In the last few years, various studies have shown that an alarming number of horses suffer from stomach ulcers, a problem referred to as Equine Gastric Ulcer Syndrome (EGUS) because of its complex nature. Estimates are that ulcers are present in 25-50% of foals, 60% of show horses, and over 90% of racehorses and other high-level performance horses – and one new study has even found a shockingly high rate of ulcers in broodmares on pasture. Experts agree that the main causes of EGUS are intensive exercise, infrequent feedings, stall confinement, trailering, the use of non-steroidal anti-inflammatory drugs (NSAIDs) and carbohydrate rich diets, but when it comes to the less tangible factor of psychological stress, opinions differ wildly.
While there is anecdotal evidence which suggests that psychological stress can play a role in the development of EGUS, and many articles and advertisements state that it does, there is little in the way of hard science to support this view. This may be due to the fact that it is extremely difficult to design a scientifically sound study around something as unquantifiable as a horse’s feelings, but some researchers are emphatic that what studies have been done on EGUS indicate that psychological stress is not a significant causative factor. Others are willing to concede that mental stress can lead to certain types of ulcers most commonly found in foals, but only in a very small percentage of adult horses. Still others are convinced that psychological stress is an important player in the mystery that is EGUS, despite the fact that the link has yet to be definitively established.
What is known without a doubt is that the stomach of the horse secretes hydrochloric acid on a continual basis, regardless of the presence or absence of food. This makes sense in an animal designed to take in small amounts of food almost non-stop throughout the day and night. But when horses are subjected to the unnatural conditions and activities imposed on them by domestication, this constant secretion of acid leaves them highly vulnerable to ulcers. The lower part of the horse’s stomach, called the glandular region, is less prone to ulceration due to a protective coating which usually keeps it from being damaged by acid. The upper portion of the stomach, called the squamous or non-glandular mucosa, does not have as much protection from acid, and this is where we find the vast majority of ulcers in horses.
Though EGUS can be caused by a number of different factors alone or in combination, the widespread practice of feeding horses relatively large but infrequent meals (2-3 times daily) is one of the most common causes, as the acidity of the stomach increases rapidly after only a few hours without food. Recent studies have shown that a horse's chance of developing ulcers increases after only six hours without the intake of food. Fasting a horse is so likely to cause ulcers, in fact, that it is the method most often used by scientists to induce ulcers for research purposes. Anything that prevents a horse from eating for any length of time, therefore, may lead to ulcers. Since many horses that are upset or worried by a given situation go off their food, most researchers would not dispute the assertion that psychological stress can contribute to the formation of ulcers if it causes a horse to stop eating.
Where opinions start to vary is when you ask whether or not the mental stress itself elicits changes in the body that can lead to ulcers. Many scientists agree with Professor Michele Doucet, DVM, Dipl.ACVIM, ACVCP, of the University of Montreal, who states, “Psychological stress itself has not been identified as a significant risk factor for EGUS in adult horses. In theory and in clinical observations, it is thought that stress can cause ulcers of the glandular portion of the stomach and in young foals, but in adult horses, if you look at the data carefully, we do not see a high prevalence of ulcers in the glandular portion – less than 5% in most studies. The type of ulcers seen in adult horses (nonglandular mucosa) have never been associated with stress. Some have looked at this via measurements of cortisol levels, which are the body's indicator of stress, and there is no apparent link.”
One study that has led many to this view was conducted at UC Davis (N.J. Vatistas, et al) in 1999. During this study, serum cortisol levels were measured at intervals in a group of horses undergoing simulated race training. All the horses developed moderate to severe ulcers, but cortisol levels stayed within normal ranges and actually decreased during the trial. Because the horses all developed ulcers in the absence of any significant differences in their cortisol levels, it was concluded that stress was not a factor in the etiology of their ulcers.
However, not all experts agree that this study disproves a link between mental stress and EGUS. According to Alison Moore, DVM, DVSc, Dipl. ACVIM, a specialist in equine internal medicine in Cambridge, ON, “The lack of ‘scientific evidence’ relating stress to ulcers may be due to the method of evaluation. The study by Vatistas only measured cortisol once a week, and cortisol on its own is quite variable. It may also be that these horses adapted quite readily to the levels of ‘stress’ (they were not actually racing) they were subjected to. Clearly in this report, other factors, i.e. feed and exercise management, had more of a part to play.”
There is also another study that calls into question the methodology used by Vatistas to measure serum cortisol levels. A pair of researchers in New Zealand (S.L. Alexander, C.H. Irvine) looked at how social stress in a group of horses affected three different aspects of serum cortisol: corticosteroid-binding globulin (CBG), total, and free (not protein bound). The study found that social stress caused CBG binding capacity to fall, and free cortisol levels to rise, but total cortisol did not change. They concluded that “No effect of stress could be detected when only total cortisol was measured. Therefore, to assess adrenal axis status accurately in horses, it is essential to monitor the binding capacity of CBG and free cortisol concentrations in addition to total cortisol levels.”
A third study, which looked at changes in cortisol during trailering, did find that the stress of trailering caused a significant increase in cortisol – and trailering is known to bring on ulcers in horses. However, since many horses will not eat during trailering, it is impossible to determine whether it is changes in the body due to stress that causes the ulcers, or the detrimental effects of not eating that are responsible. The difficulty of isolating the factor of mental stress is one of the main problems researchers face when trying to design a study to test whether or not psychological stress can cause ulcers in horses. Even if the Davis study had shown elevated cortisol levels, that would not have conclusively demonstrated that psychological stress did cause the horses’ ulcers, since there were other factors at play that are known to cause ulcers.
The bottom line is that there are no studies to date that can definitively prove or disprove the link between psychological stress and EGUS, yet some very well respected researchers believe that a link is feasible. Michael J. Murray, DVM, MS, Dipl. ACVIM with Merial Limited is one of the world’s leading authorities on the formation and treatment of equine ulcers. He says, “Psychological stress brought on by situations such as trailering, showing, stall confinement and changes in their social group may be associated with the development of gastric ulcers that often develop in adult horses in these situations. In many cases, it’s anecdotal, but there are a couple of research studies which lend support to the concept. One that I’m very familiar with – because I performed it – is one in which we took horses out of very nice pasture and put them in stalls in our barn at the equine hospital where I worked. In just a week’s time of simply being taken out of the pasture and put into the stall, with free choice hay, they developed ulcers. Now, the question is, was that due to psychological stress, or some other factor such as a change in feeding behavior, and it’s really quite difficult to say. This is just an observation, but the horse that had the worst ulcers was one that did the most hollering out towards her herd mates in the pasture. She was the queen bee, and seemed agitated that she wasn’t out there bossing them around. It seems like a reasonable conclusion that it was hardest on her to be away from her herd mates and not able to participate in her normal herd dynamics.”
Murray was also involved in another study which lends some support to the idea that psychological stress can lead to EGUS. “In this study,” he says, “we had 20 horses with no ulcers, about 2 years old or so, co-mingled in a pasture, and we took ten of those and transported them about four hours to another site where they were put into stalls. They were fed free choice hay and they had grain twice a day, and they were taken out once or twice a day and just longed. After three days of that, they were trailered back to their original farm. Of those horses that had been taken away, seven out of ten developed ulcers, in just a five day period. But what was interesting was that two out of the ten horses that stayed home also developed ulcers. It was observed by the people on the site that we had disrupted the herd dynamics, and the horses left behind had to reorder their hierarchy. Again, it’s speculation, but it was a situation where none of them had ulcers when they were all together, so we surmise that perhaps psychological stress related to herd dynamics can play a role.”
The psychological stresses of herd dynamics may also be at the root of why a recent study found a surprisingly high incidence of broodmares on pasture. The study, conducted at the University of Davis, looked at 62 broodmares on irrigated grass pasture and found that 66.6% of the pregnant mares and 75.9% of the non-pregnant mares had ulcers. This was completely unexpected, as previous studies had shown that horses in pasture were at a low risk for ulcers. When asked why the incidence was so high, lead researcher Sarah S. le Jeune, DVM, Dipl. ACVS, told HorseCare, “The farm these mares are from is extremely well managed. We do not have a good explanation as to why they had so many ulcers.” Dr. Alison Moore, however, is willing to speculate that the problem with the broodmares could be psychological stress. “I have seen fillies in race training with clean stomachs be turned out on pasture ‘for a break’, and have evaluated them on return to the stable only to find out they went from a 0 grade to a grade 3 (the worst) while on pasture. I can only surmise that the fillies, who were turned out with another filly who was boss, had hierarchical issues with the boss filly. Is this happening with the broodmares at pasture? Maybe, as mares are far more hierarchical than geldings.”
Although Moore points out that there are other considerations, such as grain rations, that could have been factors in the broodmare study, she feels that we should consider herd dynamics and other sources of psychological stress when trying to manage our horses. “My take on this is that psychological stress does contribute to the development of ulcers. It is one of many factors and may be more important for some individuals than for others.” Dr. Murray agrees, but adds, “The difficulty is that we really can’t say in many cases what the precise mechanism is that is causing the ulcers. We just know that in conditions which we can imagine are stressful, based on what we do know about horses, that we see ulcers occur frequently. The question is, what is the link between what may be stressful and the development of an ulcer. In many cases, I suspect that it involves interruption of feeding behaviors, as we know that the horse’s stomach becomes highly acidic within minutes of ceasing eating.”
Nonetheless, even if we are going to assume that psychological stress is a factor in the formation of ulcers, we cannot extrapolate that a horse whose temperament makes it more prone to appear “stressed out” is necessarily at higher risk for ulcers than individuals that seem more laid-back. As Murray explains, “There is no correlation between what we might call high-strung or nervous horses and the incidence of ulcers. “Both the high-strung ones and the apparently calm ones are equally at risk in stressful situations. That queen bee mare I mentioned was actually very laid back in general, and we had one horse that we used for a lot of our studies who was a real easygoing sweetheart, yet he could develop ulcers at the bat of an eye.”
On this point, Dr. Doucet is in complete agreement, stating, “We have observed severe ulceration in horses that were presumably considered to be calm in nature and were not in specific stress situations, and we have seen perfectly healthy stomachs in horses that were considered as very nervous and high strung by their owners or trainers. These are just observations, but we also tried to correlate temperament with the prevalence of ulcers in our studies and never found a significant association. I don't think others have either.” This is one of the reasons why Doucet does not place much, if any emphasis on psychological stress as a factor in EGUS. “I am a scientist,” she says, “and therefore I have to base my comments on valid data. I agree that we don't have the tools to measure all levels of information, but I think it's important that people understand that the association with ‘stress’ has not yet been validated either way, and based on what we know about the causes of ulcers, it is an unlikely risk factor and should be placed well below other ‘proven’ factors in decision making.”Horse owners would certainly be wise to make sure that they address the well-known risk factors when trying to prevent or manage EGUS in their horses, but that does not preclude trying to identify and reduce psychological stresses, as well. At the very least, mental stress is unpleasant and distracting to our horses, and if human data is anything to go by, it may have a whole host of health consequences for our horses. What the ongoing debate on this topic tells us is that we still have much to learn about this complicated health problem, and much to look forward to from the dedicated researchers who continue to work on this puzzle.