Enteroliths are calcifications or mineral masses that can form in the horse’s intestines, sometimes resulting in impaction colic.
These intestinal stones form when the horse consumes an indigestible object, which is not passed by the digestive system. Mineral deposits then accumulate around the foreign object.
While some enteroliths are naturally passed in the manure with time, others grow larger and can obstruct the transit of feed through the gut.
Enteroliths are most common in horses located in dry, arid regions, but can occur in any horse worldwide. A lack of pasture access and diets containing certain feeds can also increase the risk of enterolith formation.
This article will review the causes of enteroliths, clinical signs, recommended treatment and prognosis for horses affected by intestinal stones. We will also discuss how to prevent the formation of enteroliths in your horse.
Enteroliths in Horses
The term enterolith comes from the Greek terms, “entero”, which means intestinal, and “lith”, which means stone. A horse can have one or more enteroliths that develop over a period of time.
When these stones form in the horse’s gut, the condition is known as enterolithiasis. Interestingly, this condition is rarely found in animals besides horses. 
Enterolithiasis is a common cause of colic requiring surgical intervention if the stone results in a partial or complete obstruction of the large or small colon. Horses with enterolithiasis make up a large percentage of surgical colic cases in some areas, such as California. 
Intestinal obstructions typically occur when stones are transported from the large colon to a smaller-diameter part of the digestive system, such as narrow portions of the small intestines. Alternatively, the enterolith can grow large enough to cause blockage in any location. 
Enteroliths have been reported as a cause of colic since the 1800s. Historically, horses owned by millers were reported to be more affected than other horses. However, reports of enteroliths decreased in frequency until the 1980s. 
Today, California and areas of the southwestern United States have a higher prevalence of enterolithiasis than other areas of North America. 
Causes of Enteroliths in Horses
Researchers have not yet pinpointed the exact cause of enterolith development. However, we do know that stones can form when a foreign body, such as hair, twine, sand, or a small rock, is ingested by the horse and becomes trapped in the gastrointestinal system.
Over time, the object, called a nidus, is coated in layers of minerals called struvite. 
Many horses will pass an enterolith with no damage to their gastrointestinal tract. In other cases, the enterolith remains in the intestines, accumulating more layers of struvite and growing quite large. 
Horses with a small enterolith may or may not exhibit colic symptoms. Some researchers say that it takes about two years for a stone to become large enough to cause an obstruction. 
How Enteroliths Form
Enteroliths grow outward from the nidus, which acts as the nucleus of crystal growth.  The rate of enterolith formation appears to be affected by pH level, mineral availability, and the horse’s colonic motility. 
Minerals usually found in enteroliths include calcium, magnesium, sodium, sulfur, and potassium. The region where the horse lives appears to influence the composition of the stone. 
When enteroliths are removed and cut in half, there is usually a central body with rings of mineral deposits around it, resembling the rings of a tree trunk. 
Once removed, researchers can study enteroliths to understand their composition.
Enteroliths can form in various shapes, but spherical or tetrahedral (pyramid) shapes are most commonly found. Tetrahedral stones are usually observed when more than one enterolith is present in the intestines. 
Stones are generally smooth-surfaced but can have varying textures and sizes. 
The weight of stones can vary but may range from 200 grams up to 9 kilograms. The diameter can range from 1-14 centimeters. The nidus can also usually be identified when studying the stones. 
Enteroliths may also contain fragments of undigested food, hair, and mucus. 
Risk Factors for Enterolithiasis
Researchers believe that several factors may play an important role in the development of enterolithiasis in horses. Some of these include: 
- Excessive dietary magnesium, phosphorus, and/or nitrogen
- Alkaline (high pH) colonic contents
- Presence of nidi
- Slower transit time (hypomotility) of the right dorsal colon
Other specific risk factors that have been found to be linked with enteroliths include the following:
High levels of alfalfa in the diet is one of the strongest risk factors linked to enterolithiasis.
Alfalfa contains a greater concentration of minerals than either grass pasture or hay. This can lead to a significant increase in colonic pH or alkalinity. 
According to an evaluation of 15 cases, horses fed a diet consisting of over 70% alfalfa were most likely to develop intestinal stones.  In another evaluation of 900 cases, horses with a diet of 50% or more alfalfa made up 98% of cases. 
Alfalfa is also easily digested and contains lower fiber levels, which may impact gut motility. 
In contrast, horses that consume larger amounts of grass hay or fresh pasture have a lower intestinal pH and are less susceptible to the enterolith formation. 
Lack of Pasture Grazing
Stall confinement and a lack of pasture grazing may put horses at higher risk for enterolith formation.
Reduced exercise has also been linked to increased retention of matter in the intestines. Movement is important for promoting feed transit through the gut.
A study of horses in Texas found that confinement for at least 50% of the day with limited access to grazing and decreased colonic motility increased risk for enterolithiasis. 
Pasture grazing may counteract the effect of ingesting alfalfa, resulting in a reduced risk of enterolithiasis. 
Feeding wheat or rice bran, both of which contain high levels of phosphorus, magnesium, and protein, may also promote the formation of enteroliths. 
Bran needs to be balanced in the equine diet by feeding other ingredients that are high in calcium. However, if you live an area where enteroliths are common, you may want to avoid feeding bran altogether.
Certain breeds of horses appear to be predisposed to enteroliths. Researchers don’t know exactly why this is, but it may be due to anatomical features of the digestive system.
For example, Arabians and Arabian crosses have the highest rates of enteroliths requiring surgical intervention. Other breeds that appear to be at higher risk include Morgans, miniature horses, Appaloosas, Saddlebreds, and also donkeys. 
Studies show that siblings of affected horses may also be at increased risk, suggesting that a genetic factor is involved.  More research is needed, but it may eventually be possible to locate specific genes involved in enterolith formation. 
Enteroliths most often develop in middle-aged to older horses and rarely occur in young horses. This is likely due to the amount of time it takes for enteroliths to form.
However, there are occasional reports of younger horses (less than 4 years old) having enterolithiasis. 
As mentioned previously, enterolithiasis is associated with hot, dry climates. California and Arizona in the United States, as well as the Red Sea region and Persian Gulf, are areas with high rates of enteroliths.
Horses in other areas may also develop enterolithiasis, but the prevalence is rare. 
Water quality and composition may affect enterolith formation as well. Specifically, water with high amounts of magnesium and calcium may contribute to the problem. 
Signs of Enteroliths in Horses
Enteroliths don’t always cause outward symptoms, unless the stone partially or completely blocks the intestines.
Symptoms may also depend on stone location. Enteroliths in the large colon do not have as pronounced symptoms; the horse may only show signs of chronic intestinal inflammation. However, stones in the small colon usually result in more severe symptoms. 
Early symptoms of enterolithiasis include weight loss and repeated episodes of mild to moderate colic, often over a short period of time.
If the intestine is completely obstructed, pain will be severe (as with other forms of impaction colic). The horse’s heart and respiratory rates will be noticeably elevated and the horse may have gas distension. 
Other symptoms of enterolithiasis include:
- Reduced performance
- Behavioral changes
- Anorexia (loss of appetite)
- Decreased defecation
- Passage of enteroliths in manure 
If your horse shows any the above signs, your veterinarian may suspect or diagnose enterolithiasis based on breed, diet history, and location. Unfortunately, horses that pass enteroliths are likely to have more stones in their digestive system or to develop them again at some point. 
Horses exhibiting signs of a severe blockage should be treated immediately; otherwise the condition can be fatal.
Enteroliths are often suspected based on symptoms, breed, history in region where the condition frequently occurs, and diet. However, the best way to confirm the condition is by taking abdominal radiographs (x-rays). Most large veterinary clinics can perform this procedure. 
So long as the horse has not eaten within the last 12 – 24 hours, enterolith(s) can be identified by x-ray 80% of the time. If the horse has eaten recently, feed may hide the enterolith from view. 
Large colon enteroliths are easier to identify via x-ray than small colon enteroliths. If there are multiple stones, the condition may be easier to diagnose through x-ray as well. 
Rectal examination rarely reveals stones, but it is sometimes possible to find stones in the small colon this way. Ultrasound may also helpful, but this diagnostic technique doesn’t always identify stones due to the large amount of gas in the intestines. 
If x-rays, ultrasound, and rectal examination don’t reveal evidence of enteroliths, but intestinal stones are still suspected, the next option is an exploratory surgery, known as an exploratory celiotomy. 
Surgical Treatment and Recovery
Surgery is necessary for enteroliths that are too large to pass in manure. In fact, the most common cause of emergency abdominal surgery in Californian horses is for stone removal.
If left untreated, small colon stones can cause peritonitis and intestinal rupture, resulting in death. 
Horses undergoing surgery for enteroliths are fully sedated under general anesthesia. Surgeons may use the flank approach, but more commonly, they use the ventral midline approach with the horse laying fully on its back. 
During surgery, the veterinary surgeon(s) start by palpating the colon. If an enteroliths is found, they fully evaluate the rest of the colon for any remaining enteroliths. 
Due to the horse’s anatomy, it is more difficult and sometimes impossible for surgeons to explore the small colon, making this type of stone removal more difficult. There is also a high risk for peritoneal contamination with small colon enteroliths. 
Post-operative care includes monitoring and supportive treatment. Antibiotics and pain-relieving medications, known as analgesics, are given for 3-5 days after surgery in most cases. Other supportive treatments may include IV fluid therapy with electrolytes or treatments for endotoxemia. 
Most horses can be fed small amounts of food 12 – 18 hours after surgery. If the small colon was affected, feeding a complete pelleted diet is recommended to reduce the bulk of fecal material passing through the surgical site. 
Fresh grass is preferable to hay initially. Administering fecal softeners such as mineral oil, flax seed oil, or magnesium sulfate can help decrease irritation of the surgical site. 
Post-operative complications may include diarrhea and incisional complications. In rare cases, a horse may develop septic peritonitis, adhesions, or laminitis. 
Returning to Work
Stall rest is needed for about a month before the horse is turned back out to pasture.
Horses generally need three months to recover from surgery before returning to exercise. Owners should continue to monitor for signs of colic of abdominal discomfort. 
Prognosis for Horses with Enteroliths
Survival rates after surgical removal of enteroliths have improved over time, thanks to better surgical techniques and postoperative care. While early reported survival rates ranged from 47-70%, more recent reports show a short-term survival rate of 92-96% and long-term survival rates of 93%. 
Unless the intestine is badly damaged or ruptured, most surgical treatment for enteroliths in the large colon have a good prognosis. However, stones that have moved into the small colon have a poorer prognosis. 
It may not always be possible to prevent enteroliths from forming in your horse’s intestinal tract. However there are measures you can take to greatly reduce your horse’s chance of developing them.
First and foremost, limit alfalfa to no more than 50% of the horse’s overall diet. Other recommended management practices, especially for horses living in areas where enteroliths are common, include the following: 
- Add one cup of apple cider vinegar twice daily to lower the intestinal pH level
- Provide frequent access to pasture and follow these turnout suggestions
- Add psyllium to your horse’s diet, which may support intestinal motility
- Provide regular exercise to promote gut function
- Avoid feeding wheat or rice bran
- Ensure your horse drinks enough water every day
- Feed salt to promote water intake
- Test water quality and soften hard water
- Use raised feeders to keep hay off the ground and avoid ingestion of gravel, sand, or other objects
Enteroliths and the risk of impaction colic are a major source of concern for horse owners living in dry climates. However, by being aware of this condition and the factors associated with intestinal stones, you may be able to prevent enteroliths from affecting your horse no matter where you live.
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