The Equine Digestive Tract and How it Relates to Colic
By Erica Larson, News Editor • Sep 07, 2011 • Article #27894
Photo: Anne M. Eberhardt
According to a recent TheHorse.com poll, colic is many horse owners' most concerning equine emergency--and rightly so. This common ailment can be deadly or relatively easy to treat; it can clear up with an analgesic injection or it can require surgery to fix. And to boot, colic isn't picky, affecting horses of all ages, breeds, and sizes.
The good news is veterinarians and researchers have been delving deeper into understanding colic and how it relates to a horse's gastrointestinal (GI) tract. James N. Moore, DVM, PhD, recently presented on advances in understanding equine GI anatomy and the relationship it has with different types of colic at the American Association of Equine Practitioners Focus on Colic Meeting, held July 24-26 in Indianapolis, Ind.
"The term 'colic' simply means abdominal pain, but is used to describe a variety of conditions that cause the horse to exhibit clinical signs that we associate with abdominal pain," Moore began. "In order to make a diagnosis and initiate the appropriate treatment, veterinarians must understand the most clinically relevant aspects of the anatomy of the horse's gastrointestinal tract."
Moore discussed the parts of the GI tract associated with the highest risks of developing colic.
Moore started by reviewing the horse's stomach, noting that the animal "has a relatively small stomach, with a capacity of about eight to 10 liters."
The equine stomach, which is located on the left side of the horse's abdomen under the rib cage, is unique in that the junction between the esophagus and the cardia (the anatomical term for the part of the stomach attached to the esophagus) forms a "one-way valve." This allows matter to pass into the stomach from the esophagus, and then into small intestine (which is the next part of the GI tract after the stomach) but not back into the esophagus. Because horses cannot vomit, gas and fluid can accumulate in the stomach, resulting in severe dilation, unrelenting pain, and even stomach rupture.
Moore explained that since the stomach can't be palpated via a rectal exam and is difficult to identify on radiographs, abdominal ultrasonography is the most effective diagnostic tool used to evaluate the horse's stomach. Treating gastric dilation generally includes using a nasogastric tube to remove the fluid and gas from the stomach and a full diagnostic evaluation to determine the condition's underlying cause.
The Small Intestine
Next, Moore discussed the different types of diseases that can affect the horse's small intestine. Made up of three sections--the duodenum, the jejunum, and the ileum--the small intestine is one of the longest organs in the horse's body.
The duodenum is located on the horse's right side and is attached to the inner wall of the abdomen by a mesentery (a membrane that supplies blood to the intestines). Because the duodenal mesentery is short, it's nearly impossible for the duodenum to become displaced, twisted, or strangulated, Moore noted. The duodenum can, however, become distended with gas or fluid, a contrition that occurs in horses with proximal enteritis (a disease characterized by inflammation of the first half of the small intestine).
The longest portion of the small intestine, the jejunum, is about 70 feet long in most adult horses and can be the most problematic portion of the organ. Unlike the short mesentery that keeps the duodenum from being strangulated, the jejunum is connected to the inner wall of the body by a "long, wide mesentery," Moore explained, which allows the jejunum to potentially become twisted or strangulated, a condition that typically requires to colic surgery to remedy.
Finally, the ileum--which is about 18 inches long in adult horses--is much thicker and more muscular than the rest of the small intestine, a trait that decreases the size of the organ's lumen (or cavity).
Moore described the cecum as a large "fermentation vat" located on the horse's right side. The "vat" can hold about 30 liters of feed and fluid, he explained, adding that the cecum is the "primary site where the ingesta mixes with the microorganisms that are capable of digesting cellulose," which is broken down by microbes that change it into energy-producing volatile fatty acids. He noted that the cecum is attached to the inner wall of the horse's body, so strangulation or twisting isn't the main concern with this large organ. Rather, the cecum is known to become impacted, which means that dry ingesta fills the organ, resulting in abdominal pain. Unlike impactions involving the large colon, this ailment responds extremely well to medical therapy. It is not uncommon for the cecum to rupture spontaneously in horses with cecal impactions.
The Large Colon
After passing through the cecum, the ingesta enters the large colon. It first passes to the right ventral colon and then enters the left ventral colon; both sections mix and retain plant fibers until they're fully digested, Moore explained. After these areas comes the pelvic flexure, where the "left ventral colon passes caudally (moving head to tail) to the left flank area, where its diameter decreases substantially and the colon then folds back on itself," Moore said. Because of this decrease in diameter, the pelvic flexure is the most common location for impactions to occur.
After the pelvic flexure, the colon increases in diameter through the diaphragmatic flexure and is largest in the right dorsal colon. Moore explained that although the terminal part of the right dorsal colon is secured to the inner wall of the body, the majority of the rest of the large colon is not. As a result, these parts of the organ are prone to become displaced or twisted upon themselves.
The final part of the GI tract is the descending colon, which is also called the small colon. This is the portion of the horse's GI tract that is responsible for making fecal balls. Fortunately, the small colon isn't involved in many conditions that result in abdominal pain.
Other Regions of Concern
Finally, Moore discussed a few other areas of concern related to the GI tract:
- The inguinal canal (passage through which the testicles descend) is an area of concern in male horses, as the small intestine can become strangulated after passing through the opening;
- Small intestine incarcerations can occur at the epiploic foramen (a narrow opening connecting the main part of the abdominal cavity with a sac created by the omentum, fatty yellow tissue that surrounds some of the abdominal organs); and
- The renosplenic ligament, a fold of fibrous tissue that connects the capsule surrounding the left kidney to the spleen. It is not uncommon for the large colon to become displaced over this ligament and result in abdominal pain.
Understanding which parts of the equine digestive tract are associated with the highest risks of colic is can lead to a better understanding of the dangerous disease.