English
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. [1]
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. [3][7]
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 terminal colon. Alternatively, the enterolith can grow large enough to cause blockage in any location. [4][5]
Prevalence
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. [6]
Today, California and areas of the southwestern United States have a higher prevalence of enterolithiasis than other areas of North America. [6]
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. [8]
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. [3]
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. [7]
How Enteroliths Form
Enteroliths grow outward from the nidus, which acts as the nucleus of crystal growth. [7] The rate of enterolith formation appears to be affected by pH level, mineral availability, and the horse’s colonic motility. [6]
Minerals usually found in enteroliths include struvite (a magnesium compound), calcium, sodium, sulfur, and potassium. The region where the horse lives appears to influence the composition of the stone. [6]
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. [7][8]
Once removed, researchers can study enteroliths to understand their composition.
Physical Characteristics
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. [2][5][7]
Stones are generally smooth-surfaced but can have varying textures and sizes. [6]
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. [2][5][6]
Enteroliths may also contain fragments of undigested food, hair, and mucus. [1]
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: [3]
- 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:
Alfalfa
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. [5] In addition, the high levels of protein in alfalfa contribute to higher ammonia production during digestion, which is a component of struvite.
According to an evaluation of 15 cases, horses fed a diet consisting of over 70% alfalfa were most likely to develop intestinal stones. [5] In another evaluation of 900 cases, horses with a diet of 50% or more alfalfa made up 98% of cases. [9]
Alfalfa is also easily digested and contains lower fiber levels, which may impact gut motility. [5]
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. [6]
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. [11]
Pasture grazing may counteract the effect of ingesting alfalfa, resulting in a reduced risk of enterolithiasis. [6]
Feeding Bran
Feeding wheat or rice bran, both of which contain high levels of phosphorus, magnesium, and protein, may also promote the formation of enteroliths. [5]
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.
Breed
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. [3][6]
Studies show that siblings of affected horses may also be at increased risk, suggesting that a genetic factor is involved. [9] More research is needed, but it may eventually be possible to locate specific genes involved in enterolith formation. [6]
Age
Enteroliths