Enterolithiasis is the presence of intestinal calculi, called enteroliths. These calculi are deposits, or build-up, in the large intestine.

This is a problem, because they can cause blockages of varying degrees, usually resulting in colic to varying degrees.

In fact, during one study, more than 15% of horses admitted to the large animal clinic at UC Davis for colic complaints can blame enterolithiasis for the colic episode.

So what exactly are enteroliths? They are a build-up of struvite crystals, which are composed of magnesium ammonium phosphate.

The crystals are laid down in concentric rings, and the larger the ring, the bigger the blockage. The rings usually start around a foreign body in the intestine, such as a small piece of cloth or hair that has been ingested.

These rings can form into one large enterolith, or they can form into numerous small enteroliths in groups.

If one large enterolith is formed, usually the horse will experience colic to varying degrees as the calculi gets lodged in areas of the large intestine, causing bloating and digesta back-up to varying degrees.

However, if small enterolith groups are formed, usually the horse will just display symptoms of mild abdominal pain, as the small crystals usually don't cause blockages. Instead, they irritate the tract lining as they move.

Enterolithiasis risk is high in donkeys.

Which horses are at risk?

Like I mentioned before, in one study more than 15% of horses admitted to UC Davis for colic were suffering from enterolithiasis. Yet, at the same time period, less than 2% of horses admitted to a clinic in Texas for colic had enterolithiasis.

This is due to the fact that it appears horses in certain geographical regions are more prone to the disease. California and the southeastern US appear to be the areas with the highest number of cases.

However, horses in every geographical region can have enterolithiasis.

Enterolithiasis risk is high in Arabians like this stallion.

Breeds that tend to be over-represented include Arabians (and Arabian crosses), Morgans, American Saddlebreds, donkeys, and miniature breeds.

There also appears that there might be a genetic component to the disease, as the reported occurrence in siblings is high.

It used to be thought that young horses would not suffer from intestinal calculi due to the length of time it usually takes the crystals to form.

However, there have been reported confirmed cases in horses less than one year of age.

This suggests that in the right conditions, it is possible for the crystals to form to a problematic size in a short amount of time.

What causes enterolithiasis?

A number of studies have pointed to increased large intestine pH as a cause of this disease. Normal pH in the horse large intestine is usually somewhere around 6.9, while horses with substantial enteroliths usually have a pH somewhere around 7.3.

Affected horses also have a lower content of dry matter in their colon, as well as higher concentrations of magnesium, phosphorus, calcium, and potassium, as well as other nutrients in the digesta.

Also, horses that are fed large amounts of alfalfa (more than 70% of forage intake) appear to be at higher risk. This is because alfalfa has very high buffering capacities, and buffers are used to promote a higher pH. It also has more protein and magnesium than grass hays, both of which are thought to contribute to the formation of the crystals.

However, due to the large number of horses that consume large

Enterolithiasis risk is increased with alfalfa intake.

amounts of alfalfa and do not have enteroliths, alfalfa is obviously not the only factor in causing the formation of struvites.

One of the last risk factors for enterolithiasis is lack of daily access to pasture. It is not clear whether pastured horses are at less risk because of increased grass intake or increased exercise, but for one reason or the other, or maybe a combination of both, horses that have daily access to pasture are at less risk than those that are stalled for more than 50% of the day.

Nutritional Management

A diet low in magnesium, phosphorus, and protein has been shown to reduce the occurrence and size of enteroliths in dogs in cats. Likewise, in horses it appears that dietary modifications can also be used as prevention of enteroliths.

Dietary recommendations that increase the acidity of the colon that have been suggested include:

  • exclusion of alfalfa from the diet
  • exclusion of wheat bran from the diet
  • increased grain:hay ratio
  • supplementation with apple cider vinegar

Though these methods have all been suggested in one study or another, there is no solid evidence of how well they increase the acidity of the colon or prevent struvite formation.

However, in lab studies, when the colon pH was below 6.6, the size and weight of enteroliths present decreased. In another study, ponies that were supplemented with 1/2 cup apple cider vinegar daily saw a reduction in the size of their enteroliths.

So, for horses that have experienced problems with enterolithiasis, it is recommended that grass hay (instead of alfalfa) is fed, along with some form of grain (approximately 5 pounds/day for a 1000 pound horse) and 2 cups of apple cider vinegar (1 cup twice a day) to help prevent future occurrences.

This is, of course, assuming the horse has no other issues that would be negatively affected this diet.

It is also recommended that horses at risk receive daily turnout, preferably in a good pasture if possible.

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