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Equine Health Resource

Enteroliths, Intestinal Stones

Synonyms: Bezoars, Equine Pearls, Gut Rocks

Enteroliths are magnesium ammonium phosphate (struvite) stones that form in the equine colon. They can range in size, shape, color and weight depending on the unique circumstances in which they are created. Large, round enteroliths that are the weight and consistency of a bowling ball are not that uncommon. Multiple, smaller enteroliths can sometimes be found together in the intestine, and are often irregularly shaped with flat sides due to abrasion and wear against one another.

Enteroliths are very common on the West Coast of the United States and to a lesser extent, the Eastern United States. The prevalence of enteroliths in certain regions is thought to relate to magnesium content of water and hay in those regions. In areas where enterolith is common, it is high on the diagnosis list for horses showing repeated colic bouts.

The presence of enteroliths in the colon usually ultimately results in abdominal pain (colic). Horses can have a heavy enterolith in their intestine and not show signs of pain over months to years. Pain results when the stone blocks gas and flow of intestinal contents, causing gas and feed distention. The stone is thought to roll or move downstream to a point at which it blocks gas and feed flow. The horse then becomes painful and distended with gas. In many cases, the stone can roll back again, allowing gas and fluid to pass, and the pain to resolve.


Stones are found when colic diagnostics are run, or at surgery. Only occasionally are the stones palpated on a rectal exam, because they are usually out of the examiner’s reach. They are hard to locate using ultrasound because ultrasound cannot penetrate them or differentiate them well from gas or feed.

Abdominal radiography is the most useful diagnostic to locate them, but is not always available, as it requires a very powerful x-ray generator to penetrate a full-sized horse’s abdomen.

Treatment is removal of the stone at colic surgery.


  • How should I manage my horses to minimize the likelihood of this condition?

    Excessive intake of alfalfa hay has been thought to increase the possibility of enterolith formation. Do not feed pure alfalfa hay, use a grass/alfalfa mix instead.

    Some believe that acidification of feed with vinegar may be helpful in preventing formation of these stones. However, you should talk to your vet about this or any other dietary recommendation.

    Helpful Outside ResourcesCredible Equine Health Information on the Internet

    Author: Doug Thal DVM Dipl. ABVP


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