Last Updated on February 23, 2022 by Allison Price
Two hands might be required to hold large enteroliths, which are rock-like masses that can weigh up to 25 pounds and have the same size as a small watermelon. It can be hard to believe that such a large stone could have survived for many years in horse’s intestinal tract.
One horse may have 100 or more small enteroliths, while another horse might only have one or two large ones. The smaller ones can pass easily with horse manure. Larger ones may stay dormant and cause little harm to horses.
Enteroliths should not be taken lightly. These stones can sometimes cause mild colic and, in extreme cases, can lead to serious intestinal damage. You should take immediate action if you find even a few tiny pebbles in horse manure.
It is not clear why horses develop enteroliths and others don’t. In fact, there are many factors that contribute to the formation. The good news is that simple management changes can help to prevent enteroliths. Here are the facts.
Birth of a stone
When a horse swallows indigestible material, an enterolith is formed. It could be a tiny piece of gravel, rock or other substance that the horse has ingested,” Diana M. Hassel of Colorado State University, DVM, PhD says. It could be a small piece of wire, twine, hair, or seed.
These foreign particles pass normally through horses’ digestive systems and are not noticed in their manure. However, under certain conditions, the object’s progression may stop in the large colon, where the intestinal tract narrows and the ingesta slows down. Hassel states that enteroliths form in the right dorsal col, in an area with greater dimensions, the ampullacoli.
Various minerals can “stick” to the object while it sits in your gut. Concretion around the object, now called a “nidus”, continues to build up in the same manner as a pearl around a grain sand within an oyster. An enterolith is composed of struvite which is a mixture of magnesium, ammonium, and phosphates. Hassel states that there are also other minerals found in these stones in smaller amounts. “The horse’s diet, as well as the amount of feed in his gut will determine the actual content. This could contribute calcium and other minerals to these stones.
The size of the enterolith increases as the mineral layers continue to form. One horse might have one or two stones. Or, he could produce many that will rub against each other in the gut. It isn’t clear why some horses produce more stones than others. Hassel says that it is not clear why some horses form large, single-sized enteroliths. It is rare to see horses form small ones. We see more horses with large ones mixed with smaller ones. However, the majority of horses affected only form one or two large stones.
One of the following possible outcomes may eventually occur:
* The enterolith, which is still very small (less than a manure ball), may move with the ingesta to pass on harmlessly to the ground in manure. Hassel states that they may be moved around in the tract by feed modifications, such as increased fiber. They can be passed through if they are small enough. You might even see them in horse manure.
However, just because the horse’s stones appear small doesn’t mean they are not at risk of serious colic. Tyler Elliott, DVM of San Luis Rey Equine Hospital, Bonsall, California, says that all sizes of stones can cause blockage.
* The stone will not move and could grow to be very large. Hassel says that some of these stones could grow to be as large as a watermelon. As the stone moves around, mild colic may occur in intermittent episodes. As it exits the transverse colon, the right dorsal colon narrows significantly. Hassel says that larger stones can act as a hinge valve. If they are pushed against the transverse colon’s entrance, they can cause an obstruction. However, if the horse is given some treatment or hydrated, the stone might fall back, allowing the passage to be unobstructed.
Not all horses with larger colic signs will have colic. Many horses can carry the stones for many years without experiencing any symptoms. Necropsy may surprise you with the discovery of enteroliths. Hassel says that about 15% of horses with enteroliths experience recurring, intermittent colic. This could be the cause of colic in horses who have frequent colic.
The most dangerous enteroliths are those in the middle range, which is about the same size as a softball. The rectum is formed when the right dorsal colon narrows to the transverse colon.
Hassel says, “The intestine changes from a very large to a much smaller size, creating a kind of bottleneck.”
A small stone can enter the transverse colon, but it is too large to pass through to the rectum. This causes painful impaction colics. Elliott says that the right dorsal colon can hold stones up to 12-14 centimeters in size. The small colon is about 25% to a third of that size and large stones cannot pass through it.
Hassel says that if the blockage isn’t treated surgically to remove it, the intestinal tract eventually ruptures, leading to the death of the horse.
It can be difficult to determine if a horse has enteroliths. Some horses may become irritable or unthrifty, or have recurring mild colic. Elliott says that intermittent diarrhea may occur. These are just general signs that don’t necessarily point to stones. You wouldn’t see one sign to tell if a horse has stones.
Although small, easily missed enteroliths can be found on the ground. The rounded structures could be natural river rocks if they aren’t obvious while still embedded in manure. Then there’s the question of who produced them.
Troy Herthel of Alamo Pintado Equine Medical Center, Los Olivos, California, DVM, DACVS, LA, says that even though they may not see any stones in their horse’s manure, it is still a good idea. We recommend that you have abdominal radiographs done if you find stones in your horse’s stall. This will help determine if there is more. Most likely, there are many.
Another clue is the shape of any stones that you may find. Herthel states that most stones are round. However, if you find one with a flat edge, cuboidal triangular or cuboidal shape, it is likely that there are more and have been rubbing against each other.
Hassel says that radiographs of the abdomen are the only way to definitively diagnose enteroliths without having to perform surgery. “We cannot see the stones using ultrasound because the contents of the stomach look similar to the stone’s surface.” Rectal exams are not usually helpful because the stones might be too deep within the abdomen.
However, even x-rays are limited in their ability to image structures deep within the horse’s abdomen. Herthel says that radiographs cannot diagnose all cases correctly, but they can generally identify enteroliths in more than 80 percent of cases if they are located in the correct dorsal colon. Because of the anatomy of horses, it is more difficult to diagnose stones that have traveled into the small colon. The size of the patient as well as the amount of colonic fluid consumed can also be confounding. Larger horses may be more difficult to penetrate with xray beams. If the colon is obstructed by large feed particles, it can also be hard to see.
When surgery is required
Surgery to remove the stone is the only option for an enterolith impaction. It is impossible to identify the enterolith before it occurs. Hassel states that if the horse has a blockage, it will likely need surgery. If the horse does not have a blockage, the surgeon would locate the stone during surgery.
If a larger stone is found in x-rays, surgical removal is recommended. A large enterolith, even if not causing any problems, is not going to disappear on its own. The risk of colic will continue. Elliott says, “They continue to grow in size.” You may notice a mild colic in the horse, but eventually surgery is necessary. Surgery is the only treatment for large stones in horses.
Elliott recently performed surgery on a horse to remove two stones. He said that one stone weighed nine pounds while the other was around 11 pounds. We suspected partial blockage based on the history of colic and the horse’s inability to pass some manure but not very well. These stones were visible on radiographs. We recommended to the owner that the horse be taken to surgery. The horse is now doing well and has recovered.
The chances of a horse recovering from surgery to remove an impaction in the enterolith are generally very high. Hassel says that success rates for horses who go to surgery before their colon has ruptured or become compromised are around 97 percent.
Enteroliths are not as dangerous as entrapments, twists and other strangulating colics which reduce blood flow to the bowel and cause tissue death. If they are acted on promptly, they generally don’t cause any serious injury to the intestinal wall.
Hassel says that these horses have the best colics he has ever seen. It’s just an obstruction. Nothing is broken or twisted. Although the stones are generally solid and smooth, they may have spiked or pointed surfaces. Some stones are porous and more rough. However, the stones do not seem to cause any harm to the gut lining until they are lodged.
The buildup of pressure from an enterolith impaction can cause serious injury to the intestinal wall and increase the chance of rupture if it is not removed. Hassel says that if you wait too long the horse may die quickly if his intestine ruptures. There is no way to save him.
Herthel says that the majority of cases she sees have a positive prognosis. Depending on the size and location, we may need to make two or more incisions into the colon. These surgeries are usually quite straightforward.
Your veterinarian might offer other treatments if x-rays reveal that your horse may have a group of smaller stones than one large one. Elliott says that medical management may be an option. Although surgery is the best option to remove stones, you can also put horses on pasture or grass hay. For stones that are still possible to pass safely, aggressive psyllium treatment can be used. Many people try at-home psyllium treatment for a month and then return to the radiograph to check if they have cleared. These stones can be a time bomb that can be difficult to get rid of, but luck will allow you to continue with your life.
It is not clear why horses develop enteroliths and others don’t. Hassel says that these stones are formed by multiple factors, and not just one. The main factors are diet, exercise and geography. All of these can impact conditions like acidity, mineral content and slow intestinal motility. This can lead to enterolith formation.
Elliott says that the perfect storm would include a high pH in the right dorsal col, high mineral content, slow transit times, and a nidus to hold a stone. Some horses may have a slower transit time or a higher pH than others, possibly due to genetics or breed susceptibility.
These are some factors that could contribute to the formation enteroliths.
* A high mineral intake. Horses who eat a high-mineral diet are at greater risk of developing enteroliths. Enterolith risk factors have been identified as a diet that is mainly based on alfalfa, which is lower fiber and richer in minerals than grasshay.
Other feeds, and even water in certain regions, can also contain high levels of minerals. Hassel says that diet is a major factor. This may also include water supply. “Perhaps even more critical is the water supply for the feed crop, which is essential for the growth of the feed,” Hassel says. But, she also states that “the feed itself is the main risk factor since enterolith formation in at-risk horses has been linked to alfalfa Hay.”
High mineral levels are also found in brans such as wheat bran and rice bran. However, it is not clear if they are major contributors. Hassel says that bran used to be considered a risk factor. This was because bran, a byproduct from wheat milling, was fed to horses of millers. These horses often became stones and bran is high phosphorous. In all of our studies, we have not been able to identify bran, but it has been suggested that bran could be a risk factor. It could be that horses don’t consume as much bran today.
* Gut acidity is low. A higher pH environment, which is more acidic, promotes crystallization. A pH environment that is lower or more acidic will cause minerals to dissolve more quickly. Horses with higher pH levels in their colons are more likely to form enteroliths. There will always be variation between individuals. The gut acidity can be reduced by eating high amounts of alfalfa. Hassel says that alfalfa is a more alkalinizing hay than most grass hays.
* Low gut motility. Individuals will have different rates of gut motility. Those with slower speeds may take longer for enteroliths formation to occur. Management factors could play a part. Hassel says that confinement in stalls may lead to the formation of enteroliths. “Horses with enteroliths are usually well cared for and live in comfortable conditions. Regular turnout and access to pasture daily are two things that can help prevent stone formation. It doesn’t matter if it’s the exercise, or the grass; being turned out is more protective than being in a stall.
This may be because regular exercise stimulates the gut motility which allows for food to move more quickly. Elliott explains that horses who get enough exercise will pass their feed more quickly if they are getting adequate exercise. This would theoretically mean that horses have less time to eat and allow for stone formation.
* Genetic predisposition. It is possible that factors like gut pH or motility may be heritable. In a 1999 retrospective study of 900 horses, Hassel from the University of California-Davis found that about 15% of horses with stones had siblings.
Hassel says that although genetic predisposition isn’t yet proven, there are some breeds with higher levels of enteroliths. “The Arabian seems to be most affected, while Morgans are second most frequently affected breed,” Hassel says. Hassel states that you can’t have a breed predilection and not consider the dietary factors.
* Geography. Horses can be affected by enteroliths almost anywhere. However, they are more prevalent in certain areas of the country. California has a high incidence of enteroliths, but you can still see them in some areas, even though the horses are fed alfalfa. Hassel states that we see many stones in southern California. “We see horses with stones in Texas, Arizona, and Florida, but less often than in California.” The incidences in other areas of the country are lower.
Researchers aren’t sure why this happens, but they suspect that it could be due to the mineral content in local soils and water supplies. Elliott says that alfalfa grown on coastal soils, especially in California, Texas, Florida, and Arizona, tends to have higher magnesium levels. “Alfalfa already contains six times the daily recommended magnesium content. This could partially explain why enteroliths are so common in certain regions.
Your horse does not have to be in those areas to develop enteroliths. Colorado has very few enterolith problems. Hassel states that he might see three or four in a year, and not one for three years. California is sometimes the source of affected horses. We do however see cases that are not expected. It can happen anywhere.
* History of susceptibility. Although management changes can decrease the chance of horses developing enteroliths (although he may still be susceptible if he has already had them), he might still be more likely to develop new ones. Herthel says that some horses are stone factories. “We have had repeat offenders who have been admitted for another procedure. Most horses have only one to three stones. However, we sometimes find more. One horse had 100 stones. However, most were small, ranging from the size of pea gravel to silver dollar sizes. We did find 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 your horse has chronic colic and you live in one these areas or if the horse is fed alfalfahay, you might need to x-ray his abdomen. Hassel says that a stone is more likely to be seen if it isn’t obstructing the stomach than if it is. Radiographs can be used to locate the location of stones in the abdomen. The stones are more easily visible if they are sitting down than when they block the stomach.
There are ways to manage horses at high risk of developing enteroliths. These measures have proven to be effective over the past decade. Herthel says, “We are a referral facility so we receive horses from all across southern and central California.” “In the past 15 years, however we have seen a significant decrease in stone surgeries that we perform each year. In the 1980s and 1990s, the hospital used to perform a stone operation every other week. Now it does less. The decrease in cases could be due to better management and feeding plans. Our horse owners are educated about the causes of predisposing factors and proactive in trying to prevent stone formation.
These are the steps to take if you’re concerned about enteroliths.
Reduce the intake of alfalfa. Although alfalfa hay has a high level of nutrients, it may not be sufficient for horses that are susceptible to enteroliths. Herthel states that there is a risk of enteroliths and recommends not feeding more than half the alfalfa hay. “Most horse owners add pasture grass or grasshay to their horses’ diets.”
* Give your horse plenty of exercise and turnout. Activity stimulates gut motility. Turnout is especially important. Herthel states that grass pastures are great because they don’t contain as much mineral as alfalfa. Horses also get some exercise by grazing.
Alfalfa has lower fiber than grass. Grass also contains a higher amount of fiber. Elliott says that a higher fiber content “helps naturally push things through and keeps feed moving along the track.” Exercising regularly and turning out in a dry lot can help horses who are unable to graze for long periods of time.
* Use hay feeders and mats. Placing hay directly onto the ground increases the chance that horses will pick up gravel and other foreign objects while he eats. You can keep your horse’s hay cleaner by placing it in nets or other types of feeders.
* You can add psyllium. Psyllium, a high-fiber laxative, is made from the seeds of a plant called Plantago Ovata. It forms a gelatinous texture when it is ingested. This can help to capture foreign material and allow it to move through the digestive system. Horses are often given psyllium to prevent sand colic. It helps remove grains from their intestines and may also help to prevent enteroliths. Herthel states that psyllium can be helpful in removing potential stones from horses that are at high risk.
* Install a water softener. You might want to install a water softener if your water is very mineral-rich.
* Do not feed your horse too many minerals. You may be feeding your horse too many minerals if you use a commercial feed. If necessary, your veterinarian or equine nutritionist will be able to determine whether any adjustments are needed. Avoid giving horse mashes made from wheat bran, or other high-mineral ingredients.
It may not be possible to prevent every intestinal stone, especially if the horse is extremely susceptible. You can reduce the risk that your horse will be exposed to enteroliths by being vigilant and taking care.
A sour solution?
To increase the acidity of a horse’s stomach, some veterinarians suggest adding one cup of apple cider vinegar twice daily to his grain. Although we have not proven that vinegar prevents stones, it can have an effect on the pH,” Diana M. Hassel of Colorado State University, DVM, PhD states.
Others question the benefits of this practice and say it could lead to more problems. Tyler Elliott, DVM of San Luis Ray Equine Hospital, Bonsall, California, says that feeding apple cider vinegar to horses is controversial and not recommended. It is not guaranteed that it will not acidify the stomach to the point where stones don’t form. Also, it may not be a good idea for horses with ulcers.
Problems of middle age
Enteroliths are very rare in young horses, possibly because they take so long to form in the horse’s gut. Troy Herthel DVM, DACVS – LA, staff surgeon at Alamo Pintado Equine Medical Center, Los Olivos in California, says that stones are very rare in horses younger than 7 or 8. It is more common in older horses and middle-aged horses. Stones can form early in life, but they don’t become large enough to cause serious problems until horses are around 7 or 8. We removed stones from horses as young as 3-4 years old.