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Enteroliths: Is Your Horse at Risk?

October 2018 by Karen Richardson, BSc (Hons), MSc Equine Science
A graduate from the Royal (Dick) School of Veterinary Studies, University of Edinburgh, with a Master of Science in Equine Science (with Distinction), independent equine nutrition consultant, Karen Richardson specialises in formulating diets to reduce the incidence of nutrition-related disorders, maintaining performance and improving welfare for performance horses.

Enteroliths are stones that can form in the large intestine (right dorsal colon) of the horse that may cause partial or complete blockage of the intestine. They are one of the many causes of colic in horses. This article reviews the dietary factors associated with the formation of enteroliths.

The enterolith is created when struvite crystals, made of magnesium ammonium phosphate hexahydrate, form around a foreign object – such as some sand, plastic, rope, a stone, a piece of wire, or nail. The enterolith may be pea sized or grow to the size of an orange, and one or multiple enteroliths may be present.

How do enteroliths form?

Research continues to work on answering this question. What has been learned from the research is that horses with enteroliths have colon contents that are quite alkaline (pH 7.32). The normal pH resides in the region of pH 6.5-7.0. Their colonic contents also have much higher concentrations of ammonia, and minerals including magnesium, phosphorous, sulphur, sodium, calcium and potassium when compared to horses without enteroliths.

It is thought that the more alkaline environment may encourage precipitation and crystallisation of the minerals associated with struvite formation.

Factors that may contribute to these observations include diet, mineral content of water supply, genetics, and variations in the mechanisms for water and electrolyte exchange in the colon.  Colon motility may also play a role, with slowed passage of colon contents (eg. due to lack of exercise, standing in stall for long hours) facilitating precipitation of minerals.

What are common dietary factors associated with enterolith formation?

Whilst horses all around the world may develop enteroliths, and fortunately it is a relatively rare occurrence, there appears to be some geographical areas, particularly in California and the south eastern states of the USA, where the prevalence is quite high. This has enabled scientists to gain some important insight into the development of enteroliths.

Studies of dietary factors have shown the following to be significantly associated with enterolith development. It is important to point out here that an “association” does not establish “causality”. It simply means that there is a relationship between the two variables measured.

1. Feeding large quantities of lucerne

Studies conducted at a veterinary clinic in California observed that, when compared to horses presenting at the clinic with colic caused by “other” factors, horses presenting with colic due to enteroliths had diets that contained a large proportion of lucerne (on average, greater than 80% of the total diet).
In contrast, the horses presenting without enteroliths received, on average, less than 60% of their diet as lucerne. Further studies were conducted and it was proposed that horses consuming >50% of their total daily diet as lucerne had a significantly greater risk of developing enteroliths than horses consuming less than 50% of their diet as lucerne.

Lucerne’s association is due to it containing high levels of protein and higher mineral concentrations (particularly magnesium) than other hay types. The pH of the colon of horses fed large quantities of lucerne is more alkaline than when fed smaller quantities of lucerne, and this in combination with its nutritional profile may promote enterolith formation.

It is important to remember that the horse populations examined in these studies are small, geographically limited, and focus on colic patients. There is an even larger population of horses around the world fed large portions of their total daily diet as lucerne that do not develop enteroliths.

2. Mixing Hay Types

These studies also identified that feeding less than 50% of the total diet as oaten hay or less than 50% of the diet as grass hay, increased the risk of developing enteroliths. Oaten and grass hay types have a lower protein and magnesium content than lucerne and the digestion of the higher fibre content of these hay types may produce a more acidic colonic environment believed to be less favourable for enterolith formation.
The study authors proposed that including oaten hay or grass hay types in a diet containing lucerne may provide a “protective effect”, reducing the risk of enterolith formation, to an extent greater than simply reducing the quantity of lucerne in the diet.

3. Lack of daily access to pasture grazing and stall confinement

The risk of developing enteroliths is increased when horses do not have daily access to pasture and spend time confined to a stall or small yard. This may be related to reduced activity levels, and differences in patterns of ingestion. Also, the products of grass digestion may have beneficial effects for the prevention of enteroliths.

4. Vitamins and minerals

The absence of daily vitamin/mineral supplementation was found to increase the risk of developing enteroliths.

5. Carrots

A trend (close to statistical significance) was observed for daily feeding of carrots. The chance of developing enteroliths when carrots were regularly fed was nearly 3 times higher, in this study population.

6. Brans and grains

Feeding bran and grain-containing products are commonly implicated as causes for enterolith formation. According to the scientific literature, no scientific evidence has been identified to associated wheat bran, rice bran, oat bran or grain as risk factors for enterolith formation.

Is your horse at risk of developing enteroliths?

Horses considered high-risk for development of enteroliths include:

  • Horses with a past history of surgical treatment of enterolithiasis,
  • Horses that have passed small enteroliths in their manure,
  • Horses living at a facility with a high incidence of enterolithiasis,
  • Relatives of horses with the disease,
  • High risk breeds (eg. Arabians) with a history of a long term lucerne diet and chronic recurrent colic,
  • Horses kept in a geographical region with a high prevalence of enterolith formation, and
  • Horses fed a diet primarily of lucerne.

Management of high risk horses

If you have a horse at risk of developing enteroliths there are several measures that can be taken to reduce the risk of enterolith formation.

  • Remove lucerne, or reduce the quantity of lucerne in the diet to less than 50% of the total diet.
  • Feed at least half of the total daily diet as oaten or grass hay.   
  • Provide daily access to pasture grazing.
  • Appropriately balance the mineral ratios in the diet and ensure vitamin intake is adequate.
  • If appropriate, include some grain in the diet to facilitate a more acidic colon environment
  • Addition of 1 cup of apple cider vinegar twice daily (12 hourly) has been shown to reduce colonic pH although its effectiveness at preventing enteroliths is not proven.
  • If your horse is not considered high risk for enterolith development then don’t be too quick to cut the lucerne from the diet.

Lucerne is highly nutritious and a great accompaniment to other components of the diet. Remember that it may also provide a protective effect on the non-glandular portion of the stomach against gastric ulcers, and provide necessary calcium for horses grazing on oxalate-containing pastures (e.g. kikuyu).

Quick links to relevant scientific studies: