Proximal enteritis refers to an inflammation of the duodenum and jejunum, which are the first segments of the horse's small intestine. It is a serious cause of severe abdominal pain, or colic and usually has a very acute onset – the horse is typically described as having been completely normal within the past 24 hours.

The cause of proximal enteritis is unknown. Certainly many clinicians suspect that there is a bacterial cause of proximal enteritis.

Proximal enteritis seems to have different levels of severity in various areas of the country, with the most severe cases being seen in the southeast of the United States. Certainly, each locale has its own resident population of normal bacteria, and it may be that for unknown reasons, these normal bacteria can become pathogenic under certain circumstances.

Although proximal enteritis is seen in all ages, it certainly seems to be more common in older horses.

In the normal horse, the contents of the intestines are continually propelled away from the mouth and toward the anus by peristalsis – a wave of worm-like contractions passing along the entire digestive system. In the normal condition, the small intestine also resorbs large amounts of fluid on a daily basis, both fluid that the horse drinks, and fluid that normally enters the intestines from the rest of the body. With proximal enteritis, whether the cause is bacterial or not, the ensuing inflammation causes damage to the intestinal walls. This causes the intestines to stop their peristaltic action – termed ileus – and to stop resorbing fluid. This in turn causes the horse to become extremely dehydrated, and causes the intestines to become severely distended. The distension causes the horse to experience abdominal pain that can be extremely severe, and thus results in signs of colic.

Because the walls of the intestines have become compromised, bacterial toxins, especially one called endotoxin, can seep into the body. Endotoxin can cause fever, high heart rate and pain.

The most important differential diagnosis for proximal enteritis is any other cause of small intestinal damage that requires surgical intervention. This may include a twisted or entrapped small intestine. Both proximal enteritis and surgical problems cause signs of colic, voluminous backed-up fluid in the stomach and small intestine, and distended small intestine on examination per rectum. However, small intestinal twists or entrapments usually cause the affected portion of the small intestine to die, thus requiring surgery, whereas the damage caused by proximal enteritis can usually be repaired by the body as long as supportive therapy is provided.

The horse with proximal enteritis has a good prognosis; however, this is usually a very expensive disease to treat.