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Enteroliths in HorsesBy Kentucky Equine Research Staff · February 14, 2011

Enteroliths are mineral masses that form in the colon of a horse. They are also known as intestinal stones or calculi. Usually these stones build up in thin layers around a bit of foreign matter (a small piece of wood, wire, hair, or other material) that the horse has swallowed. Although mineral content varies, about 90% of a typical enterolith consists of struvite (a hydrous phosphate of magnesium and ammonia) and vivianite (a hydrous phosphate of iron). Combinations of sulfur, sodium, potassium, calcium, titanium, aluminum, and nickel make up the remaining 10%. A horse may have one or more enteroliths ranging in diameter from pea-sized to softball-sized or larger.

Although enteroliths have been found in foals less than a year old, they are more common in animals over the age of 10. Horses of Arabian breeding seem to be most often affected, but stones are not uncommon in Quarter Horses, Morgans, Saddlebreds, and donkeys. Scattered diagnoses have been reported throughout America, particularly California and Florida. There have also been cases reported in France and the United Kingdom, and there is at least one report of enteroliths in a zebra herd.

Some horses pass small enteroliths with their manure, but masses larger than a golf ball usually cause recurring colic symptoms as passage of gut contents is impaired. Affected horses may show decreased appetite resulting in a drop in weight. Ultimately large stones lodge in the gut, frequently where the intestine narrows at the pelvic flexure or right dorsal colon, and the gut wall begins to necrotize from abrasion and pressure. Unless the stone is removed, the intestine eventually ruptures and the horse develops fatal peritonitis. When the problem is detected before the intestine perforates, about 95% of horses make a full recovery from surgery to remove the enterolith.

In areas where this is a common problem, veterinarians suspect enteroliths if a horse has frequent colic episodes. Sometimes the stone can be felt during rectal palpation, especially if the horse is positioned with its head uphill, and radiographs can reveal the mineral mass in about 65% of affected horses. Management history can also identify horses that seem to be predisposed.

While no one knows exactly why a particular horse develops intestinal stones, three factors seem to be associated with an increased risk.

Diet. A study of 900 horses at the University of California-Davis showed that two-thirds of horses with enteroliths were fed diets with very high levels of alfalfa (lucerne) hay, while alfalfa made up only about 60% of the diet of unaffected horses. Some theories link phosphorus-rich wheat bran, water with a high content of dissolved minerals, or iron-laden feeds to enterolith formation.

Management. Horses turned out on unlimited fresh forage seem to have little trouble with enteroliths. Stall-kept horses on limited amounts of easily digested hay and grain may develop stones because there is less dietary residue in the gut, and this factor coupled with a lack of exercise leads to longer transit time for intestinal contents. Stall bedding choice may also play a role. It has been suggested that horses bedded on straw have a chance to nibble on material with a higher level of insoluble substances, while horses on shavings or rubber stall mats lack this access to a substance that can enhance gut motility.

Genetics. About 10% of horses suffering from enteroliths have at least one sibling with the same problem. One theory is that some horses have a genetic defect that causes cells in the intestinal wall to mishandle electrolyte transport. Diets of alfalfa and wheat bran load the gut with calcium, magnesium, phosphorus, and protein. Calcium from the alfalfa and ammonia released by protein digestion allow minerals to precipitate and attach to foreign objects in the gut instead of being absorbed through the wall of the intestine. While this theory has not been proven, it could explain why horses undergoing surgery for obstruction caused by enteroliths had less acidic bowel contents and higher levels of calcium, magnesium, phosphorus, sodium, and potassium than horses with obstructions from conditions other than enteroliths.

Several management steps have been suggested to lessen the risk of enterolith development. One is decreasing the mineral overload by switching from alfalfa to grass hay and eliminating or cutting down on wheat bran in the horse's diet. Another is encouraging gut movement by increasing exercise and grazing time for horses that have been kept in the stall. A third is preventing ingestion of foreign objects by cleaning up pastures and turnout paddocks.

Including a cup of vinegar in the horse's feed has been shown to decrease intestinal pH, but there is no hard evidence of an effect on enterolith formation. Like the other management tips, this idea is formulated on a “won't hurt, might help” basis.

As with any change in a horse's diet, gradual adjustments should be made over a period of several days or weeks. However, histories of many horses admitted to clinics for enterolith surgery have shown significant dietary changes in the previous weeks, suggesting that such alterations may have caused the stones to shift and create a blockage. Therefore, if a horse shows recurrent colic signs consistent with enterolith problems, researchers strongly advise taking diagnostic radiographs before making major adjustments to the diet. Unfortunately, exploratory abdominal surgery is usually required to diagnose and resolve intestinal obstruction caused by enteroliths.

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