Gastric Ulcers: A Pain in the Gut

Over the last 10-12 years, veterinarians and researchers have documented an alarmingly high prevalence of gastric ulcers in horses. Worse, the prevalence seems to be even higher in horses in training, particularly racehorses. Of course, the rise in prominence of this condition has a lot to do with advancements in diagnostic technology--ante-mortem (before death) diagnosis of gastric ulceration only became possible with the advent of endoscopes long enough to see the stomach of an adult horse.

While we now have a much better handle on the prevalence of gastric ulcers in various horse populations, as well as effective methods for treatment, we have a fairly poor understanding of what causes them. It is also fair to say that our understanding of the effects of gastric ulceration on the horses' health is incomplete. Given that the prevalence of gastric ulcers is highest in horses in heavy training, two questions arise. First, is the stress of training one factor that contributes to the development of gastric ulcers? Secondly, do gastric ulcers adversely affect exercise performance?

In this article, we will address both of these questions. Bear in mind, though, that relatively little research has been done in this area and more research is required to fill in some of the gaps in our understanding.

Stomach Anatomy and Function

Let's quickly review the anatomy of the horse's stomach and the common sites of ulceration. The equine stomach has two distinct anatomical regions--glandular and non-glandular. The non-glandular mucosa occupies the top portion of the stomach and covers about one-third of its total area. This mucosa contains no glands and is covered by a special squamous (scale-like) epithelium. The glandular mucosa covers the bottom two-thirds of the stomach. As the name implies, this mucosa contains glands that secrete acid and pepsin, which are important aids in the early digestion of food. The glands also produce bicarbonate and mucus, which help form a protective barrier over the mucosal surface.

This is a key point, because the glandular mucosa is reasonably well-protected from the damaging effects of acid and pepsin. On the other hand, the non-glandular or squamous mucosa has few defenses and is particularly susceptible to injury caused by stomach acid. This helps explain why most ulcers occur in the squamous mucosa or along the margo plicatus (a narrow, raised area that separates the non-glandular and glandular portions of the stomach).

Another important thing to know about stomach function is that the horse continuously secretes gastric acid, whether food is present in the stomach or not--that's 24/7 in today's vernacular. This situation is markedly different from that in people--we produce gastric acid mostly when we eat. So, in people, gastric acid secretion essentially shuts down during periods of fasting, whereas the horse will continue to produce acid regardless of feeding regimens imposed by humans.

Research studies have shown that the mature horse produces approximately 0.4 gallons (1.5 liters) of gastric juice per hour. Under natural grazing conditions, this high and continuous rate of acid production presents few problems. When the horse grazes, he produces an equally large amount of bicarbonate-rich saliva that is swallowed with the chewed food. The bicarbonate neutralizes or "buffers" the stomach acid, thus keeping the gastric pH (a measure of acidity) near neutral and minimizing the insult to the squamous mucosa. The presence of food in the stomach, particularly forage (grass or hay), also can protect the mucosa. Compared to an equivalent meal of forage, consumption of a grain meal produces only half as much saliva. Forage itself acts as a buffer against stomach acidity.

We know that present-day horse management is at odds with the physiology of gastric acid production. Indeed, our tendencies to feed horses on a schedule, limit forage intake, and keep horses indoors likely contribute to the development of stomach ulcers. Two meals a day with no snacks in between mean that there will be long periods of time with little or no food in the stomach. Yet, all that time the horse's stomach is producing acid. That means there will be prolonged exposure of the squamous mucosa to high acidity without buffering.

Performance Horse Problems

Although researchers suspect that feeding management is an important factor in the development (and control) of gastric ulcers, other factors are as important, if not more important, in performance horses. Several endoscopic surveys have revealed some startling statistics concerning the prevalence of gastric ulcers in performance horses. (Prevalence is the number of affected animals within the study population.)

In one study, 91% of Thoroughbreds in race training had gastric ulcers of varying severity. When looking just at horses actively racing, 100% had gastric lesions (Murray et al. 1996). In another study of Thoroughbred horses in active race training, 166 of 202 horses (82%) had gastric ulcers. An association between severity of ulceration and racing activity has also been demonstrated--horses actively campaigning have more severe ulcers.

What about other types of performance horses? Certainly, show horses are subject to many of the management factors that might contribute to ulcer development--they are trained frequently, transported long distances, and, as with racehorses, are often fed meals with or no little access to pasture. Available data, however, suggests that the prevalence of gastric ulcers in show horse populations is somewhat lower compared to racehorses.

One study from Purdue University (McClure et al. 1999) examined 50 horses which were engaged in active training and showing, performing activities including driving and pulling, hunter/jumper classes, dressage, and Western performance (rodeo, cutting). These horses had been transported to at least one event in the 30 days prior to examination. Gastric ulceration was detected in 58% of the horses. Interestingly, horses judged to have a nervous disposition were more likely to have gastric ulcers.

Another study (Bertone et al. 2000) examined 156 Western performance horses which were three years of age or older. These horses were subjected to a fairly rigorous competition schedule--all horses included in the study had competed in six events and traveled a minimum of 300 miles during the 28-day period before examination. In this population, there was a comparatively low prevalence of gastric ulcers--62 of 156 horses, or about 40%.

Performance Stress and Ulcers

One weakness shared by these studies is the lack of a control group of horses. A control group would be horses which are housed and fed in a manner similar to the performance horses, but not subject to the rigors of training and transportation. Inclusion of such a control group would strengthen conclusions regarding the effects of performance training on the development of gastric ulcers, and allow researchers to separate the influences of feeding management from the effects of conditioning and showing. For now, we are left with the impression that gastric ulcers are more prevalent in racing and performance horses.

In Thoroughbred racehorses, there is evidence that the severity of ulcers increases with a step-up in the intensity of training. In a group of 2-year-olds first examined about one month after arriving at a racetrack, lesion severity had increased markedly by the second endoscopic examination, which was performed two to three months later. At the time of the first examination, the horses were in light training, but they were race-ready by the time of the second examination (Murray et al. 1996).

Treadmill studies have shown a relationship between exercise intensity and the severity of ulceration. Horses which received galloping exercise had a greater number and severity of squamous lesions compared to horses given only light (trotting) treadmill exercise (Vatistas 1998). Importantly, these two groups of horses were managed identically apart from the conditioning program. Therefore, it is fair to conclude that training load contributed to the observed difference in ulcer severity.

That the rigors of race training are important in the development of gastric ulcers is also evidenced by the fact that in some horses there will be spontaneous (i.e. without treatment) ulcer healing when they are taken out of race training.

Why does exercise contribute to the development and maintenance of gastric ulcers? The short answer is that we don't know. Current research efforts are being directed toward an improved understanding of how exercise--particularly intense effort--can alter stomach function. Intense exercise might delay gastric emptying. If so, gastric acid would accumulate in the stomach, increasing the risk of injury to the squamous mucosa. The tendency to train horses in a fasted state, without the buffering effects of saliva and food in the stomach, might compound this problem. It is also possible that physical pressure on the stomach during galloping results in the "splashing" of acid over the squamous mucosa.

Because excessive use of non-steroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone (Bute) can result in ulceration of the glandular mucosa, veterinarians have wondered whether use of these drugs contributes to the high prevalence of ulcers in racehorses. However, several studies have failed to demonstrate a relationship between NSAID use and the severity of gastric ulcers of the squamous mucosa (Murray et al. 1996; Vatistas et al. 1999). Although it's prudent to guard against excessive use of NSAIDs in horses, it seems likely that these drugs aren't responsible for the high prevalence of gastric squamous ulcers in performance horses.

Health and Performance

Although we have evidence that most, if not all, performance horses have gastric ulcers, the effects of these lesions on health and performance are less clear. Clinical experience and research studies have indicated that many adult horses with gastric ulcers appear to be otherwise healthy. In a recent study of Thoroughbred racehorses, only a little more than 40% of horses with endoscopic evidence of gastric ulcers displayed clinical signs attributable to the ulcers (Vatistas et al. 1999). An earlier study (Murray et al. 1989) reported a similar proportion of horses with gastric ulcers that had clinical problems (87 of 187 horses, or 46%).

Horses with moderate to severe gastric ulcers can exhibit poor appetite, poor body condition and hair coat, recurrent low-grade colic, and a change in attitude--e.g., the horse becomes more aggressive or grouchy.

Less clear is the effect of gastric ulcers on exercise performance. Certainly there is the perception that gastric ulcers contribute to poor athletic performance. In one study, trainers were asked to rate a horse's performance as at expectations, above them, or below expectations before the endoscopic examination was performed (Vatistas et al. 1999). These researchers found an association between the presence of gastric ulceration and decreased performance. However, contrary to expectations, the effect of gastric ulcers on performance was "all or nothing." Even mild ulceration appeared to adversely affect training and racing performance. In a study of Western performance horses, those with gastric ulcers were often reported to have "low energy" and a "loss of attention during competition" (Bertone 2000).

More research is required to clarify the relationship between gastric ulcers and athletic performance. Nonetheless, it is reasonable to assume that a horse's performance will be subpar when he is eating poorly and not maintaining body condition, possibly as a result of gastric ulceration. Basically, the horse will not have enough "groceries" to withstand the rigors of training and competition. As well, it is possible that pain associated with gastric ulcers adversely affects a horse's performance.

Treatment and Prevention

The very high prevalence of gastric ulcers in performance horses sparks considerable debate among veterinarians concerning treatment and prevention of these lesions. Some veterinarians question the clinical importance of a few small ulcers in a horse which otherwise appears to be quite healthy. Others will argue that, regardless of the number and severity of gastric lesions, ulcers are abnormal and should be treated. These latter arguments are supported by the knowledge that gastric ulcers do not resolve spontaneously in horses maintained in training. In fact, it is likely they will worsen over time.

Fortunately, effective treatments for gastric ulcers are widely available (see "Ulcers" in the November 1999 issue of The Horse, online at Decisions regarding the need for ulcer treatment and the type of therapy must be made in consultation with your veterinarian. In some horses, a break from training and competition might be required for complete ulcer healing.

You might be aware that treating a horse for gastric ulcers can be a very expensive proposition. Therefore, we must give some consideration to prevention. In this regard, you should carefully consider the horse's diet and feeding schedule. Horses at pasture, grazing for upwards of 18 hours per day, don't tend to develop gastric ulcers. Two factors might be involved in that scenario. First, constant grazing will result in the production of large amounts of saliva that will buffer gastric acidity. Second, the horse remains very active while it grazes. This movement might promote better stomach emptying, again reducing the accumulation of damaging acid in the stomach.

Contrast the grazing horse with a horse maintained in a stall for much of the day. Apart from formal conditioning, he has no opportunity for exercise. As well, it is common practice to feed a diet high in concentrates and low in forage, with food offered in the morning and evening. This is a "triple threat" with respect to stomach health--a high-grain diet will result in lower saliva production and higher gastric acidity compared to a forage diet. As well, a twice daily feeding schedule means there will be long periods without food in the stomach, increasing the risk for acid-induced injury of the gastric mucosa.

As much as possible, you should try to alter the feeding schedule to mimic a grazing situation by giving the horse access to hay at all times. Recent studies have shown that compared to a bromegrass hay diet, a diet of alfalfa hay and grain provided greater buffering of gastric acidity (Nadeau et al. 2000). Therefore, it is recommended that you include some alfalfa in the diet, perhaps by mixing chopped alfalfa with grain.

Also take advantage of the horse's natural buffering mechanisms. Where available, provide the horse as much pasture turnout as possible. Even large yard turnout is better than stall confinement. The horse will get a little more exercise and will no doubt have a better attitude. Instead of two meals per day, feed smaller concentrate meals at more frequent intervals. If grazing is not available, including an equine antacid such as in the diet might help buffer gastric acidity.

Yes, these changes will dictate a change in the daily schedule. However, given the potential for gastric ulcers to adversely affect health and performance, it is your responsibility to make this extra effort.


Bertone, J.J. Prevalence of gastric ulcers in elite, heavy use Western performance horses. Journal of Veterinary Internal Medicine, 14, 366 (abstract), 2000.

McClure, S.R; Glickman, L.T.; Glickman, N.W. Prevalence of gastric ulcers in show horses, Journal of the American Veterinary Medical Association, 15, 1130-1133, 1999.

Murray, M.J.; Grodinsky, C.; Anderson, C.W.; Radue, P.F.; Schmidt, G.R. Gastric ulcers in horses: a comparison of endoscopic findings in horses with and without clinical signs. Equine Veterinary Journal, Supplement 7, 68-72, 1989.

Murray, M.J.; Eichorn, E.S. Effects of intermittent feed deprivation, intermittent feed deprivation with ranitidine administration, and stall confinement with ad libitum access to hay on gastric ulceration in horses. American Journal of Veterinary Research, 57, 1599-1603, 1996.

Murray, M.J.; Schusser, G.F.; Pipers, F.S.; Gross, S.J. Factors associated with gastric lesions in Thoroughbred racehorses. Equine Veterinary Journal, 28, 368-374, 1996.

Nadeau, J.A.; Andrews, F.M.; Mathew, A.G.; Argenzio, R.A.; Blackford, J.T.; Sohtell, M.; Saxton, A.M. Evaluation of diet as a cause of gastric ulcers in horses. American Journal of Veterinary Research, 61, 784-790, 2000.

Sellnow, L. Ulcers, The Horse, November 1999, 32-50.

Vatistas, N.J. Gastric Ulceration in the Racing Thoroughbred. PhD Thesis, University of California, Davis, 1998.

Vatistas, N.J.; Synder, J.R.; Carlson, G.; Johnson, B.; Arthur, R.M.; Thurmond, M.; Zhou, H.; Lloyd, K.L.K. Cross-sectional study of gastric ulcers of the squamous mucosa in Thoroughbred racehorses. Equine Veterinary Journal, Supplement 29, 34-39, 1999.

About the Author

Ray Geor, BVSc, PhD, Dipl. ACVIM

Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University

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