Horse ulcers are non-healing sores. These sores can develop outside the body in skin and inside the body in the lining of the stomach and intestines (colon). Ulcers in horses mostly occur in the stomach (gastric ulcers).
The horse stomach is different from the human stomach. The horse stomach has two sections: an upper section lined with squamous cells that does not have a thick mucous coat, and a lower glandular-secreting section that is lined by mucus. Most gastric ulcers occur in the upper non-glandular section because it is not protected by mucus. Mucus is such a good barrier that the lower glandular section of the stomach where hydrochloric acid is produced is well protected. Some horses also have ulcers in the intestine, especially in the first section of the intestine (duodenum) that accepts food directly from the stomach. A few horses have ulcers throughout the intestines (colonic ulcers).
Ulcers affect horses by causing pain and poor performance. Ulcers can be an irritating illness—or they can be fatal if they erode through the stomach or colon and cause bleeding and peritonitis.
According to Dr. Michael Murray in the Equine Veterinary Journal, about 60% of foals and 60% of performance horses have ulcers. Other equine veterinarians believe the number of foals with ulcers reaches 90%. Performance horses include event and dressage horses, and horses transported to trials or shows. Veterinarians who examined the stomachs of racehorses and weanlings confirmed that over 90% of racehorses and over 90% of stalled weanlings have ulcers. Stallions kept in the vicinity of females, including colts kept in the vicinity of their mothers, develop ulcers. Horses housed at home but undergoing training are also prone to ulcers.
It takes about five days for stressed horses to develop ulcers. This includes horses hauled to shows, horses housed in stalls they are unfamiliar with, and horses fed twice a day rather than allowed to graze throughout the day.
Several factors contribute to the development of ulcers, including stress, high carbohydrate diets, being fed twice a day instead of grazing, and physical activity:
Since horses evolved with a nervous system that is highly responsive to stress, they are exceptionally sensitive to the stress hormone, cortisol. It's the interaction of cortisol with the vagus nerve, which controls digestion, that causes ulcers. Circumstances that contribute to ulcer development are those that increase cortisol: shipping, performing, changing diets, and changing schedules. Thus, horses in training that are shipped to events are prone to ulcers even though they enjoy performing and may appear unstressed.
High carbohydrate diets can predispose horses to ulcers and acid reflux. Carbohydrates release volatile fatty acids that enter stomach cells and encourage ulcer development.
Horses fed twice a day rather than allowed to graze on pasture are prone to ulcers. This is because grazing 8-16 hours a day stimulates constant saliva flow. Saliva has a basic pH and is the opposite of the stomach acidic pH. A constant flow of saliva neutralizes stomach acid. Horses evolved eating almost continuously, so their stomachs evolved to release acid steadily during the day. When horses are fed only twice a day, gastric acids are produced continuously, but there is insufficient saliva to buffer the stomach contents and keep the lining of the stomach ulcer free.
Horses undergoing physical activity, such as racing, may develop ulcers because exercise causes abdominal muscles to contract. Abdominal contractions force bile and acidic contents from the intestine back into the stomach where they irritate the stomach lining and cause ulcers.