Imagine having a stone the size of a melon in your gut. Enteroliths—rocklike objects that form in the intestines of horses—are a leading cause of colic in some parts of the United States, and they can be fatal. Their cause has long been a puzzle, but new research has uncovered some answers and produced concrete guidelines to help prevent the stones.
Enteroliths form in the colon when mineral layers build up around a foreign object, such as a pebble or a bit of wood, that the horse accidentally ingests. While the stone is small, it may not bother the horse; he may even pass it. But as more minerals are laid down, the stone grows larger—stones weighing 15 pounds have been reported. Eventually it blocks the intestine, leading to colic. Surgical removal is the only option, says Diana M. Hassel, DVM, who led two recent studies of enteroliths at the University of California at Davis.
Enteroliths are common in California and some other states, including Texas and Louisiana. They occur in other areas but not as often. They're also more common in horses of certain breeds. "Arabians and Morgans are most susceptible. American Miniatures also seem to be slightly overrepresented," Dr. Hassel says. "All breeds are potentially susceptible, however."
Several factors—diet, management, genetic predisposition—may work together to cause the stones. In a 2008 study, Dr. Hassel and her colleagues gathered information on horses brought to Davis with colic—61 cases with stones and 75 with other causes. They found that horses with enteroliths were more likely to get alfalfa as half or more of their diets. The risk of stones was actually five times lower when oat or grass hay made up 50 percent of the diet. Enteroliths were also significantly more common in horses who didn't have access to pasture daily.
Other studies have implicated alfalfa as a risk factor, Dr. Hassel notes. Compared to grass and oat hay, it has more magnesium—a key component of enteroliths. High levels of magnesium in the diet also tend to make the colon contents less acidic, a good environment for stone formation. The risk may be especially high for California horses. "In the 1980s, when California-grown alfalfa hay was analyzed and compared to the national average for alfalfa hay, it was found to contain a much higher quantity of magnesium," says Dr. Hassel, who is now an assistant professor at Colorado State University.
The researchers didn't find evidence of some other suspected risk factors: There were no significant links to a horse's age, gender, type of bedding, number of times fed daily or exercised per week, or drinking-water source. Feeding grain or bran didn't increase the risk of stones. Adding vinegar to feed, long thought to prevent stones, made no difference.
But not every horse on an alfalfa diet develops enteroliths. A second study, published in October this year, was designed to find out what goes on in the guts of horses who do. Six horses, three with a history of enteroliths and three without, were used. Surgeons made a small opening into the colon of each horse, so the contents could be sampled. Then each horse went through four feeding trials: grass hay and untreated water, alfalfa hay and untreated water, grass hay with filtered/softened water and alfalfa hay with filtered/softened water. When colonic contents were analyzed, horses with a history of stones had higher concentrations of stone-forming minerals and less acidity than others. In other words, they processed their food in ways that made the formation of stones more likely.
The findings point the way for future research into hereditary and other factors. Meanwhile, diet and management changes may help protect horses at high risk, such as those with a history of (or relatives with) stones. For these horses, eliminate alfalfa, feed oat or grass hay and provide pasture grazing. A daily feeding of grain will do no harm and may help—grain tends to make gut contents more acidic, so stones may be less likely to form.
This article originally appeared in the December 2009 issue of Practical Horseman magazine.