Key Points The horse's digestive system is unique among domestic animals. Equine organs of digestion work together in a finely orchestrated fashion and are very sensitive to disruption. Cecal fermentation allows horses to extract nutrients from cellulose, a plant carbohydrate that is not digested by mammalian enzymes. The digestive system of modern-day horses evolved approximately 57 million years ago. 1 Despite standing the test of time, it is highly sensitive to changes in feed regimen, which can prove catastrophic. A good understanding of the complex equine digestive system is essential to both home and hospital care of horses. A thorough appreciation of the horse's unique digestive system will enhance the technician's ability to assist veterinarians and horse owners in caring for their equine charges. Evolutionary History Animals of the Perissodactyl order were the dominant ungulates during the Eocene epoch approximately 57 million years ago. 1 By the time of the Miocene epoch 33 million years later, they had largely died out, to be replaced by the ruminating artiodactyls. 1 There are numerous theories for this change, one of which is that the digestive system of the perissodactyls was too inefficient for long-term survival. 1 However, the representatives of the perissodactyls that survived have been very successful. The horse is the best-known member of this group. Dietary Influence All organisms must obtain energy from nutrients to carry out their daily activities and eventually reproduce, ensuring survival of the species. Nutrients found in plant materials are plentiful and easily available in the environment. The challenge for mammals lies in harvesting and converting plant nutrients to a form that is available to body tissues. Fibrous plant material is one of the most difficult nutrient sources to use because mammalian digestive enzymes are unable to break down cellulose, the most abundant
polysaccharide in plants. 2 Many different digestive strategies have evolved in order to meet this challenge. The equids and their surviving relatives, the rhinoceros and tapir, use cecal fermentation. 1 This process depends on microbes living in the animal's cecum and colon that can digest or break the polysaccharide bonds in cellulose, releasing volatile fatty acids. The animal is then able to absorb and use the volatile fatty acids as an immediate energy source or in the synthesis of glucose and fat. 3 Cecal fermentation therefore gives horses a chance to harvest nutrients left untouched by enzymatic digestion, which occurs in the small intestine. Another substance that poses difficulty to mammalian digestion is lignin, a noncarbohydrate polymer that protects and provides structural strength to the living tissues of plants and encrusts components of herbaceous cell walls. Neither enzymatic nor fermentative digestion can successfully harvest nutrients from lignin directly. 4 The grinding and liquefying that take place in the equine mouth and stomach mechanically release cellulose from the tough, ligneous cell walls encasing it, thereby increasing its availability to microbes of the large intestine. For this reason, adequate preparation of ingested feed not only is essential to the successful passage of feed but also assists digestion. The Digestive Tract Oral Cavity The mouth and teeth of horses are well adapted to the task of collecting and grinding forage. Horses have sensitive, muscular, prehensile lips that assist in the selection and collection of feed and full sets of upper and lower incisors that are able to trim grasses protruding mere millimeters from the ground. Their muscular tongue helps position the feed for grinding by highly specialized molars and premolars. The circular chewing motions of the horse include wide lateral excursions that grind laterally as well as crush vertically, efficiently pulverizing the forage. During chewing, salivary glands release copious amount of saliva, which mixes with the crushed plant fibers, preparing them for the next stage of digestion.
Forages contain highly abrasive silicates that, over time, wear away the grinding surfaces of the teeth. To compensate for this wear, the horse has continuously erupting hypsodont teeth. These teeth have a very long crown and continue to erupt through the gum line at a rate of approximately 2 to 3 mm (1/8 inch) per year, matching the normal rate of wear. 5 In order for a horse's teeth to be worn down in a uniform fashion, the horse must be able to chew both vertically and laterally. Anatomic abnormalities and injuries may lead to abnormal chewing patterns and subsequently alter the grinding surfaces of the teeth. Diets that alter the normal biting and tearing actions of the incisors or the normal lateral excursions of the grinding molars may also contribute to abnormal tooth wear patterns. 6 For example, a horse that always eats hay may not wear down its incisors as well as a horse grazing on pasture, or one fed a diet high in concentrates may not perform enough of the wide lateral excursions necessary for proper wear of the molars. Other dental abnormalities include missing teeth and "wave mouth," which occurs in geriatric horses. Even a slight deformity of the chewing surface may limit the lateral movements of the jaw, and as the horse ages, minor malocclusions may worsen until they become severe enough to limit the animal's ability to chew, highlighting the need for good dental care. 6 Horses rarely have trouble with tooth decay or gum disease, but dental malocclusions may prove fatal if they become severe enough to prevent effective mastication. 5,7 Another problem encountered by geriatric horses is that their teeth do not erupt indefinitely. Horses' teeth are fully formed within the alveoli by the age of about 7 years. 7 The long reserve of crown embedded within the jaw then continues to erupt until
it has been completely consumed, but sometimes it is not enough. The modern domestic horse may live much longer than its wild counterparts, sometimes to 25 to 30 years or more. In some animals, the teeth may completely wear away, making it difficult, if not impossible, for the horse to ingest enough feed to sustain itself. 5 Esophagus The esophagus is an area in which the horse is unique among domestic animals. 2 The musculature of the esophagus is composed of striated voluntary muscle until it reaches the level of the heart; from this point on, the musculature of the digestive tract is composed of smooth or involuntary muscle. For this reason, unlike other domestic species, horses cannot voluntarily reverse peristaltic movement of the stomach and distal esophagus in order to vomit. Therefore, once swallowed, feed must pass through the entire digestive tract. 2 To begin this process, feed must first move down the lengthy esophagus, and successful passage through the esophagus requires the coarse forage to be well chewed and moistened. If the bolus is too dry or too large, it may become lodged in the throat, a condition commonly referred to as choke. Choke often resolves spontaneously but may require the assistance of a veterinarian to clear the blockage. In addition to causing the horse discomfort, choke may lead to the development of scar tissue at the site of blockage, making recurrence more likely. In the most severe cases, the esophagus may tear. Bacterial infection and the poor healing ability of the esophagus may contribute to a very poor prognosis in this situation. 8 Stomach A horse's stomach is designed to suit the needs of a continuous grazer. It is small, roughly 12 L, compared with the horse's digestive tract, which holds roughly 120 L. 3 Horses on pasture spend up to 16 to 17 hours a day grazing in order to allow enough forage to pass through their small stomach. 9
The stomach comprises a nonglandular area followed by a glandular region where digestive acids are produced and released. The two areas are visibly divided by the margo plicatus. 10 The process of liquefaction initiated by the grinding teeth and saliva continues here. As the ingesta is mixed within the confines of the stomach, the smaller particles settle and combine with the gastric acids released within the lower glandular region. This feed is now ready for enzymatic digestion in the duodenum. Two factors play a role in the rate of gastric emptying: peristaltic movements of the lower stomach and changes of the muscle tone in the upper, nonglandular region. 10 (see Table). Difficulties that may be encountered within the stomach include ulceration and, rarely, impaction. Because the digestive strategy of the horse revolves around grazing behavior, that is, eating small amounts frequently, the gastric parietal cells constantly secrete hydrochloric acid. Medical conditions (e.g., anorexia) or the need to withdraw feed (e.g., colic surgery recovery) increases the risk of gastric ulcer formation in the sensitive upper stomach region, an important issue that must be addressed for the equine patient. Working or stabled horses that cannot be fed several meals over the course of the day are also at greater risk of developing gastric ulcers. Several other factors that may contribute to ulcers have been identified, including stress, NSAID therapy, and glandular disorders. 10 If the digesta cannot enter the duodenum, impaction may occur and be worsened by the horse's normal swallowing of copious amounts of saliva and inability to vomit. Impaction of the stomach is rare but is one of many syndromes associated with colic in horses. 10 It has also been associated with stomach rupture. Parasites, particularly blood worms,
small strongyles, tapeworms, roundworms, and bot larvae, may alter the health of the intestine and stomach, also playing a role in the development of colic. 2,11 Small Intestine In the next step of the digestive process, the digesta (now referred to as chyme) enters the duodenum and enzymatic digestion begins. Within the small intestine, large volumes of fluid are added to the chyme, liquefying it further. Bile is released from the bile duct and acts to emulsify fats. The pancreas and liver release enzymes such as carbohydrase, lipase, and peptidase into the duodenum; these enzymes assist in the digestion of carbohydrates, fats, and proteins. The chyme then travels through the jejunum, which is suspended from the dorsal wall of the abdomen by a long mesentery. The small intestine is approximately 22 m (72 ft) in length, and the mobile, loosely suspended jejunum accounts for 20 m (67 ft). 2 The mobility of this portion of the intestine makes it susceptible to torsions, intussusceptions, and hernias. Finally, the chyme passes through the ileum. When the chyme leaves the small intestine, enzymatic digestion and absorption of most fats and variable amounts of carbohydrates and proteins are complete. 2 However, only nonstructural carbohydrates are digested by
enzymes and absorbed from the small intestine. 12 Transit time through the small intestine can be rapid, as little as 3 hours. Cecum The chyme now enters the large intestine and cecum, an area often referred to as the hindgut. The cecum, largely unused in many species, is highly developed in horses and acts as a fermentation vat. Located on the right side of the abdomen in a fixed position, it is joined to the proximal end of the right ventral colon, forming a large, blind pocket. 2 Chyme enters and exits through a common opening, the ileocecal valve, while muscular contractions of the cecum walls aid mixing and regulation of cecal volume. The function and contents of the cecum are regulated by a complex system of checks and balances; if this system is suddenly altered, the cecum may become dilated or impacted or may even rupture. It is another area in which difficulties may lead to colic. Large Colon After the chyme leaves the cecum, it enters the large colon, where fermentation continues and more nutrients are absorbed. The chyme then passes through the transverse colon; it is here that large quantities of water are reabsorbed. The small intestine excretes a large volume of water, approximately 114 L/day, which must be reabsorbed so that it can be recycled. The diameter of the large colon rapidly decreases as it reaches the small colon, where the soupy chyme is compacted into relatively dry fecal balls. 9 The large colon changes directions as well as diameter several times. Impaction may occur where the diameter is reduced, again causing colic. The narrowed areas that are most commonly associated with impaction are the ileal orifice, the pelvic flexure, and the beginning of the transverse colon. 13
Small Colon The strong, muscular walls of the small colon compact the waste products of digestion and absorb most of the remaining fluid. The resulting relatively dry fecal balls enter the rectum and are expelled through the anus. Feeding Considerations Water Water is essential for life, and horses are no exception. A horse consumes at least 20 to 30 L of water every 24 hours. 14 Most of this water is lost as sweat. For example, moving at a fast trot, a horse may lose up to 12.5 L of water per hour in an attempt to dissipate heat generated by working muscles. 15 Digestion also demands a large commitment of fluid resources. Successful digestion depends on a highly fluid environment to aid passage of coarse, fibrous plant material. 2 For example, impaction may occur in a number of locations within the lengthy intestinal tract if the fluidity of digesta is not maintained. To maintain fluidity, 95% of the horse's extracellular fluid volume passes through the digestive tract over the course of 24 hours. This means that in an average 1,000-lb (450-kg) horse, about 125 L of water passes through the digestive tract each day. Most of this water is reabsorbed within the large colon. Approximately 85 L is absorbed from the cecum and an additional 22 L within the small colon, with the remainder being lost in the feces. 2 Starch A problem associated with fermentation, starch bypass occurs when soluble starch is fed in a quantity that cannot be fully digested and absorbed within the small intestine. Patient history often includes such factors as excessive grain intake or sudden access to lush, fresh pasture. The excess starch passes into the large intestine and is subjected to microbial fermentation, an end product of which is lactic acid, which acidifies the contents of the cecum and colon. 9 This acidic environment kills the resident microbes, releasing endotoxins that may then be absorbed through the gastrointestinal
epithelium and enter the general circulation. The resulting complex syndrome often proves catastrophic to horses. Diarrhea, colic, and laminitis often develop as a result of starch bypass. 9,16 Fiber As in humans, fiber plays an important role in maintaining digestive health in horses. To maintain good digestive health, horses must ingest at least 1.5% of their body weight in
fiber per day. 3,16 The quality or type of the fiber is also important. The quantity of soluble fiber (starch) provided in an equine feeding program should not exceed the amount that can be digested and absorbed within the small intestine, limiting the activity of the fermentative microbes to the digestion of insoluble fiber. 4 Finally, some fibrous materials that cannot be digested (e.g., lignin) play an important role in supplying bulk, thereby assisting the passage of digesta. Plant fiber also plays an important behavioral role. Horses evolved as continuous grazers; in other words, they are programmed to spend 16 to 17 hours a day eating. Researchers estimate that horses normally chew an average of 3,600 times per hour. Over 16 hours, this equals 57,600 chews, which keep the horse very busy with the business of eating. Horses that are denied this "chew time" are at increased risk of developing behavior abnormalities such as cribbing and wind sucking. 4 Supplying the stabled horse with sufficient feed to satisfy its instinctive need to graze can be difficult, but it is essential to the animal's digestive and psychologic health. Conclusion Regardless of whether the horse's digestive system is a truly inefficient, archaic accident looking for an opportunity to happen or a system ingeniously adapted to the needs of a nomadic grazing lifestyle, it is worth understanding for its effect on equine health. Knowing the requirements for maintaining efficient equine digestion enables veterinary technicians to assist not only in developing adequate diets for different equine life stages but also in treating horses with choke, colic, or other digestive problems. Acknowledgment The author would like to thank Dr. C. Belan, DVM, for his assistance and guidance. REFERENCE: 1. JANIS C: THE EVOLUTIONARY STRATEGY OF THE EQUIDAE AND THE ORIGINS OF RUMEN AND CECAL DIGESTION, IN EQUINE FUNCTIONAL ANATOMY. GUELPH, ONTARIO, CANADA, EQUINE RESEARCH CENTRE, 2002, PP 73-90.
2. LIVESEY M: THE STRUCTURE, FUNCTION, AND DYSFUNCTION OF THE EQUINE DIGESTIVE SYSTEM, IN NUTRITION. GUELPH, ON, CANADA, EQUINE RESEARCH CENTRE, 1999, PP 1-4. 3. LAWRENCE LM: DIGESTION IN THE HORSE, IN DISTANCE LEARNING FROM THE EQUINE RESEARCH CENTRE MODULE II, NUTRITION [VIDEOTAPE]. GUELPH, ON, CANADA, EQUINE RESEARCH CENTRE, 1996. 4. CUDDLEFORD D: FIBRE, IN NUTRITION. GUELPH, ON, CANADA, EQUINE RESEARCH CENTRE, 1999, PP 89-92. 5. OGLESBY RN: DENTISTRY AND FLOATING THE TEETH OF HORSES. ACCESSED OCTOBER 2006 AT WWW.HORSEADVICE.COM/SBS/ARTICLES/ HORSECARE/DENTISTRY.HTML. 6. SELLNOW L: EQUINE DENTISTRY UPDATE [ARTICLE #543]. THE HORSE 1998. ACCESSED OCTOBER 2006 AT WWW.THEHORSE.COM. 7. HIMENS MG JR: UNDERSTANDING YOUR HORSE'S TEETH. ACCESSED OCTOBER 2006 AT WWW.MYHORSEMATTERS.COM. 8. RAKESTRAW PC: PATHOLOGY OF THE SALIVARY GLANDS AND ESOPHAGUS. ACCESSED FEBRUARY 2006 ATWWW.IVIS.ORG/PROCEEDINGS/GENEVA/2003/RAKESTRAW1/CHAPTER_FRM.ASP?LA=1. 9. GEOR R: FROM START TO FINISH [ARTICLE #917]. THE HORSE 2001. ACCESSED OCTOBER 2006 AT WWW.THEHORSE.COM. 10. EVERS SL, CHURCH SL: AAEP WRAP-UP: THE EQUINE STOMACH [ARTICLE #5068]. ACCESSED OCTOBER 2006 AT WWW.THEHORSE.COM. 11. RESEARCH CENTRE: COLIC UPDATE, IN NUTRITION. GUELPH, ON, CANADA, EQUINE RESEARCH CENTRE, 1999, PP 57-63. 12. PAGAN J: SUGAR: AN ESSENTIAL INGREDIENT IN A HORSE'S DIET, IN NUTRITION. GUELPH, ON, CANADA, EQUINE RESEARCH CENTRE, 1999, PP 69-78. 13. KAINER RA, MCCRACKEN TO: HORSE ANATOMY: A COLORING ATLAS, ED 2. LOVELAND, CO, ALPINE PUBLICATIONS, 1998. 14. BALL M: LIFE'S CURIOUS BREW: FLUIDS AND ELECTROLYTES [ARTICLE #656]. ACCESSED OCTOBER 2006 AT WWW.THEHORSE.COM. 15. RIDGEWAY KJ: THERMOREGULATION AND ELECTROLYTE MANAGEMENT IN THE ENDURANCE HORSE, IN FOCUS ON ENDURANCE I & II. GUELPH, ON, CANADA, EQUINE RESEARCH CENTRE, 1995, PP 5-13. 16. NATIONAL RESEARCH COUNCIL: NUTRIENT REQUIREMENTS OF HORSES, ED 5. WASHINGTON, NATIONAL ACADEMY PRESS, 1989