Intestinal calculi that form in the large intestine are termed enteroliths. Although all breeds are affected, Arabians are over-represented. The vast majority of animals with enteroliths are older than 5 years of age. Enteroliths occur rarely in Western Australia.
The calculi form by depositing layers of magnesium ammonium phosphate (struvite) around a dense nidus, such as hair, stone, or metal. Animals that are fed diets high in magnesium and protein are most vulnerable. The intestinal pH is also important, with values > 7.0 commonly observed in horses with enterolithiasis.
Clinical signs include intermittent, mild-to-moderate pain that may be episodic in nature, separated by weeks to months. Many of the signs are similar to intraluminal obstruction due to impaction.
Rectal palpation may reveal intestinal distention with gas and/or ingesta ahead of the obstruction. If the enterolith is in the descending colon, it may be palpated directly. Rectal palpation on a slope may increase the chances of palpating the “rock”. Radiography is also of diagnostic value in many cases, but failure to image the enterolith radiographically does not eliminate the diagnosis. Peritoneal fluid will be normal unless the overlying bowel becomes necrotic.
Treatment is through surgical removal of the offending stones.
Enterolithiasis can be prevented by restricting the percentage of alfalfa or Lucerne hay in the diet (<50%). The daily feeding of vinegar to decrease the intraluminal pH may prevent rock formation by lowering the luminal pH.