You might need two hands to hold a large enterolith, a rocklike mass that can weigh 25 pounds or more and reach the size of a small watermelon. And it can be difficult to imagine that such a huge stone may have persisted for years undetected inside a horse’s intestine.
A horse can carry 100 or more smaller enteroliths in his gut, or he may develop only one or two large ones. Often, the small ones pass uneventfully with the horse’s manure, or larger ones may remain dormant, doing little apparent harm to the horse.
But enteroliths aren’t to be taken lightly. Sometimes, these stones can result in recurrent mild colics, and in the worst-case scenarios, they can cause serious impactions that lead to intestinal rupture. Finding even one or two tiny pebbles in your horse’s manure is a sign that you need to take action to protect him.
Why some horses develop enteroliths while others do not isn’t fully understood, and in fact, multiple factors probably contribute to the formation of the stones. But the good news is that basic management changes can go a long way toward preventing enteroliths. Here’s what you need to know.
Birth of a stone
An enterolith begins when a horse swallows something indigestible. “It might be a small piece of rock, gravel or something else the horse has ingested,” says Diana M. Hassel, DVM, PhD, DACVS, DACVECC, of Colorado State University. “It might be a seed, or hair, a coin or a tiny piece of wire or twine.”
Normally, these bits of foreign matter pass harmless through the horse’s digestive system and go out unnoticed in his manure. Under certain conditions, however, the object’s progress comes to a halt in a part of the large colon where the intestinal tract widens and the ingesta naturally slows down. “Enteroliths are formed in the right dorsal colon,” Hassel says, “in an area of greater dimension in that location, called the ampulla coli.”
While the object sits in the gut, various minerals may begin to “stick” to it. The concretion around the object—now called a “nidus”—continues to accumulate, much in the same way a pearl forms around a grain of sand inside an oyster. The minerals in an enterolith consist of struvite, a combination of mainly magnesium, ammonium and phosphates. “There are other minerals in these stones, in smaller quantities,” Hassel says. “The actual content will depend on the horse’s diet and the feed that is present in the gut—which might contribute some calcium and other minerals that could be found in these stones as well.”
As the layers of mineral continue to form, the enterolith grows in size. A horse may have only one or two stones, or he may produce many, which will shift and rub against each other in his gut. Why some horses form more stones than others is not fully understood. “We don’t know why certain horses form many small enteroliths versus solitary large ones,” says Hassel. “It is not common to see horses that just form little ones; more often we see big ones mixed with little ones, but the majority of affected horses just form one or two large stones.”
Eventually, one of three possible outcomes will occur:
• While still small—less than the size of a manure ball—the enterolith may move with the ingesta and pass harmlessly onto the ground in manure. “They may get moved along in the tract by feed changes, such as increased fiber,” Hassel says. “If they are fairly small they can pass on through, and you might see them in the horse’s manure.”
But just because a horse’s stones are small does not mean he is not at risk for serious colics. “All sizes of stones have the possibility of causing blockage,” says Tyler Elliott, DVM, of San Luis Rey Equine Hospital in Bonsall, California.
• The stone will remain in place and may grow very large. “Some of these may get as big as a watermelon,” says Hassel. The horse may show intermittent signs of mild colic, as the stone shifts around. The right dorsal colon narrows considerably as it exits to the transverse colon. “The bigger stones can be like a hinge valve,” says Hassel. “If they get pushed up against the entrance to the transverse colon, they may create an obstruction, and then if the horse gets a little treatment or is rehydrated, the stone may fall back down, no longer blocking the passage.”
But not all horses with larger enteroliths will show signs of colic. Many can carry the stones for years without any indication of trouble—the enteroliths may be a surprise finding at necropsy. “Intermittent, recurring colic occurs in about 15 percent of horses with enteroliths,” says Hassel. “So if you have a horse who colics a lot, this might be a possible cause.”
• Enteroliths in the mid-range—about the size of a softball—are by far the most dangerous. After the right dorsal colon narrows into the transverse colon, the gastrointestinal tract narrows again to become the small colon, which is the last section before the rectum.
“The intestine goes from a very large diameter to a much smaller diameter, creating a sort of bottleneck,” says Hassel.
A stone that is small enough to enter the transverse colon but too big to pass on through to the rectum becomes lodged in place, creating a complete blockage that leads to painful impaction colics. “The right dorsal colon can accommodate a stone up to 12 or 14 centimeters in diameter, whereas the small colon is only about a quarter to a third that size, and a large stone cannot fit through it,” says Elliott.
“If the blockage is not treated surgically to remove the stone, the intestinal tract eventually ruptures, causing the death of the horse,” says Hassel.
Determining that a horse is developing enteroliths can be difficult. Some may be generally unthrifty or irritable, or develop recurrent mild colics. “There may be intermittent diarrhea,” says Elliott. “These are all very general signs, however, that don’t point specifically to stones. There’s no one sign you’d see and know that the horse has stones.”
Finding small enteroliths on the ground would be an obvious indication, but they are easily overlooked. If they aren’t visible while still embedded in manure, the rounded structures may resemble naturally occurring river rocks. And then there is the question of which horse in the herd produced it.
Still, “I tell people that if they notice stones in the manure, even if they are tiny, it’s a good idea to check the horse,” says Troy Herthel, DVM, DACVS-LA, of Alamo Pintado Equine Medical Center in Los Olivos, California. “If you see stones while cleaning your horse’s stall, we definitely recommend having abdominal radiographs to determine if there are more inside. If the horse has one, most likely there are more.”
The shape of any stones you might find provides another clue. “Most of the stones we see are round, but if you see some with a flat edge or cuboidal triangular shape, there are probably more of them and they have been rubbing against one another,” Herthel says.
“The only definitive way to diagnose enteroliths, without surgery, is to take radiographs of the abdomen,” says Hassel. “We can’t see the stones with ultrasound, because the content of the gut looks similar to the surface of the stone.” Rectal exams usually aren’t helpful either, because the stones may be too deep in the abdomen to feel.
But even x-rays have limitations when it comes to imaging structures deep inside the horse’s abdomen. “Diagnostics with radiographs are not 100 percent although we are generally able to correctly diagnose enteroliths on radiographs more than 80 percent of the time, if they are still in the right dorsal colon,” says Herthel. “The stones that travel down into the small colon are harder to diagnose with radiographs, due to the horse’s anatomy. Other confounding factors are the size of the patient and the amount of ingesta within the colon. The larger-bodied horses can be more difficult to adequately penetrate with x-ray beams, and if the colon has a large feed impaction the stone can be obscured from view.”
When surgery is needed
The only treatment option for an enterolith impaction is surgery to remove the stone. At that point, identifying the enterolith beforehand is practically moot. “Usually the horse will need surgery anyway, if there’s a blockage—and then you would find the stone during the procedure,” Hassel says.
Surgical removal is also recommended when a larger stone is identified in x-rays. Even if it’s not currently causing trouble, a large enterolith isn’t going to go away on its own, and the risk of colic will always remain. “They continually build in size,” says Elliott. “You may see the horse become mildly colicky, but surgery will eventually be needed. If the horse has a large stone, the only curative treatment is surgery.”
Recently, a horse in Elliott’s practice underwent surgery to remove two stones. “One was about nine pounds and the other weighed about 11 pounds,” he says. “Based on the history of recurrent colic and the horse not doing very well, though still able to pass a little bit of manure, we suspected a partial blockage. We took some radiographs and these stones showed up—so our recommendation to the owner was to take the horse to surgery. That horse has now recovered and is doing very well.”
A horse’s chances for recovery after surgery to remove an enterolith impaction are usually very good. “If the horse goes to surgery early, before the colon is compromised or ruptured, success rates are about 97 percent,” says Hassel.
Unlike the twists, entrapments and other strangulating colics that squeeze off blood flow to the bowel and cause the tissue to die, enteroliths generally do not cause serious injury to the in-testinal wall if acted upon in time.
“These horses are some of the healthiest colics we ever see,” says Hassel. “It’s a simple obstruction, with nothing twisted or injured. The stones are usually fairly smooth and solid, though they can have spiculated surfaces [with spikes or points]. Some are more porous and rough. The stones don’t seem to injure the gut lining, however, until they get lodged.”
If an enterolith impaction is left in place, the built-up pressure can seriously injure the intestinal wall and increase the risk of a rupture. “If you wait too long, the horse will die rapidly if the intestine ruptures because there is nothing we can do to help the horse at that point,” says Hassel.
However, says Herthel, “The majority of the cases we see have a good prognosis. We have to make at least one incision into the colon, sometimes two, depending upon the size and location of the stone or stones. Otherwise, these surgeries are generally fairly straightforward.”
If x-rays show that your horse has a cluster of smaller stones rather than one big one—all of which appear to be small enough to pass in the manure—your veterinarian may offer alternative treatments. “Medical management is sometimes an option,” says Elliott. “Surgery to remove the stones is best, but you can put the horse on pasture or grass hay and try aggressive psyllium treatment for stones that still have a possibility of passing safely. People often try a month of at-home psyllium treatment and dietary management, then come back for a re-check radiograph to see if they’ve cleared. These stones are a ticking time bomb, but with luck they can be passed on through.”
Why some horses develop enteroliths while others do not is still a bit of a mystery. “We think these stones form as a result of multiple factors working together, rather than just one cause,” says Hassel. Some of the major factors include diet, exercise, geography and genetics—all of which can have an impact on conditions such as the acidity in the gut, mineral content of the ingested feed and slow gut motility, which can enable enteroliths to form.
“The perfect storm would be to have a high pH within the right dorsal colon, a high mineral content in the feed, slow transit time and a nidus for a stone to build around,” says Elliott. “Some horses, perhaps due to individual genetics or breed susceptibility, may have decreased transit time as well as a naturally higher pH than other horses.”
Here are some of the factors that can contribute to the formation of enteroliths:
• High mineral intake. The risk of developing enteroliths increases when a horse’s diet includes high levels of minerals. A diet based primarily on alfalfa hay, which is lower in fiber and higher in minerals than grass hay, has long been known as a risk factor for enteroliths.
But other feeds—and even water, in some regions—can also be high in minerals. “We know diet plays a huge role, and this may include the water supply as well as the feed,” says Hassel. “Perhaps even more important is the water supplied for growth of the feed, such as the hay crop.” Still, she adds, “The feed itself is the primary risk factor, since alfalfa hay has been clearly linked to enterolith development in at-risk horses.”
Brans, such as wheat and rice bran, also have high mineral levels but it is not clear that they are major contributors to the problem. “People used to say that bran was a risk factor because in earlier times millers’ horses were fed a lot of bran [as a byproduct of the wheat milling] and often formed stones, and bran is high in phosphorus,” says Hassel. “We’ve never been able to identify bran as a risk factor, however, in all the studies we’ve done. This might be because our horses today don’t eat as much bran.”
• Low acidity in the gut. An en-vironment with a higher pH, which is more alkaline, encourages the crystallization of minerals. A lower, or more acidic, pH environment is more corrosive and dissolves minerals more readily. There will always be natural variation among individuals, and horses with a higher pH in the contents of their colons may be more prone to forming enteroliths. A high-alfalfa diet can also reduce acidity in the gut. “Alfalfa is an alkalinizing hay compared to most grass hays,” says Hassel.
• Low gut motility. The speed with which ingested food moves through the gut will also vary somewhat among individuals, and those with slower rates may offer more time for enteroliths to begin forming. Management factors may play a role. “Stall confinement may contribute to formation of enteroliths,” says Hassel. “The horses we see with enteroliths are often well taken care of, living in pampered conditions. Things that help prevent stone formation include regular turnout and daily pasture access. Whether it’s the exercise or the grass, being turned out rather than in a stall seems to be protective.”
The reason for this may be that regular exercise stimulates gut motility, so ingested foods move through faster. “If horses have adequate exercise they will be passing their feed on through more readily,” explains Elliott. “In theory, this would mean less time for the ingesta to sit in the colon and provide the opportunity for stone formation.”
• Genetic predisposition. It’s possible that factors such as gut pH and motility are heritable traits—in a 1999 retrospective study of 900 horses conducted by Hassel at the University of California–Davis, about 15 percent of the horses with stones also had siblings with stones.
“Genetic predisposition hasn’t been proven yet, but there are certain breeds that tend to have more enteroliths,” says Hassel. “The most affected breed seems to be the Arabian, and Morgans are the second most commonly affected breed.” Other breeds identified in Hassel’s study include Arabian crosses, Appaloosas, Saddlebreds and Miniature Horses. “You can’t just have the breed predilection without the dietary factor, however,” Hassel says.
• Geography. Enteroliths can appear in horses just about anywhere, but they are more common in certain regions of the country. “California has high incidence, but there are areas in California where you don’t see this occurring, even though those horses are fed alfalfa. We see a lot of stones in southern California,” Hassel says. “Texas, Arizona and Florida are states where we occasionally see horses with stones, but not as often as in California.” Incidences are much lower in other parts of the country.
The reasons for this pattern are not clear, but researchers suspect it may be related to the mineral content of local water supplies and soils. “It seems that the alfalfa grown in coastal soils, and especially in certain areas of California [as well as Arizona, Texas and Florida], tends to have a higher magnesium content,” says Elliott. “Alfalfa already has six times the daily required magnesium content. This could be partially why there is such a region-specific incidence of enteroliths.”
But your horse doesn’t need to live in one of those regions to develop enteroliths. “Here in Colorado we see enterolith problems only sporadically. I might see three in one year and then not see any for the next three years,” Hassel says. “Sometimes the affected horses come from California. But we do see some cases just out of the blue. It can happen anywhere.”
• History of susceptibility. Management changes can reduce the risk that a horse will develop enteroliths, but if he has had stones before, he may be more susceptible to developing new ones. “Some horses seem to be stone factories,” says Herthel. “We’ve had repeat offenders that have come in for another surgery. Most of these horses have one to three stones, but occasionally we find more. We had one horse that had about 100 stones, but most of them were very small—from the size of pea gravel on up to silver dollar size, with a few larger ones.”
Talk to your veterinarian if you suspect your horse may have, or be at risk for, enteroliths—especially if you find a stone in his manure. “If you have a horse that experiences chronic colic problems and you live in one of those regions or the horse is being fed alfalfa hay, this might be a reason to x-ray the abdomen. You are much more likely to see a stone when it is not obstructing the gut than when it is,” says Hassel. “The location where stones form are visible with radiographs in the lower part of the abdomen and the stones show up very readily when they are sitting down there, as compared to when they are causing a blockage higher up.”
Horses who are likely to develop enteroliths can be managed to reduce the risks with measures that appear to have worked well over the past decade. “We are a referral hospital so we get horses from all over southern and central California,” says Herthel. “Within the last 15 years, however, incidence of stone surgeries we do each year has decreased significantly. Back in the 1980s and early 1990s the hospital was doing a stone surgery every week or two, and now it’s significantly less. This decrease in cases may be due to improved management and feeding plans. Most of our horse owners are well educated regarding the predisposing causes and are proactive in trying to prevent stones.”
Here are some steps you can take if you’re worried about enteroliths:
• Reduce alfalfa in the diet. Alfalfa hay is rich in nutrients, but for horses susceptible to enteroliths, the levels of minerals it supplies may be too high. “With the risk for enteroliths, we don’t recommend feeding more than half the hay content as alfalfa,” Herthel says. “Most of our horse owners now add pasture grass or grass hay to the diet.”
• Provide ample turnout and/or exercise. Activity helps to stimulate gut motility, and turnout can be especially helpful: “Grass pastures are great, because grasses don’t have as high a mineral content as alfalfa, and the horse gets some exercise out grazing,” Herthel says.
Grass also has a higher fiber content than alfalfa. “A higher fiber content helps naturally push things through and keeps the feed moving along the tract,” says Elliott. If unlimited grazing is unhealthy for your horse for other reasons—such as the risk of laminitis or obesity, for example—then turnout in a dry lot, combined with regular exercise, will also help.
• Use hay feeders or mats. Placing hay directly on the ground increases the risk that the horse will pick up bits of gravel or other foreign objects as he eats. Placing hay in nets or some other type of feeder will keep it cleaner and also reduce waste.
• Add psyllium to the ration. Psyllium is a high-fiber laxative made from the seed husks of an herb called Plantago ovata. When ingested, it forms a gelatinous texture that can capture foreign material and help it to move on through the digestive tract. Psyllium is often given to horses to prevent sand colic because it helps to remove the grains from the intestine, and it may help prevent enteroliths, too. “If a horse is at risk for having stones, adding psyllium to the diet may be beneficial to help remove potential niduses from the colon before stone formation occurs,” Herthel says.
• Install a water softener. If your local water is high in minerals, you may want to consider installing a water softener to filter the water that your horses will drink.
• Avoid feeding too many minerals. If you are using a commercial feed and adding supplements to your horse’s diet, he may be getting high levels of some vitamins and minerals. Your veterinarian or an equine nutritionist can determine if any changes are necessary. Also, avoid giving your horse mashes made with wheat bran or other ingredients high in minerals.
Preventing every intestinal stone may not always be possible, especially if your horse is unusually susceptible to them. But with vigilance and attentive care, you can greatly reduce the risks that enteroliths will pose a serious threat to your horse’s health.
A sour solution?
Some veterinarians recommend adding a cup of apple cider vinegar to a horse’s grain twice a day to increase the acidity in his gut. “We haven’t proven that the vinegar helps prevent stones, but it does have an impact on the pH,” says Diana M. Hassel, DVM, PhD, of Colorado State University.
However, others question whether the practice is beneficial and warn that it may contribute to other problems. “Feeding apple cider vinegar is somewhat controversial, and we don’t recommend it,” says Tyler Elliott, DVM, of San Luis Ray Equine Hospital in Bonsall, California. “There is no guarantee it will acidify the gut to the degree that stones won’t form, and it might not be a good idea if the horse is prone to ulcers.”
A problem of middle age
Very few young horses have problems with enteroliths, perhaps because of the time it takes them to form in the equine gut. “In the cases we see, it’s pretty rare to find stones in horses under the age of 7 or 8 years old,” says Troy Herthel, DVM, DACVS-LA, staff surgeon at Alamo Pintado Equine Medical Center in Los Olivos, California. “It’s more a problem of middle-aged to older horses. The stones probably start forming early on but do not get large enough to cause problems until the horse is about 7 or 8. The youngest horse we surgically removed stones from was a 3 or 4 year old.”
This article first appeared in the October 2017 issue of EQUUS (Volume #481)