The purpose of the following article is to provide an overview of the equine intestinal tract, including its anatomy, how it functions in a healthy state, the most commonly seen intestinal diseases, clinical signs of these illnesses, as well as therapeutic options to address the illnesses. Finally, preventative measures which can be used through proper diet and husbandry practices will conclude this article.
The Anatomy and Physiology of the Equine Intestinal Tract
Horse Stomach
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Horse Small Intestines |
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Horse Small Colon Transverse |
Horse Cecum |
Horse Colon |
Horse Rectum |
As previously discussed in the initial article of the gastrointestinal series, the equine digestive tract is comprised of two sections--the foregut which consists of the stomach and small intestine, and the hindgut, which consists of the components of the large intestine including the cecum, the large colon ( right ventral colon, left ventral colon, left dorsal colon, and right dorsal colon), and the small colon (transverse colon and descending colon).
Horses are herbivores, and unlike ruminants (such as cattle, sheep and goats) which are considered “foregut fermenters”, they are “hindgut fermenters”. Horses must be able to extract nutrients from the hay, grass and feeds that they consume. This extraction takes place through the process of fermentation which occurs within the large intestine and results in the formation of absorbable volatile fatty acids which can subsequently be used for energy. It is the actions of the hindgut which make it possible for horses to generate energy from the consumption of grasses.
While digestive enzymes are secreted in the stomach, only a small amount of digestion actually occurs there and no nutrients are absorbed. The digestive enzymes combine with the stomach contents and help to begin the process of breaking down particles of feed, with the enzyme pepsin beginning protein digestion. This combination of food, liquid and enzymes passes from the stomach through the muscular pyloric sphincter and into the small intestine, which is comprised of three sections, (the duodenum, the jejunum and the ileum) and is approximately 60 to70 feet in length. The duodenum is only about 1 meter in length and it is into this section of the small intestine that the pancreatic and bile ducts empty. The pancreas secretes the digestive enzyme amylase into the small intestine in order to aid in the breakdown of carbohydrates. The small intestine is the main site for the absorption and digestion of sugars, starches, minerals, fatty acids and proteins (which have initially been processed in the stomach). The small intestine is also the location where absorption of the fat soluble vitamins (A, D, E and K), calcium and phosphorous occur. The small intestine transit time ranges from as little as 30-60 minutes up to as long as 8 hours. Anything that causes an
increase or decrease in transit time through the small intestine will ultimately alter the absorption activities as well.
After passing through the jejunum and ileum, the intestinal contents enter the large intestine which normally contains enormous numbers of bacteria and protozoa and acts as a fermentation site. The comma-shaped cecum is the first section of the large intestine which is located on the right side of the abdominal cavity, is roughly 4 feet in length, and holds up to 8 gallons. Following the cecum, the contents pass through the large colon and then finally the small colon. Each section of the colon measures approximately 10-12 feet in length. The transit time through the hind gut can range from less than 1 hour to as many as 3 days depending upon the diet and the overall health of the horse. The normal microbial population (i.e. the good bacterial flora) assists in the fermentation of complex carbohydrates (fiber) into usable products. These microorganisms generate volatile fatty acids as a source of energy as well as produce B vitamins, Vitamin K and some amino acids. The colon then absorbs the resulting nutrients in addition to water which has accompanied the food throughout the digestive tract. Once the water is efficiently absorbed from the large intestine, any remaining waste which is not usable by the horse will be passed into the rectum and defecated via the anus.
