The esophagus is the muscular tube that carries feed from the rear of the mouth to the stomach. In normal horses, this tube synchronously contracts and relaxes in the swallow reflex to carry a bolus of feed downstream.

Swallowing requires intact and functioning nerves and muscles of the reflex. In megaesophagus (meaning "big esophagus"), there is dysfunction of the esophageal muscle, and the tube loses its ability to contract. The result is a widely opened "bag" that fills with feed and fluid and overflows back into the mouth and nasal passages.

This is a somewhat rare congenital condition, seen more commonly in the Shire and Friesian breeds. The congenital form of the disease is usually seen in young horses. That said, loss of esophageal function resulting in megaesophagus can also result from a variety of neuromuscular diseases or primary damage to the esophagus itself.