The little gray mare was known for her touchy gut. No matter how carefully her owner measured and monitored her rations, it seemed that every few months she would develop a mild bout of colic. The incidents always passed with just a dose or two of Banamine. Still, her owner constantly worried that the next episode might be “the big one.”

Down the aisle, a chestnut gelding had a reputation for his robust appetite, and he’d never had any digestive difficulties—until one night, when he was found down and thrashing in his stall and rushed to emergency surgery.

Both of these cases of colic had the same source: enteroliths, rocklike concretions—similar to gallstones in people—that form in the large intestine. A horse may carry hundreds of these small rocks (calculi) in his gut, or he may develop only one or two that grow to huge proportions—as big as a cantaloupe or even larger. The stones might remain in place for years while causing few, if any, outward difficulties. Or, they may trigger intermittent mild colics, collecting to cause an impaction and then shifting away to allow the intestinal contents to pass by once again. In the worst-case scenario, an enterolith becomes lodged in a spot that completely blocks the intestine, creating a life-threatening obstruction that requires emergency surgery. 

Researchers do not yet know exactly why some horses develop enteroliths while others kept in similar circumstances do not. “At this point, we only know about risk factors that can contribute to enterolith formation,” says Diana Hassel, DVM, PhD, DACVS, DACVECC, of Colorado State University. “The condition appears to occur as a result of multiple factors, likely a combination of genetic predisposition with environmental factors—both diet and management.”

Still, even if your horse is susceptible to enterolith formation, you can take measures to reduce his chances of being troubled by these stones. 

A rock in a soft placeAn enterolith begins when a horse ingests some small object—usually a pebble or grain of sand but sometimes a piece of hardened grain, metal, twine or another material. Normally, these indigestible bits would simply pass through the horse’s intestine and exit with the manure. Under the right conditions in the horse’s colon, however, layers of a mineral called struvite (magnesium ammonium phosphate) may begin to crystallize around the foreign object (called a nidus when it forms the center of a stone). Sometimes, the stones may also incorporate other minerals from the horse’s gut, including calcium, potassium, sodium or sulfur.

Enteroliths typically form in a portion of the gut called the right dorsal colon, which is the last section of large intestine before it narrows into the funnel-shaped transverse colon and enters the small (or descending) colon, which leads to the rectum. “The right dorsal colon has an ampulla—this is an area of the colon that has a larger diameter than the colon preceding and after it,” says Rebecca Pierce, BVetMed, MRCVS, DACVS, of the University of Tennessee. “One thought is that, because this area is a bit larger, it may allow accumulation of minerals that form enteroliths.” Slowed motility in this area—meaning that the ingesta moves less quickly here—may also give the concretions more time to develop.

If the object remains in the colon, additional layers of struvite will form over it in concentric rings, and the new enterolith will continue to grow in size. While the stone is relatively small, less than the diameter of a baseball, it can still be pushed safely out of the horse with the manure. If a horse has multiple stones rubbing against each other in his gut, they may have flat sides or become more pyramid-shaped. You may find rounded stones that look like river rocks in the stall and pasture.

A stone that remains in the colon will continue to grow indefinitely. As one grows—or enough small ones accumulate—the horse may begin to develop colics in the right dorsal or transverse colon. “The enterolith basically acts like a valve and can prevent ingesta from passing through the colon, which results in an impaction and gas accumulation,” says Pierce. 

Yet because the right dorsal colon is 12 to 14 inches wide, even very large enteroliths seldom cause complete blockages. “Enteroliths that are obstructing the right dorsal or transverse colon tend to show mild colic signs,” Pierce says. “These include being off feed, lying down, pawing and decreased manure production. The colic signs generally respond to basic treatment by your veterinarian. Many horses demonstrate these same signs with other causes of a mild colic. The difference is horses with enteroliths tend to be ‘repeat’ or chronic colickers. This means they may show colic signs repeatedly—sometimes within a week and sometimes over months.”

More worrisome is a stone that is little enough to enter the small colon—which is only around four inches wide—but too big to pass all the way through. “The smaller, baseball-size enteroliths usually result in a one-time colic event and often have more severe signs when they become lodged in the descending colon,” says Hassel. In these cases, the horse will be rolling, sweating and showing other signs of more severe pain, which will not respond to the veterinarian’s treatment.

“The biggest concern with enterolith obstruction is that many will go on to rupture the intestine before they are detected and surgically removed,” says Hassel. “We estimate about 15 percent of horses with colic due to enteroliths die or are euthanized as a result of rupture of the colon at the site of obstruction.”

Diagnostic difficultiesDetermining whether a horse has enteroliths before colics develop can be tricky, because the signs—if there are any—are nonspecific. Finding small stones in the manure is, of course, obvious, but these are easily missed. 

Some horses who carry stones may be generally irritable and unthrifty, while others may show no outward evidence of them at all. “I have seen several horses who didn’t demonstrate classic colic signs but were evaluated for poor performance,” says Pierce. “Once the enterolith was identified and removed, owners reported the animal came back at a higher level of work and performance than before surgery.”

And when a blockage develops, the signs are the same as for any other impaction colic. “The problem with enterolithiasis is it really is not very distinguishable from other forms of simple obstruction, such as feed impactions and foreign body obstructions,” says Hassel. “One consideration is a history of chronic recurrent colic. This occurs in about 30 percent of cases.”

And because the enterolith impactions so often occur deep within the horse’s abdominal cavity, the stone can’t always be detected on x-rays or by rectal exams. “Digital and computed radiography use has resulted in slight improvements in presurgical diagnosis with a reported overall sensitivity of 85 percent,” says Hassel. “However, the smaller, baseball-size enteroliths that lodge in the descending colon remain a diagnostic challenge, with a sensitivity of only 50 percent. Fortunately, these horses often have more severe colic signs that lead them to surgery regardless of the results of the abdominal radiograph. To my knowledge, there are no other new technologies to help identify enteroliths presurgically.”

The only treatment option for removing an enterolith impaction is surgery. Fortunately, when enterolith impactions are caught before the colon wall is damaged, the chances of a full recovery are very high—90 to 95 percent of horses return to their previous level of activity. “Horses undergoing surgery for enterolithiasis have an excellent prognosis for full recovery,” says Hassel. “At the other extreme, if the colon ruptures it is uniformly fatal.”

Risk factorsAny horse can develop enteroliths, but the stones occur much more commonly under certain conditions. “The rate of formation seems to be variable and may be affected by pH, mineral availability and colonic motility,” says Pierce. In other words, here are the conditions under which enteroliths are most likely to form:

• Higher than average pH within the right dorsal colon. The high pH creates an alkaline environment, which encourages crystallization of minerals. (An acidic environment, in contrast, is more corrosive and dissolves minerals more readily.

Why some horses have a more alkaline environment within the colon is not completely understood, but one of the biggest contributors is a diet rich in alfalfa. “Alfalfa hay is higher in protein and has a higher buffering capacity than most other hays due to its mineral content,” says Hassel. “This contributes to more effective alkalinization—higher pH—in both the stomach and colon. A higher pH provides a more ideal environment for the mineralization of struvite around a nidus.” In one survey, 98 percent of horses with enteroliths were receiving a ration that consisting of at least 50 percent alfalfa hay.

That said, however, some horses seem to have a naturally higher pH than others. “Horses who are prone to enterolith development will have a slightly higher pH in their colon than those who are not, even when on an identical food, water and management protocol,” Hassel says. “So there is something unique in some horses that increases their risk for enterolith formation.”

Genetics and heredity clearly play some role. In one study from the University of California–Davis, Arabians and Arabian crosses accounted for more than 40 percent of the enteroliths seen there, even though they represented only 14 percent of total hospital population. The same study suggested that Appaloosas, Saddlebreds, Morgans, Miniature Horses and donkeys are also somewhat more likely to develop the stones. “The proof for the predisposition of enterolithiasis in certain breeds—the Arabian being the most prominent—leads me to strongly suspect a genetic component,” says Hassel. “In a large retrospective study done on 900 horses, we found that about 15 percent of affected horses have one or more siblings also affected.” Horses who have had enteroliths in the past are more likely to develop them again.

• Higher concentrations of minerals, especially magnesium, in the colon. When dissolved minerals super-saturate (become overabundant) in a liquid, especially in an alkaline environment, they crystallize easily. “The formation of struvite crystals has been linked to magnesium supersaturation and the presence of ammonia and phosphates in an alkaline colon,” says Pierce. “In experiments, ponies fed magnesium oxide had an increase in colonic pH.”

How and why some horses develop these high mineral concentrations isn’t fully understood, but diet clearly plays some role, as does geography. Enteroliths are far more common in the Southwest and on the West

Coast, especially in California, than in other parts of the country—hays and other feeds, especially alfalfa, grown in these regions tend to be higher in minerals. 

“Alfalfa hay, especially in California, has excess magnesium, protein and phosphorus, and increased colonic mineral content was reported in horses fed a high alfalfa diet,” says Pierce. 

Water, too, can be higher in magnesium in California. The relationship between drinking water and enteroliths has not been well established—yet it may still play some role. “Dietary magnesium, from alfalfa hay, may contribute more to enterolith formation, but the combination of diet and water sources may have an additive effect,” says Pierce.

Do not assume, however, that your horse is automatically safe if you live in other areas. Enteroliths have been reported in many states as well as other countries.

• Slower gut motility. Ingested food does not move as quickly through the gastrointestinal tracts of some horses, a factor which gives the stones more time to form. Genetics may play some role in this factor, too, since horses may have some natural variation in gut motility rates.

However, the amount of turnout and exercise a horse gets also has an effect: “Decreased exercise has been linked to increased retention time for particulate matter within the intestinal tract,” says Pierce. This effect on enterolithiasis has been documented: “Stalled horses without access to pasture have a slight, but statistically significant, increased risk for enterolithiasis,” says Hassel. 

It is likely that the exercise a horse gets moving around the pasture, combined with the grazing, helps any small stones that form to pass with the manure before they can get big. “Theoretically, it is the combination of activity and grass pasture that helps,” Hassel adds. “Grass is generally good for reducing gastrointestinal stasis, and it is likely that some degree of gastrointestinal stasis is needed for effective enterolith formation.” 

Grazing may also help simply by reducing the total amount of alfalfa in the horse’s diet. “Because horses without access to pasture were found to have higher colonic mineral concentrations, there may be a potential dilutional effect of grass on ingested alfalfa,” says Pierce.

Stopping stonesFor horses who are susceptible to enteroliths, vigilance is key to preventing a serious colic episode. Talk to your veterinarian about the possibility of intestinal stones if your horse seems vaguely unwell and/or develops mild, passing colics two or three times a year. You have more reason to be concerned if you find even one small, rounded stone in his manure. 

Horses prone to enteroliths may benefit from several management changes—“some of which are based on science, and some on hunch and clinical experience,” says Hassel:

• Eliminate alfalfa from the diet. “This is probably the single most effective thing that can be done to prevent enteroliths,” says Hassel. Be aware, however, that abruptly increasing the roughage content in a horse’s diet may cause a previously stationary stone to begin moving. “If an enterolith is present and the feed change occurs, it is not uncommon that the horse obstructs a few weeks later due to the higher fiber content moving the stone,” she adds. She suggests introducing grass hays gradually, over several months.

• Provide as much turnout as possible. The steady roughage from grazing fresh grass helps to keep ingesta moving. If a horse should not graze for other health-related reasons—such as, he is prone to laminitis—the turnout in a dry lot combined with light exercise will also help. Moving around also helps stimulate gut motility. 

• Use feeders to dispense hay. Placing hay directly on the soil increases the risk that a horse will ingest sand, bits of gravel and other foreign substances along with his forage. Placing hay in a commercial or homemade feeder will keep it cleaner and can also reduce waste. 

• Supplement with psyllium. Not all horses with sand colic develop enteroliths, and vice versa, but the two conditions may occur together. Psyllium, a high-fiber dietary laxative made from the husk of seeds of an herb called Plantago ovata, is often given to horses as a preventive measure against sand colic because it helps to “capture” sand grains and move them out of the gut—and it might help prevent enteroliths, too. “Sand and pebbles sometimes make up this central nidus, so preventing sand accumulation is a good recommendation,” says Pierce. “This includes reducing access to sand and feeding psyllium—one to two eight-ounce measuring cups twice daily for one week out of the month.”

• Soften hard water. Researchers speculate that some high levels of minerals, especially calcium and magnesium, in the water may contribute to enterolith formation. “Consider a barn water filtration system to remove minerals and neutralize the pH of the supply,” says Hassel.

• Avoid mineral-rich feeds. Wheat bran is a common ingredient in mashes that will not harm healthy horses when used in moderation, but wheat contains high levels of phosphorus that can contribute to enteroliths in those who are prone to them. 

•Add vinegar to the ration. “If you know your horse is high risk—a sibling is affected, he is passing enteroliths in his feces, or he lives at a barn where others have been affected—consider adding one cup of apple cider vinegar in a small amount of grain to the diet twice daily,” says Hassel. “While conducting my research, I did find that feeding vinegar in this amount to adult horses will reduce the pH in the colon slightly.” This advice is somewhat controversial, because feeding vinegar has not been proven to actually prevent the formation of enteroliths—yet veterinarians generally agree that this practice falls into the category of “can’t hurt; might help.”

“I can remember several horses who were supplemented with vinegar yet still developed enterolithiasis,” says Pierce. “However, it will not harm the horse so it can be used.”

There’s still much to be learned about enteroliths, but enough is now known to identify those horses most susceptible to them as well as the factors that increase their risk. And, while it may not always be possible to prevent all intestinal stones, with vigilance and attentive care, you can reduce the chances that they will pose a serious threat to your horse’s health.

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