Possible Causes and Prevention of Equine Entroliths (intestinal stones)

Dr. Harold Hintz, PhD.
Dr. Hintz is a professor in the Department of Animal Science at Cornell University. His areas of interest include equine and companion animal nutrition. Research interests in equine nutrition include energy metabolism and requirements of working horses, the use of dietary fat by the equine athlete and protein and amino acid metabolism and requirements of the growing horse. At the Equine Nutrition Conference in October 2001 he presented current research on entroliths as part of his discussion on digestive function. The proceedings from this conference are available from the Equine Research Centre.

One of the primary issues when addressing diet and hind gut function is the hay to grain ratio. The control of pH within certain limits is essential. Excess grain in the hind gut can cause founder and colic. When excess grain is fed, more starch reaches the hind gut and the fermentation of the starch decreases the pH (makes the gut content more acid).

On the other hand, chronic alkalosis could increase the incidence of enteroliths (intestinal stones). Several other factors, however, such as the intakes of magnesium, phosphorous and nitrogen, presence of a nidus (a foreign body such as a stone), breed of horse, and alfalfa intake may also be associated with enteroliths.

The intakes of magnesium, phosphorous and nitrogen are of interest because the majority of enteroliths are struvite, a material composed of the above three elements. Struvite is more likely to be formed in a pH of 6.5 or higher. This is why the pH of the hind gut is a factor.

A nidus is needed to start the precipitation of the struvite. Arabians are the horses that have the highest incidence of enteroliths; the reason is unknown. I think the Arabian horses in California have a higher incidence than breeds such as Thoroughbreds, in part, because Arabians are fed less grain and therefore are more likely to have a higher pH in the hind gut.

But other factors may also be involved. Results of several studies in regions where enteroliths are not common have suggested an association between Arabian horses and colic. Reeves et al. (1996) studied data supplied by veterinary hospitals at the University of Guelph, The Ohio State University, University of Pennsylvania and Tufts University.

Arabian horses were more than twice as likely to be colic cases. Cohen and Peloso (1996) reported that Arabians were more likely to have colic than other breeds based on data from cases treated by Texas veterinarians. In their study, the Arabian breed was not associated with any particular type of colic. Dart et al. (1997) studied data from Veterinary Medical Teaching Hospital, University of California at Davis. They found that Arabian, Morgan and Appaloosa horses were more at risk for cecal impaction than other breeds. White and Edwards (1999) suggested the association of colic and Arabian horses may be related to differing management practices for Arabian horses, a greater concern about colic and its management by owners of Arabian horses, or perhaps a genetic predisposition to gastrointestinal disorders among Arabian horses. Cohen and Peloso (1996) suggested that perhaps Arabians "may be more likely to manifest signs of pain than other horses."

But not all studies show that Arabians have a higher incidence of colic. A prospective study with horse owners in Virginia and Maryland found that the incidence of colic was almost one-third lower in Arabian horses (Tinker et al., 1997).

Studies in California indicate most horses with enteroliths have been fed alfalfa. Of course, most horses in California are fed alfalfa. However, grass hay is likely to produce a lower pH in the hind gut of horses because of the buffering capacity of alfalfa (Hintz et al., 1988). Furthermore, alfalfa provides the hind gut with significant amounts of nitrogen, and lfalfa raised in some parts of California contains a higher magnesium content than found in alfalfa raised in the midwest and east. Also, alfalfa usually has a higher digestible energy content than grass hay and therefore less grain is needed when feeding alfalfa. Less grain could lead to higher pH.

The addition of vinegar has been reported to decrease the pH in the hind gut of ponies (Hintz et al., 1989). Therefore, 2 cups of vinegar per day has been recommended as a preventative method. No clinical trials have been conducted to prove that vinegar would be of value, but I have had California horse owners tell me that the incidence of enteroliths in their hands has decreased since they have been using vinegar.

Hassel et al. (1999) conducted an excellent evaluation of 900 cases of enteroliths from the medical records at the University of California Veterinary Medical Teaching Hospital at Davis. They found that 7.7% of the study population had a recurrence of enteroliths. Horses with recurrent enterolithasis had less reduction of alfalfa in the diet (P = 0.1) and less dietary vinegar (P=0.09). The authors rightly concluded that "conclusions cannot be made regarding the effectiveness of vinegar supplementation on the prevention of enterolithiasis on the basis of results of this retrospective study." However I think the anecdotal reports and the association observed in the Hassel et al. (1999) report could be a basis for justification of clinical studies on the use of vinegar to prevent enterolith formation.


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Updated: October 2005.