths in horses publications

References

• Lavoie, J. P., Hinchcliff, K. W. Blackwell's Five-Minute Veterinary Consult: Equine John Wiley & Sons
• Mair, T. S., Divers, T. J., Ducharme, N. G. Manual of Equine Gastroenterology Elsevier Health Sciences
• Robinson, N. E., Sprayberry, K. E. Current Therapy in Equine Medicine Elsevier Health Sciences