Equine gastric ulcer syndrome

Equine gastric ulcer syndrome (EGUS) is a common cause of colic and decreased performance in horses. Horses form ulcers in the mucosa of the stomach, leading to pain, decreased appetite, weight loss, and behavioral changes. Treatment generally involves reducing acid production of the stomach and dietary management. Unlike some animals, however, stomach rupture is rare, and the main goal of treating is to reduce pain and improve performance of animals used for showing or racing.

The digestive system of the horse evolved for its grazing lifestyle, where it would almost constantly eat small amounts of roughage throughout the day. Unlike carnivores, who produce stomach acid during meals, horses constantly secrete acid to help digest this source of grass, leading up to 9 gallons produced per day. Unchecked, the stomach acid can lower the pH to levels that will damage the gastric mucosa, leading to ulcers.

The stomach is divided into 2 main sections: a squamous region at the upper 1/3 of the stomach near the cardiac sphincter, and a lower glandular region. These 2 regions are separated by a band of tissue called the margo plicatus. The pH of the stomach contents varies by location. The most dorsal part of the stomach has the highest pH, usually close to 7, dropping to a pH of 3.0–6.0 near the margo plicatus, and reaching as low as 1.5–4.0 in the glandular regions. In foals, pH is uniform since all gastric contents are liquid, and pH rises for a around one hour after milk ingestion.

The esophagus and dorsal stomach is made of stratified squamous epithelium, which is only weakly protected from the effects of hydrochloric acid, and those cells deeper in the layer of tissue transport hydrogen ions intracellularly, leading to death. This region is therefore especially vulnerable, and accounts for 80% of all gastric ulcers. The glandular portion produces hydrochloric acid and enzymes such as pepsinogen, as well as bicarbonate and mucus that helps prevent self-digestion. Mucosal blood flow is also an important factor in glandular epithelium health, since it provides oxygen and nutrients to the cells and helps to remove excess hydrogen ions.

When a horse is on a diet high in roughage, the fibrous mat of chewed roughage provides a physical barrier and helps prevent splashing of acid up onto the squamous region of the stomach. Additionally, the horse's saliva is alkaline, and provides a chemical buffer that is produced during constant chewing and swallowing.

...
Wikipedia