NEWS
Veterinary Topics: A long way from entry to exit
Posted: Saturday, October 23, 2004
Without the ability to vomit, whatever a horse eats must pass through its entire digestive tract
I BECAME a fan of the horse's digestive system early in my career as a veterinarian. In those early days, I did not have a lot of work, and one or two cases of colic were always welcome. As time went on and I became busier, I could have done without colic, but cases still came, unwelcome though they were.
It would have been okay if they occurred at convenient times in convenient places, but that was never the case. A colic would raise its ugly head in the southeast portion of my practice range when I was 20 miles away in the northwest portion, or a colic would appear on a Sunday or a holiday (many times on holidays) when I was trying to relax and enjoy life.
Colic is a fact of life for an equine veterinarian. And most colics are the result of a problem in the horse's digestive system.
The digestive system, also called the gastrointestinal tract, consists of the mouth, esophagus, stomach, small intestine, cecum, and large intestine. The horse is classified as a monogastric herbivore, meaning it has a simple stomach (unlike ruminants, which have four compartments making up their stomachs) and eats vegetation.
The mouth
A horse uses its mouth to gather and chew feed and with a grinding action uses its teeth to reduce the particle size of the feed to make it more easily digested lower in the digestive tract. Feed the horse does not chew will remain undigested in the small intestine and might result in digestive problems.
The salivary glands secrete saliva, which moistens the feed to assist in swallowing, and amylase, a salivary enzyme, begins the digestion of carbohydrates.
The esophagus
From the mouth, feed is swallowed and enters the esophagus, a tube measuring four to five feet long that runs from the mouth through the thoracic area to the stomach.
There is a band of smooth muscle around the esophagus where it enters the stomach. This muscle usually prevents a horse from being able to vomit. In school, we were told that a horse will vomit only when death is near, but I once saw a mare vomit and she did not die. Recently, I read a statement that said, "Vomiting [in a horse] is rare," so I guess it does occur at times other than at impending death.
The stomach
The stomach is a muscular sac lined by glands. It receives ingesta from the esophagus, and physical and chemical digestion occur there. The stomach is roughly J-shaped and has a capacity of eight to 15 liters. The main secretions are pepsin, the enzyme that digests protein, and hydrochlorides, the acids that aid in protein digestion. The stomach also regulates the rate of passage of feed into the small intestine.
The small intestine
The small intestine is 45 to 65 feet long and is divided into three parts: the duodenum, the jejunum, and the ileum. Most digestion occurs in the duodenum, where starch is broken down by glycanase, fat is broken down by lipase, and protein is broken down by protease.
Digestion continues in the jejunum; whereas, the ileum is mainly for absorption.
Types of colic that occur here are spasmodic (hypermotility), volvulus (rotation and twisting) and accompanying strangulation, and, in the ileocecal region, intussusception (telescoping of the intestine upon itself).
Fat and protein are readily absorbed here, but starch may not be in certain cases. Undigested starch that enters the hindgut (the cecum and large intestine) is fermented there by resident bacteria, which causes an accumulation of lactic acid, which can lead to colic or laminitis.
The cecum
The cecum and the large intestine make up the hindgut. The overall capacity of the hindgut is approximately 100 liters and contains about 60% of the capacity of the digestive tract.
The cecum is a large sac analogous to the appendix of humans, and it contains a huge microbial population. These bacteria, called protozoa, and fungi digest dietary fiber by fermentation. The fiber is converted into volatile fatty acids, which, when absorbed, are used as a source of energy.
The cecum is a site of tympany (excessive gas production), which is a common cause of colic.
The large intestine
The large intestine is more than 100 feet long and is composed of the large colon, the small colon, and the rectum. It also has bacterial activity but is most important as the site of water absorption. The rectum, the sacculated posterior end of the small colon, is used for waste storage. The sacculations are responsible for the shape of equine feces (balls).
Obviously, being as long as it is, the large intestine is coiled considerably within the abdominal cavity. The material within it normally passes along without a problem, but there are two bends in which the lumen narrows. These are called the diaphragmatic and the pelvic flexures; if there is not enough moisture in the material, blockage can occur at these points, which causes colic.
Other causes of colic involving the large intestine are tympany (gas distention), impaction, and torsion (twisting) with strangulation.
Material moves through the entire tract by peristalsis (waves of contraction) and when the peristalsis reaches the rectum you have a stall to clean.
Capacities
The relative capacity of the equine stomach is about 8% of the gastrointestinal tract, the small intestine about 30%, the cecum about 15%, and the large intestine about 47%. Of domestic animals, the horse's stomach has the smallest relative capacity and the cecum and the large intestine are by far the largest.
Brent Kelley, D.V.M., is a retired veterinarian living in Paris, Kentucky.
