Equine Enterolithiasis: A Review and Results of a Retrospective Study

Diana M. Hassel, DVM
Equine Surgery, Veterinary Medical Teaching Hospital
University of California, Davis


The word 'enterolith' is derived from the Greek words 'entero' meaning intestinal, and 'lith' meaning stone. Reports of stones, concretions, or calculi arising from the large intestine of horses have been cited in the veterinary literature since the ea rly 1800's. The first successful report of surgical removal of an enterolith was in 1877 (Lloyd, 1987). An unusually high number of enterolith cases occur in the southwestern regions of the United States; the majority being reported from California (Mu rray, 1992). However, enteroliths also occur in other regions of the United States including Ohio, Texas, Florida, Missouri, and Minnesota. Internationally, enteroliths are common in Tahiti (Murray, 1992), and also have been reported in France and the United Kingdom.


Structural and chemical composition analysis indicates that enteroliths are composed primarily of magnesium, ammonium, and phosphate (Blue, 1981). The combination of these three elements into a crystalline form is commonly referred to as struvite. Oth er minerals such as calcium, titanium, iron, aluminum, and nickel comprise less than 10% of the enterolith (Blue, 1981). The struvite crystals are laid down in concentric rings. Although enteroliths derived from horses from California are highly simila r, some degree of variation exists in enteroliths from other parts of the country (Murray, 1992). This may be reflective of the mineral composition of soil and feed materials from these regions.


The number of horses presenting to the VMTH with a diagnosis of enterolithiasis has steadily increased over the past 25 years (Figure 1). Nine-hundred cases of enterolithiasis have been diagnosed at UC Davis from 1973-1996. In 1973, 2 horses were admi tted to Davis with a diagnosis of enterolithiasis. This number increased to 18 by 1982, and to 106 by 1992.

Etiology and Prevention

Significant advances in the study of risk factors and mechanisms involved in enterolith formation have not occurred in recent years, but are the subject of ongoing research at the University of California. Proposed etiologic factors for development of enteroliths include dietary or genetic factors contributing to increased mineral content and an alkaline pH within the colon, increased exposure to “nidi” (foreign materials providing a central core for the enterolith), and conditions promoting reduced i ntestinal motility in the large intestine. Theories for enterolith formation have been derived from studies on the chemical composition of enteroliths and the interactions of these components with the large intestine of the horse. Previous studies show ed that the mineral content of both the water supply and alfalfa hay in several parts of California far exceeded the recommended magnesium requirements for horses (Lloyd, 1987). Alfalfa hay may also contribute to enterolith formation by providing a high level of protein. An excessive amount of free ammonium may be released into the large colon of the horses from the digestion of high protein feeds such as alfalfa (Blue, 1979; Murray, 1992). This free ammonium could combine with magnesium and phosphat e ions to form ammonium, magnesium, and phosphate crystals (struvite) and contribute to enterolith formation. Retrospective analysis of horses with enteroliths revealed approximately 98% had a diet which consisted of ≥ 50% alfalfa hay. Although the wat er supply in California is also high in magnesium, the daily intake of magnesium from drinking water is less than one-tenth of that which is ingested in a daily feeding of alfalfa hay.

The source of phosphorus for enterolith formation has been attributed to brans. Although grains provide a significant amount of dietary phosphorus, rice, wheat, and rye brans have been reported to have higher protein, phosphorus, and magnesium content than their whole grain counterparts (Lloyd, 1987). Further studies identifying whether bran feeding is a risk factor for enterolith formation are pending. The pH of large colon intestinal contents from horses with enteroliths has been reported to be higher than in horses without enteroliths (Hintz, 1985). Alterations in intestinal pH have been shown to occur as a result of dietary changes. Increasing th e grain to hay ratio resulted in a decrease in colonic pH, and feeding alfalfa with magnesium oxide supplementation resulted in an increase in pH in colonic fluid (Hintz, 1985). In addition, adding 1 cup of vinegar to the diet daily has been shown to re duce colonic pH (Hintz, 1989). An additional factor which could play a role in enterolith formation is environmental and feeding management factors. Reduced movement of bulk feed material through the large intestine may provide a favorable environment for stones to incubate and grow . Feeding highly digestible, lower fiber feedstuffs, such as alfalfa hay, may contribute to reduced intestinal motility in the large intestine. Other factors which may negatively affect motility include restricted physical activity (i.e. stall confinem ent) and infrequent feeding (i.e. feeding 1-2 times daily). With regards to stall confinement, the type of bedding used may play a role in enterolith formation. Straw bedding provides an opportunity for horses to nibble on a high fiber, bulk feed mater ial throughout the day, which is also low in magnesium, phosphorus, and protein, as opposed to horses bedded on shavings or without any bedding. All of these factors will be more thoroughly investigated to determine whether they play a role in enterolit h formation.

Genetic predisposition may play a role in the development of enteroliths. Within the retrospective study, 8.4% of horses with enteroliths had siblings affected. In accordance with previous reports, the Arabian breed remains the most commonly affected b reed. Arabians and Arabian crosses comprised 40.3% (353) of the enterolith case population, yet they only represented 14.2% of the control hospital population (Figure 2). Other commonly affected breeds included Quarter Horses (238), Thoroughbreds (84), Appaloosas (47), Morgans (35), and American Miniatures (22). Warmbloods were underrepresented compared with the control group.

Clinical Signs

Important clinical signs are related to the stone causing an obstruction to feed passage in the large intestine. Horses exhibit general colic signs with a mild to moderate degree of pain that increases as the large intestine becomes more gas distended. In the retrospective study, 13% of horses diagnosed with enteroliths had a history of passing stones in their feces. There were also a number of behavioral abnormalities that were observed less commonly including lethargy or depression (5.6%), cranky behavior (5.3%), weight loss (2.8%), occasional loose stool (2.2%), and poor performance or reluctance to jump or go down hills (3.4%). Horses with a single large enterolith in the large colon commonly have a history of chronic, intermittent colic. Hor ses with smaller diameter stones which become lodged within the small colon are less likely to have a history of colic. These enteroliths tend to cause a slower onset of more severe colic signs and many will go on to rupture the intestinal tract. 15.9% of all horses diagnosed with enteroliths had evidence of intestinal rupture at the time of admission or intra-operatively. Once an enterolith has caused an intestinal rupture, the horse either dies or must be euthanized due to severe peritonitis.


Definitive diagnosis of enterolithiasis is made by surgical exploration, abdominal radiographs, or at the time of necropsy. Several factors such as breed, age, diet, and history may provide clues to lead a clinician to suspect enteroliths. The average age of horses presenting with enterolithiasis in the retrospective analysis was 11.5 yrs. This is higher than in previous reports which describe the highest incidence in horses from 5-9 years of age. Abdominal radiographs are an important screening to ol when attempting to make a diagnosis of enterolithiasis in horses with colic. In a retrospective study of 141 horses with enteroliths, 67% were confirmed by abdominal radiographs (Yarbrough, 1993). The smaller the enterolith, and the larger the horse , the less likely the diagnosis will be confirmed by radiography alone. Fasting horses for 24-36 hours to decrease the amount of feed material within the large colon is sometimes helpful to assist with penetration of the x-ray beam to allow visualizatio n of the enterolith. There is currently ongoing research to help find alternative, non-invasive methods to identify enteroliths and help detect horses at risk for developing stones.


The only successful treatment for horses with colic due to enteroliths is surgical removal. The success rate at UC Davis for a horse with enteroliths in good physical condition is 90-95%. Complications which may occur post-operatively are similar to t hose seen with other forms of colic. These include diarrhea, fever, incisional infection, incisional hernias, laminitis, peritonitis, and pneumonia. The current cost for surgical treatment is $3500-$5500, but most horses with enteroliths will be discha rged from the hospital at the lower end of the estimate. Further research into the feasibility of dissolution of enteroliths in horses without active colic signs via dietary and management changes is necessary.


Enteroliths are a common cause of colic in horses in California. The cause of enterolith formation likely consists of many interrelated factors including diet, genetic predisposition, and management practices. Evaluating the medical history, performin g a complete colic examination, and obtaining abdominal radiographs is important for a complete diagnostic work up. Surgery sometimes becomes the final diagnostic tool and is the only means of relieving the obstruction caused by enteroliths. The large number of horses requiring euthanasia due to intestinal rupture or financial constraints further supports the need for continued study into the causes and prevention of enterolithiasis.


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