Oysters and mollusks form pearls as a protective reaction to irritants, like a parasite or grain of sand invading their shells. A similar concept is at work with enterolith formation in the horse, but instead of a valuable pearl, the end result may lead to colic or surgery.

Enteroliths are mineralized intestinal stones that sometimes form within the horse’s colon. The condition can become serious if a stone gets too large to pass through the digestive tract. When an enterolith causes obstruction, surgical removal is the only solution to avoid a fatal gastrointestinal rupture.

It takes at least a year—but more likely several years—for an enterolith to become large enough to form an obstruction.

The process under which enteroliths gradually form is known as enterolithiasis.

What’s Causes Enteroliths in Horses?

Like a pearl, an enterolith starts by forming around something. This small, indigestible object could be a pebble, coin, piece of rubber or wire, or even a piece of baling twine, rope, or fabric the horse has swallowed.

Mineral salts in the horse’s intestine or colon sometimes form around the object. These struvite crystals are comprised mainly of magnesium, ammonium and phosphate.

To support this struvite mineralization, there must be the correct environment within the colon, which includes a higher pH/more alkaline colonic contents, combined with a rich combination of minerals.

Diets high in alfalfa hay are most likely to create this optimal environment for mineralization.

“Alfalfa is a forage with a relatively high buffering capacity compared with other forages, thus it contributes to the more alkaline colonic environment needed for enterolith formation,” says Diana M. Hassel, DVM, Ph.D., Dipl ACVS, a colic surgeon at Colorado State University Veterinary Teaching Hospital and professor in the Department of Clinical Services. “The other big contributing factor is likely the mineral content in alfalfa. Magnesium has been implicated, and it is particularly high in alfalfa grown in certain areas versus others.”

Some studies have recognized higher magnesium and other minerals in the water in regions of the country where enterolithiasis is more common.

The enterolith that forms around a hard object like a pebble, coin, or wire can be round, spherical, triangular, or flat in shape. Enteroliths can vary in size from as small as a walnut to larger than a grapefruit. Among the most dangerous are those that are baseball-sized.

When the horse swallows a soft foreign body like baling twine, it can become mineralized, resulting in the formation of hundreds of tiny enteroliths along the piece of twine.

Risk Factors

Risk factors for enterolith formation include:

• Geographical region
• Diet high in alfalfa
• Stall confinement
• Sibling that developed the condition

California, Texas, Florida and Arizona are the primary areas for enterolithiasis in the U.S. This may be related to minerals in the soil, and/or the fact that many horses in these states eat a high or all-alfalfa diet.

“It’s not known whether it’s the water, the soil, the alfalfa itself, or simply the amount of alfalfa owners feed in these regions that makes these areas at higher risk,” notes Hassel. “Within the state of California, there are areas with very few enterolith cases, even when alfalfa is fed frequently, and other areas where [cases are] commonly found.”

Hassel points out that stall confinement and diets high in alfalfa are the only repeatably documented risk factors for enterolithiasis, with high alfalfa diets being the most impactful.

“Alfalfa is higher in protein than most other forages, and this may provide a source of nitrogen for the ammonium content in enteroliths—and contribute to the higher pH in the colon,” says Hassel.

She notes that the roles of phosphorus and other minerals, such as calcium and zinc, are not well established.

Studies have shown that Arabians and Morgans are most commonly affected, but Hassel notes that no breeds are “immune” to developing enteroliths. Any breed may be affected when rich alfalfa hay is fed.

Interestingly, the condition is found in mares more often than in male horses.

Watch for Signs of Enteroliths in Your Horse

Clinical signs of enterolithiasis can include:

• Recurrent bouts of colic/abdominal pain
• Depression/lethargy
• Loose stool
• Change in behavior/“crankiness”
• Weight loss
• Resentment when girth is tightened
• Reduced performance
• Reluctance to go downhill or jump

On occasion, a horse will pass small enteroliths in manure. If you see this happen, it’s a red flag that should not be ignored. This is an indication there may also be a larger stone that can’t pass.

Pay attention when cleaning the stall; a small enterolith passed in the manure will be hard as a rock.

Any time a horse passes a small enterolith, Hassel strongly recommends making an appointment at a veterinary referral facility to have abdominal radiographs performed.

“If there are no large enteroliths present (all are smaller), it may be possible to manage these cases long-term without surgery,” says Hassel. “If larger enteroliths are present, surgical management on an elective basis is a lot less risky and less expensive than an emergency colic surgery.”

Surgical Resolution

When abdominal radiographs reveal the presence of an enterolith too large to pass through the GI tract, surgery is the only solution.

This is not a “wait and see” situation. Without surgery to remove the obstructing enterolith, the horse’s intestine can rupture, causing a painful death.

Fortunately, when the horse is in good condition and without other issues, surgery for enterolith removal has a high success rate.

Reducing Risk of Enteroliths

Horse owners can be proactive and lower their horse’s risk of forming enteroliths.

“The most critical advice is to reduce the quantity of alfalfa fed to 50 percent or less of the diet,” says Hassel. “If the horse is at high risk (for example, already had an enterolith removed), I would recommend taking that horse off alfalfa entirely.

“Keep in mind that if your horse has an enterolith and you change his diet to a different hay, that may contribute to movement of the stone downstream in the gastrointestinal tract, resulting in obstructive colic,” she adds.

Hassel encourages any owner who suspects their horse has an enterolith to first have abdominal radiographs done before making diet changes.

Other preventative management techniques include:

• Daily access to pasture/turnout
• Keeping hay and feed free of foreign objects
• Feeding small amounts of grain twice daily with a half-cup of apple cider vinegar (ACV) to attempt to decrease pH level in the gut

Although ACV has a small but reliable impact on decreasing pH in the gut, it can’t dissolve an enterolith that has already formed.

Hassel finds that horses who regularly graze on pasture are at lower risk for developing obstructive enteroliths.

If grazing is not an option, it’s beneficial to keep hay available throughout the day using a slow feeder so the horse doesn’t go for many hours with an empty gut.

Encourage water consumption by ensuring clean, cool water is available 24/7. Electrolytes can help promote drinking when horses consume less water during cool weather.

Because enteroliths form around small objects the horse has swallowed, keep feed areas free of debris. If you use rubber mats, sweep them daily. Make it a routine to clean hay bunks and feed bins frequently.

Careful management, taking proactive measures, and being observant can protect most horses from enterolithiasis.

This article about enteroliths in horses appeared in the May 2024 issue of Horse Illustrated magazine. Click here to subscribe!