Horses At Risk For Ulcers

Horses moving faster than a walk could be at greater risk of developing gastric ulcers. Alfred Merritt, DVM, MS, and Mireia Lorenzo-Figueras, DVM, recently discovered that changes in gastric tension during intense exercise can push acidic stomach contents up into the vulnerable, squamous-cell-lined portion of the stomach--a circumstance that hints at why ulcers often develop or worsen when horses are in intensive training. This work done at the Island Whirl Equine Colic Research Laboratory at the University of Florida's College of Veterinary Medicine has provided another piece of the complex puzzle that is equine ulcers. It is a follow-up to the work that described the ability of the equine stomach to adapt to meals of various sizes and compositions.

Merritt and Lorenzo-Figueras used an ingenious method to explore what happens in the equine stomach of a live, exercising horse. The University of Florida's College of Veterinary Medicine is fortunate to have at its disposal three cannulated research horses--animals with permanent external access to their stomachs. The three Thoroughbreds (two mares and a gelding) had Mylar bags, similar to the balloons sold at some florists’ equipped with barostats, temporarily inserted in the proximal portions of their stomachs before exercising on a treadmill.

The barostat maintained a constant pressure in the bag, releasing air when the stomach contracted and injecting air into the bag when it relaxed. Because the Mylar bag followed the movements of the gastric wall, the measurable changes in the volume of the bag gave an indirect measurement of changes in the volume of the stomach itself. Specially designed software kept continuous track of the barostat measurements over the course of each exercise test.

The object of the study was to examine what sort of influence exercise might have on the contraction and relaxation of the stomach, in both horses which had been fed two hours previously and those for whom feed was withheld for 18 hours before exercise. Over a period of five weeks, the three cannulated animals were put through increasingly intense sessions on the University of Florida's treadmill, culminating in a gallop of about 3.2 km on an uphill slope.

It didn't take maximum exertion to produce a notable result. As soon as each of the study horses moved from a walk into a trot, the volume of air within the balloon started to rapidly decrease, to the point of almost emptying. It remained deflated when each of the horses was galloping, and didn't regain its original volume until they came back to a walk. This effect was most dramatic in the fasted horses, but it was observable in the fed horses as well, although the volume of food in the stomach meant that the initial volume of air in the balloon was less.

Merritt's team concluded that during movement, at any gait faster than a walk, either the gastric wall was becoming more rigid than normal, or some external pressure was being exerted on the stomach. To investigate the latter possibility, they inserted a catheter into the abdominal cavity through the right flank to measure intra-abdominal pressure, while a solid-state pressure transducer kept track of intra-gastric pressure. As soon as each horse moved from walk to trot, both measurements shot up and stayed there for the duration of the exercise routine, most likely thanks to a tensing of the abdominal muscles associated with the faster gaits.

It's estimated that more than 80% of high-performance equine athletes suffer from gastric ulcers. The external pressure on the stomach during exercise, Merritt's team hypothesizes, forces the liquid contents of the lower portion of the stomach upward, exposing the sensitive mucosa of the squamous-lined gastric wall to the stomach acids and inducing ulcerous lesions. Left to their own devices, horses rarely exert themselves for long periods of time, so the upper portion of their stomachs are not naturally exposed to these corrosive elements. Given that strenuous exercise can trigger gastro-esophageal reflux disease (GERD)--more familiar to most of us as heartburn--in human athletes, the University of Florida findings provide an interesting parallel.

Merritt's team further tested their hypothesis by inserting a pH electrode into the stomach via a nasogastric tube, positioning it just below the esophageal sphincter to continually monitor pH during an exercise session. When the horses were standing or walking, the pH remained in the 5.0 to 6.0 range, but as they moved into a trot or canter, the pH plunged, sometimes down to 1.0, and remained on the severely acidic side until they were brought back to a halt. This result was a strong indicator that stomach acids were indeed "splashing" on the sensitive upper portion of the stomach in response to strenuous exercise.

The discouraging part of this study is the realization that gastric ulcers might be more the rule than the exception as long as we continue to ask our horses to exert themselves on our behalf. Ongoing studies should continue to provide more ways--both dietary and pharmaceutical--in which we can normalize the pH or cushion the equine system to minimize the damage.


About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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