Vet Vibes: Enteroliths

Enteroliths are at their most dangerous when they become large enough to block the colon, or have sharp edges that could potentially cut the bowel and cause peritonitis.

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VET VIBES

The problem with enteroliths

Enteroliths are mineral masses that can form in a horse’s colon. Veterinarian DOUG ENGLISH explains the how, the why, and what you need to know.

Enteroliths, also known as fecaliths or phytobezoars, are stones composed of magnesium ammonium phosphate hexahydrate, or struvite. They mostly form in the right dorsal colon in horses and can subsequently cause partial or complete blockage of the right dorsal, transverse, or descending colon.

Many cases of enteroliths were reported in the 19th century, but the number greatly decreased in the early 20th century. However, in recent years cases in certain parts of the United States, California for example, have greatly increased.

Although several reasons for enterolith formation have been suggested,

why, exactly, these stones form in the intestinal tract remains unclear.

The occurrence of enteroliths varies quite a lot all over the world. They are rare in many areas and endemic in others. I think one of the main issues comes down to water quality and the volume consumed. Water with a high mineral content, and excessive addition of minerals to the diet are factors. The intake of nitrogen, magnesium and phosphorus (the primary components of enteroliths) from water and food is a likely contributor to their formation. Sodium chloride (common salt) is not implicated.

Clean pure water in clean troughs is essential for many good reasons, and one at the top of the list is that if your horse likes the water, they’ll drink more, and that’s good. I cringe when I see some of the stagnant pools that horses have to drink from. No good aye!

The problem with small dams is that horses can walk into them and foul them – and I’m very much not in favour of any animal being able to walk into a dam that they must drink from. It’s best to fence the dam off and pump the water to a clean trough. But that creates the need for daily checks to make sure the trough is full. There is a trade-off between optimum health and convenience.

Most enteroliths are formed by minerals being deposited in concentric layers around a central spherical or pyramid shaped piece of ingested material, often a small bit of baling twine or a stone, known as a nidus. In fact a nidus has been found in every enterolith that has been examined. The enteroliths themselves were found to consist primarily of ammonium magnesium phosphate. Most, if not all, involve a single slow growing enterolith which can exist for years without causing any problems because the diameter of the intestine where it sits is large enough to allow contents to pass. Occasionally a smooth egg sized one is found passed in manure. Limited studies evaluating dietary and environmental management practices have identified an association between enterolithiasis and certain breeds (Arabian or miniature breeds had an increased risk), the feeding of lucerne hay, and spending less than 50 per cent of time outdoors. However, many other factors including water supply, exposure to nidi such as bits of plastic baling twine, and the feeding of bran, grain, and other supplements have been suspected of playing a role in enterolith formation. However, these factors have not yet been properly evaluated. horses with enterolithiasis, 29 per cent of horses received bran and 52 per cent were fed grain regularly, but the significance of these values is unknown without studying a comparative population. Brans are rich in phosphorus and magnesium, and grains are rich in phosphorus, but they contribute to a more acidic environment in the colon, which is less conducive to enterolith formation. Excessive levels of dietary magnesium, nitrogen, and phosphorus, combined with alkaline conditions within the ascending colon are believed to contribute to the formation of enteroliths.

A bunch of keys next to a pyramid shaped enterolith gives a sense of its size.

manipulation to maximise carbohydrate (grass, cereal or grass hays) fermentation and minimise protein fermentation in the large intestine may help prevent enterolithiasis.

The following factors were found to be significantly associated with enterolithiasis: feeding more than 50 per cent of the diet as lucerne; feeding less than 50 per cent of the diet as oat hay; feeding less than 50 per cent of the diet as grass hay; and lack of daily access to pasture grazing, resulting in a lack of fibre from grasses.

Furthermore, researchers surveyed 900 horses with enteroliths that were admitted to the University of California in the US, and found:

• 15.1% of the patients were admitted for the treatment of colic. • 27.5% of patients required surgery for correction of colic. • Arabian and Arabian crosses,

Morgans, American Saddlebreds, and donkeys were significantly overrepresented, and Thoroughbreds,

Standardbreds, Warmbloods, and stallions were significantly underrepresented. • The horses’ average age was 11.4 years. • The most common historic findings were signs of intermittent colic (33.3%) and passage of enteroliths in the faeces (13.5%). • 15% developed a gastrointestinal tract rupture caused by an enterolith, which necessitated euthanasia. • Short-term and one-year survival rates for horses undergoing surgery for the treatment of enterolithiasis and recovering from anaesthesia were excellent (96.2 and 92.5% respectively), and postoperative complications were uncommon. • Recurrence of enterolithiasis was identified in 7.7% of the study population. Most cases are asymptomatic and affected horses can carry an enterolith for years without problems, but when issues do occur the prominent clinical features are recurrent mild abdominal pain, lack of appetite, gaseous distension, and minimal intestinal motility due to the partial or complete blockage caused by the enterolith.

Although symptoms are generally less severe than with other forms of colic, if there is a complete blockage then the signs are severe and the only solution is surgical removal, except in cases where an experienced veterinarian can feel, manipulate, and remove the mass through the rectum by hand.

Twice in the past I have seen cases where an enterolith has formed in separate pieces that had moulded into a roundish shape before some force separated them causing sharp edges to become exposed, cutting the bowel and causing peritonitis and certain death.

Interestingly, a dead male bottlenose dolphin was found with an enterolith that completely obstructed their intestinal lumen (the opening inside the bowels). Further examination of the enterolith revealed a stingray spine nidus. Most enteroliths occurring in land animals are composed primarily of struvite (magnesium ammonium phosphate). However, the enterolith discovered in the stranded dolphin was mainly composed of calcium phosphate carbonate.

Diagnosing enterolithiasis is often very difficult and is frequently arrived at either by conjecture, rectal palpation or ultrasound, or by x-ray if a very high quality equine x-ray machine (usually only located at a university or other large institution) is available.

Polished slices of equine enteroliths showing concentric layers of mineral deposited around a central spherical or pyramid shaped nidus.