Gastric Ulceration in Adult Horses
Dr. Melissa Mazan
It is important to remember that horses are herbivores, meaning that they are true vegetarians. Consequently, the anatomy and physiology of their gastrointestinal system is much different from ours.
It is also important to remember that horses have very small stomachs when you consider their size - only approximately 4 gallons at holding capacity. This, again, reflects the way that they would eat in the wild – very frequent small meals, so that the stomach is never stretched to full capacity.
To start with, the horse's stomach is made up of two different parts. The primary difference between these two parts is that they have different types of cells lining them. Epithelium is a general term for the covering of any surface of the body, and it consists of many cells tightly joined to each other.
The first type of epithelium encountered in the stomach is stratified squamous epithelium, after which a glandular epithelium is found. A distinct margin, called the margo plicatus, separates the two.
The glandular epithelium, as its name suggests, contains many glands that produce gastric secretions. The squamous epithelium contains no glands, and merely serves to contain food, without aiding in any chemical digestion.
The purpose of the stomach is to help in the long process of digesting food. In order to do this, the stomach must not only mix food, but produce secretions that help to break down food.
Two digestive factors, hydrochloric acid (gastric acid) and pepsin, are produced in the glandular portion of the stomach. Both hydrochloric acid and pepsin begin the digestive process in the stomach before food reaches the small intestine.
The glandular portion of the stomach also secretes factors that help to protect the stomach. One of these, a mucus-bicarbonate layer serves to protect the stomach lining both by preventing acid from physical contact with the stomach surface, and by buffering gastric acid at the level of the stomach lining .
This mucous-bicarbonate layer protects only the glandular portion of the stomach, and not the squamous portion.
Other protective factors include -prostaglandin E, which causes increased blood flow in the stomach lining, increased secretion of the mucus-bicarbonate layer, and also causes decreases in hydrochloric acid production -various growth factors
Unlike humans, horses produce gastric acid continually, regardless of whether they are eating regularly. If horses do not eat, then their stomachs become more and more acidic because acid production cannot be 'turned-off'.
The squamous portion of the stomach is at the greatest risk from increased acid production, because it does not benefit from all the protective factors that the glandular portion of the stomach enjoys.