Animals are usually classified into two groups on the basis of their digestive physiology; ruminants and monogastric or non-ruminants. This is really an over-simplification, since some non-ruminants, in addition to their simple stomachs, have an important symbiotic relationship with a microbial population in their hind-gut.

Animals such as the horse and the rabbit are able to take advantage of the nutrients from many of the same feeds as ruminants.

The major difference between these animals and ruminants is that their fermentation takes place at the end of the gastrointestinal tract while in the ruminant it occurs at the beginning.

These non-ruminant herbivores also have requirements for dietary essential amino acids and vitamins, since although they are synthesized by the microbes, the animal's ability to absorb them from the large intestine is severely limited.

Canines and felines have simple stomachs, short intestinal tracts and undeveloped caeca. All ingested food passes quickly through the digestive system with little opportunity for microbial fermentation. Thus, these animals are carnivores, feeding almost entirely on animal tissue although some nutrients can also be obtained from high quality plant material like hulled grains or tubers that contain starch rather than cellulose.

Omnivores, like humans or pigs, also possess simple stomachs but their digestive systems are capable of extracting nutrients from readily digested plant tissue if the plant cell wall is thin or the fibrous coat surrounding grain seeds or nuts is disrupted or removed.

Omnivores vary considerably in ability to obtain nutrients from plant cell walls. In pigs, the sacculated colon and caecum make up about half of the total digestive capacity, providing substantial volume for microbial fermentation.

This allows extensive break-down of complex plant tissues and liberation of sufficient nutrients from reasonable quality forages to satisfy maintenance requirements of mature pigs.

Some herbivores like cows, sheep, goats, buffaloes and deer, posses a complex fore-stomach rumen, reticulum and omasum populated by micro-organisms. This is a symbiotic relationship since the bacteria and protozoa inhabiting the rumen posses the enzymes necessary to ferment and break down cellulose, liberating volatile fatty acids fore absorption directly through the rumen wall and used as an energy source by the host.

The micro-organisms also synthesize nutrients into their own tissues and, as their population increases, large numbers are flushed into the true stomach abomasum and intestines for digestion by the animal.

Alternately, complex or protected proteins, peptides and even some carbohydrates may by-pass the rumen and escape fermentation.

These pass directly into the abomasum and intestines to release nutrients through conventional mammalian digestion-absorption processes. Species with this modification are ruminants and can obtain all essential nutrients from plant cellulose.

Non-ruminant herbivores such as equines, cameloids and rabbits can also derive substantial nutrients from plant materials. These animals have well developed caeca and large intestines that contain numerous symbiotic micro-organisms possessing the ability to ferment cellulose.

Ruminant Digestive System Ruminants, unlike most other mammals, do not have upper incisor teeth, but possess a tough dental pad on which the bottom incisors can put pressure. In cattle, the tongue is used to pull long feed into the mouth, while the lips are used when the animal is grazing or eating smaller feeds such as grains.

Usually enough chewing occurs to mix the feed with saliva and form a bolus which may be swallowed. Domesticated ruminants produce large amounts of alkaline saliva, which contains a little amylase an enzyme required for starch digestion. The saliva aids the chewing and swallowing of dry feeds, allows for breakdown of short chain triglycerides by salivary lipase, and buffers acids produced during rumen fermentation.

The volume of saliva produced is affected by the form of the feed being consumed and the water intake of the animal. The rapid movement of the bolus through the esophagus is probably due to the presence of striated muscle in the ruminant esophagus.

Three sphincters are also present but they may be more important during regurgitation than in swallowing. The bolus is deposited into the anterior portion of the rumen or the area of the reticulo-rumen fold.

The ruminant stomach is divided into four compartments, the reticulum, rumen, omasum and abomasum. The reticulum and rumen are joined by a fold of tissue; the reticulo-rumen fold, with the result that digesta may move freely between the two compartments.

The ruminant stomach contains a large microbial population that exists in a symbiotic or mutually beneficial relationship with the animal. The walls of the rumen are lined with epithelial cells which terminate in papillae small, finger-like projections.

These increase the surface area for absorption and for microbial contact with the ingesta. The rumen microflora ferment cellulose, liberating volatile fatty acids. These volatile fatty acids mainly propionic, butyric and acetic acid are absorbed directly through the wall and serve as the main energy source of ruminants.

Rumination, or cud chewing the ability to do so being one of the characteristics of ruminants , is the regurgitation, chewing to reduce particle size and swallowing of rumen contents. More time is spent chewing and more saliva is secreted during rumination than when the animal is eating.

Animals ruminate much more during the night than they do during the daylight. Contraction of the muscular pillars in the rumen and the reticulum produce rhythmic mixing and moving of the ingesta throughout the rumen and reticulum.

In young ruminants the reticular or esophageal groove is a continuation of the esophagus which forms a tube to bypass the reticulo rumen. This allows milk to pass directly into the omasum and then to the abomasum where it may be easily digested. The next compartment is the omasum whose interior is partially filled by a number of longitudinal folds or leaves.

The spaces between the leaves are packed tightly with finely ground ingesta, giving the whole compartment a round, hard appearance. The function of the omasum has not been clearly defined, but it is thought that it may act as a filter through which water and fine particles may enter the abomasum, but coarse particles may not.

It also absorbs water and some ions from the digesta. The last stomach compartment is the abomasum, which is very similar in structure and function to the glandular stomach in non-ruminants. Acid and some digestive enzymes are added to the stomach contents as they are moved through by contractions of the muscular wall.

The acidic ingesta passes into the duodenum where it is mixed with bile and pancreatic secretions. There is little digestion of soluble carbohydrates and lipids since most have been destroyed in the rumen.

The small intestine is very efficient in absorbing amino acids. The large intestine and caecum may be of lesser importance in the ruminant animal, since many of the nutrients were released from dietary plant fibers in the rumen. Absorption of water, minerals, some nitrogen, volatile fatty acid and carbohydrate occurs in this region.

The terminal end of the gastrointestinal tract is the rectum. Its function is involved with defecation or elimination of the materials remaining after passage through the digestive system.

Non-ruminant Herbivore Digestive Tract The digestive system of the non-ruminant herbivore such as the horse, rabbit and guinea pig combines features of both the ruminant and monogastric systems.

The proximal part stomach, small intestine of the gastrointestinal tract is very similar to that of the monogastric. The digestion of the dietary fractions which would also be available to the monogastric ie..

the non-structural carbohydrates such as starch, proteins, minerals and vitamins takes place in the stomach and small intestine. The more fibrous part of the feed, which in some situations is the majority of the diet, is passed through the small intestine to the hindgut caecum and colon where it undergoes digestive processes similar to those occurring in the rumen.

The bacteria which inhabit the caecum and colon of the horse are, in most cases, identical to those found in the rumen of cattle. The end products of bacterial fermentation are also very similar to those of the ruminant animal. Since the major site of bacterial fermentation in the hind-gut fermenters is distal to the small intestine the non-ruminant herbivore may not be as efficient in using fibrous foodstuffs as is the ruminant.

Much of the bacterial protein and cell wall constituents which would be available to the ruminant are lost to the horse.

There is absorption from the hind-gut of by-products of bacterial fermentation such as volatile fatty acids, free amino acids, B-vitamins and minerals released from plant sources, but losses of these nutrients in the feces is greater in the horse than in the cow.

The rate of passage of ingesta through the gastrointestinal tract of the horse is, however, more rapid than in the ruminant.

This allows the horse to meet a great part of its requirements from fibrous sources by ingesting large quantities, digesting what is available and eliminating the remainder from the tract relatively quickly.

For horses with high nutrient requirements ie.. pregnant, high performance and working horses a diet with a high proportion of cereal grains and supplements is necessary to meet their nutrient requirements.

Monogastric Digestive Systems In animals like pigs, the lips, cheeks, palate and tongue are all involved with prehension and movement of feed in the mouth. Small feeds are usually ingested with the tongue, while the teeth shear off any large or long pieces.

Pigs can also suck up slurry feeds and water. Saliva is produced by three primary glands, and its volume and consistency can change with the nature of the food. Once food has been chewed and mixed with saliva to a proper consistency it is swallowed and passed down the esophagus to the stomach.

The motility of the stomach is necessary to mix the digesta with the gastric juices and to move the digesta into the small intestine. The small intestine, comprised of the duodenum, jejunum and the ileum, is the site where the majority of digestion takes place and most, if not all, nutrient absorption occurs.

The duodenum is the site for the mixing of digesta with intestinal, liver and pancreatic secretions. These secretions serve to buffer the contents as they leave the stomach, and to lubricate the bolus for ease of movement through the intestines. The caecum and colon hind gut retrieve any nutrients, primarily water and electrolytes, remaining in the digesta as it leaves the small intestine.

The caecum is a blind sac arising at the junction of the ileum and colon. Anaerobic fermentation of fiber in the caecum and colon produces some utilizable energy in the form of volatile fatty acids. The amount of energy produced is small in relation to the pig's total requirement but hind gut fermentation liberates substantial nutrients in horses and rabbits.

Go to Digestive Physiology of Herbivores , a site provided by Colorado State University, for more information. Avian Digestive Tract The avian oesophagus has an outpocket, the crop, which occurs about two-thirds of the way down its length, right before it enters the thorax.

There are large numbers of mucous glands above the crop which serve to augment the small amount of saliva produced. The crop functions mainly to store feed. From the oesophagus, the bolus moves into the proventriculus, where HCl and pepsinogen are produced at higher levels per unit body weight than most mammals.

The gizzard, a very hard, muscular organ is the site for grinding of the ingesta and also for much of the gastric digestion.

Movement of ingesta is related to a coordinated effort of the crop, lower oesophagus and gizzard, with little muscular contraction of the proventriculus involved. The level of fibre in broiler rations has an effect on the size of the proventriculus and gizzard. The more fibre, the slower the rate of passage through the proventriculus.

Thus the amount of gastric juices produced is less, the more the gizzard must grind the feed, so the larger the gizzard. The gizzard further degrades the ingesta to prepare for digestion in the small intestine. The ingesta is mixed in the gizzard and when the particles are fine enough they are pushed out into the small intestine.

Intestinal contents are continually refluxed back into the gizzard which differs from most animals since the pylorus effectively blocks back-flow of fluid from the small intestine into the stomach. Grit was thought to be necessary for the digestion of coarse feedstuffs, but there seems to be no advantage to adding grit to a mash diet.

The small intestine, about cm long in a mature chicken, is made up of the duodenum, jejunum and the ileum. The duodenum ends where the bile and pancreatic ducts empty into the intestine. The jejunum and the ileum cannot be readily differentiated. The large intestine absorbs some of the nutrients, usually water and electrolytes, remaining in the digesta as it leaves the small intestine.

The colon in avian species is very short and the ceca are much longer and are paired. Only fluids and very fine particles enter the ceca and once they are evacuated from the ceca are rapidly excreted. Volatile fatty acids are by-products of anaerobic fermentation of fibre in the ceca. Ducks and geese have longer digestive tracts than do chickens.

Thus, these species can be productive on diets that are higher in fiber. The energy derived from volatile fatty acid production is small compared to the chickens' total requirement but can be substantial in ducks and geese.

The colon acts more as a passage of ileal and cecal digesta than as a fermenter or absorber. The cloaca is the terminal end of the gastrointestinal urinary and reproductive systems. Voiding moves feces and uric acid out of the cloaca together. Check the Colorado State University site for additional information on avian digestion.

Other Major Organs of Digestion. The pancreas and liver are two other organs intimately associated with digestion. In addition to producing several hormones, the pancreas secretes a number of digestive enzymes lipase, amylase, trypsinogen, chymotrypsinogon that are essential for breaking down complex feed components and liberating the nutrients in forms that can be absorbed.

The liver adds bile to the ingesta as it passes through the small intestines. The bile emulsifies fats so they become soluble and detoxifies potentially harmful materials that gain entrance to or are formed within the body. Digestion and liberation of nutrients from animal tissues occurs quickly and efficiently during passage through the stomach and intestines.

In contrast, digestion of complex plant tissue requires retention of ingested feeds for a considerable period under conditions suitable for microbial fermentation.

The gastrointestinal anatomy and related ability to promote fermentation allow classification of animals into general groups as shown in the following table.

The breakdown of lignocellulose in the rumen and other parts of the digestive system is limited by the crystallinity of cellulose fibres and the barrier to microbial attack created by the lignin and other related components of the mature plant wall.

Various fungi with high lignin degrading activity are currently under study to increase understanding of the physical and enzymatic mechanisms employed in these processes. On the basis of this knowledge, solid-substrate fermentation technologies have been developed and significant improvement of in vitro digestibility can now be achieved through use of fungi carefully selected for activity on particular substrate.

A specific gene coding for lignin peroxidase has been isolated, cloned, incorporated into and expressed by bacteria, so genetic engineering to construct strains with increased in vitro lignin-degrading potential, and subsequent introduction of these into the rumen, may be feasible in the future.

Crude plant by-products are abundant throughout the world, with annual production of cereal grain straw, corn stover and sugar cane tops and bagasse each accounting for many millions of tonnes. However, since these all contain high proportions of lignin protected hemicellulose, and cellulose, digestibility is low even in ruminants.

Considerable research is being conducted on developing techniques that improve digestibility of low quality roughage grinding, chopping, pressure, explosion, irradiation, chemical treatments, etc.

and successful methods to increase the digestibility would contribute dramatically to better livestock nutrition. Animals must receive sufficient amounts of all essential nutrients water, energy, amino acids, vitamins, minerals to remain healthy, to grow and to produce.

Ration formulation involves combining the various ingredients so that an animal's nutritional requirements are met. Producers must always be aware that the nutritional requirements vary with species, age, production and even the season of the year. One of the major challenges facing the livestock production industry today is providing adequate amounts of a balanced ration at a reasonable cost.

Livestock feeds can be classified as roughages, cereals and supplements. Examine the individual components and mixed feeds to determine ingredients and which animal species might utilize these.

Roughages The roughage or forage utilized in Canada comes from native rangelands and pastures, improved pastures, crops harvested and stored specifically for feeding to animals or residues of crops grown for other purposes.

Roughages are usually feeds that have a low bulk density, but because of the variety of crops which are considered to be roughages, there is a great deal of variation in composition.

Most feeds in this group have a high crude fibre content and a low digestible energy content. Corn silage is one exception since, although it has high levels of crude fibre, it also has high levels of digestible energy.

The protein, mineral and vitamin contents can vary substantially between roughages and also between crops of one roughage. Forages form the major part of most ruminant rations, since they are less expensive than other feeds and the ruminant animal is able to digest most plant components. However, some ruminants are fed very little roughage.

Cattle and lambs being readied for market are often fed a ration based almost entirely on cereal grains. Secretions from blood to gut and absorptions from gut to blood occur simultaneously. In clinically healthy animals, absorption exceeds secretion resulting in a net absorption.

If the amount exuded exceeds the absorptive capacity of the intestines, diarrhea results. The size of the material that leaks through the mucosa varies, depending on the magnitude of the increase in pore size. Large increases in pore size permit exudation of plasma protein, resulting in protein-losing enteropathies eg, lymphangiectasia in dogs, paratuberculosis in cattle, nematode infections.

Greater increases in pore size result in the loss of RBCs, producing hemorrhagic diarrhea eg, acute hemorrhagic diarrhea syndrome Acute Hemorrhagic Diarrhea Syndrome in Dogs Acute hemorrhagic diarrhea syndrome in dogs is characterized by both acute vomiting and diarrhea.

Prompt IV fluid therapy is the main read more , parvovirus infection Canine Parvovirus Canine parvovirus is a highly contagious virus that commonly causes GI disease in young, unvaccinated dogs. Presenting signs include anorexia, lethargy, vomiting, and diarrhea, which is often read more , severe hookworm infection Hookworms in Small Animals Hookworms Ancylostoma spp, Uncinaria stenocephala are common infections of dogs and cats, particularly puppies and kittens.

Some species are zoonotic. Adult parasites reside read more. Hypersecretion is a net intestinal loss of fluid and electrolytes that is independent of changes in permeability, absorptive capacity, or exogenously generated osmotic gradients.

Enterotoxic colibacillosis is an example of diarrheal disease due to intestinal hypersecretion; enterotoxigenic Escherichia coli produce enterotoxin that stimulates the crypt epithelium to secrete fluid beyond the absorptive capacity of the intestines. The villi, along with their digestive and absorptive capabilities, remain intact.

The fluid secreted is isotonic, alkaline, and free of exudates. The intact villi are beneficial because a fluid administered PO that contains glucose, amino acids, and sodium is absorbed, even with hypersecretion. Osmotic diarrhea is seen when inadequate digestion or absorption results in a collection of solutes in the gut lumen, which cause water to be retained by their osmotic activity.

It develops in any condition that results in nutrient malabsorption or maldigestion eg, exocrine pancreatic insufficiency Exocrine Pancreatic Insufficiency in Dogs and Cats Exocrine pancreatic insufficiency is caused by decreased production of digestive enzymes by the pancreas.

The most common clinical signs are polyphagia, weight loss, and a large volume of loose Malabsorption Diseases of the Stomach and Intestines in Small Animals See also Malassimilation Syndromes in Large Animals Malassimilation Syndromes in Large Animals is failure of absorption due to decreased absorptive capacity, enterocyte damage, or mucosal infiltration.

Several epitheliotropic viruses directly infect and destroy the villous absorptive epithelial cells or their precursors eg, coronavirus, transmissible gastroenteritis virus of piglets Porcine Coronaviral Enteritis Coronaviral enteritis affects pigs of all ages and typically manifests as an acute watery diarrhea.

Multiple coronaviruses cause enteric disease in pigs, and clinical differentiation is difficult read more , and rotavirus of calves. Feline panleukopenia virus Feline Panleukopenia Feline panleukopenia is a parvoviral infectious disease of kittens typically characterized by depression, anorexia, high fever, vomiting, diarrhea, and consequent severe dehydration.

Adult cats read more and canine parvovirus Canine Parvovirus Canine parvovirus is a highly contagious virus that commonly causes GI disease in young, unvaccinated dogs. read more destroy the crypt epithelium, which results in failure of renewal of villous absorptive cells and collapse of the villi; regeneration is a longer process after parvoviral infection than after viral infections of villous tip epithelium eg, coronavirus, rotavirus.

Intestinal malabsorption also may be caused by any defect that impairs absorptive capacity, such as diffuse inflammatory disorders eg, inflammatory bowel disease, histoplasmosis or neoplasia eg, lymphosarcoma.

The ability of the GI tract to digest food depends on its motor and secretory functions and, in herbivores, on the activity of the microflora of the forestomachs of ruminants, or of the cecum and colon of horses and pigs.

The flora of ruminants can digest cellulose; ferment carbohydrates to volatile fatty acids; and convert nitrogenous substances to ammonia, amino acids, and protein. In certain circumstances, the activity of the flora can be suppressed to the point that digestion becomes abnormal or ceases.

Incorrect diet, prolonged starvation or inappetence, and hyperacidity as occurs in engorgement on grain all impair microbial digestion. The bacteria, yeasts, and protozoa also may be adversely affected by the oral administration of drugs that are antimicrobial or that drastically alter the pH of rumen contents.

The location and nature of the lesions that cause malfunction often can be determined by recognition and analysis of the clinical findings. In addition, abnormalities of prehension, mastication, and swallowing usually are associated with diseases of the oral mucosa, teeth, mandible or other bony structures of the head, pharynx, or esophagus.

Vomiting is most common in single-stomached animals and usually is due to gastroenteritis or nonalimentary disease eg, liver disease, kidney disease, pyometra, endocrine disease.

The vomitus in a dog or cat with a bleeding lesion eg, gastric ulcer or neoplasm may contain frank blood or have the appearance of coffee grounds.

Horses and rabbits do not vomit. Regurgitation may signify disease of the oropharynx or esophagus and is not accompanied by the premonitory signs seen with vomiting.

Large-volume, watery diarrhea usually is associated with hypersecretion eg, in enterotoxigenic colibacillosis in newborn calves or with malabsorptive osmotic effects.

Blood and fibrinous casts in the feces indicate a hemorrhagic, fibrinonecrotic enteritis of the small or large intestine, eg, bovine viral diarrhea, coccidiosis Overview of Coccidiosis in Animals Coccidia are single-celled obligate intracellular protozoan parasites in the class Conoidasida within the phylum Apicomplexa.

The main clinical sign of coccidiosis is diarrhea. Oocysts can be read more , salmonellosis Salmonellosis in Animals Salmonellosis is infection with Salmonella spp bacteria. It affects most animal species as well as humans and is a major public health concern. The clinical presentation can range from read more , or swine dysentery Swine Dysentery Swine dysentery is a mucohemorrhagic diarrheal disease of pigs that is limited to the large intestine.

Swine dysentery is most often observed in growing-finishing pigs and is associated with Black, tarry feces melena indicate hemorrhage in the stomach or upper part of the small intestine. Tenesmus of GI origin usually is associated with inflammatory disease of the rectum and anus.

Small amounts of soft feces may indicate a partial obstruction of the intestines. Abdominal distention can result from accumulation of gas, fluid, or ingesta, usually due to hypomotility functional obstruction, adynamic paralytic ileus or to a physical obstruction eg, foreign body or intussusception.

Distention may, of course, result from something as direct as overeating. A sudden onset of severe abdominal distention in an adult ruminant usually is due to ruminal tympany. Ballottement and succussion may reveal fluid-splashing sounds when the rumen or bowel is filled with fluid.

Varying degrees of dehydration and acid-base and electrolyte imbalance, which may lead to shock, are seen when large quantities of fluid are lost eg, in diarrhea or sequestered eg, in gastric or abomasal volvulus.

Abdominal pain is due to stretching or inflammation of the serosal surfaces of abdominal viscera or the peritoneum; it may be acute or subacute, and its manifestation varies among species. Throughout the years, it has become a broad term for a variety read more is common.

Subacute pain is more common in cattle and is characterized by reluctance to move and by grunting with each respiration or deep palpation of the abdomen. Abdominal pain in dogs and cats may be acute or subacute and is characterized by whining, meowing, and abnormal postures eg, outstretched forelimbs, the sternum on the floor, and the hindlimbs raised.

Abdominal pain may be difficult to localize to a particular viscus or organ within the abdomen. A complete, accurate history and routine clinical examination can often determine the diagnosis. In outbreaks of GI tract disease in farm animals, the history and epidemiologic findings are of prime importance.

In small animals, travel history or other details such as recent adoption from a shelter or recent kenneling or exposure to other animals in dog parks might give clinical suspicion to certain infectious diseases. If the history and epidemiologic and clinical findings are consistent with GI disease, the lesion should be localized within the system, and the type of lesion and its cause determined.

The abnormality may sometimes be localized to the large or small intestine by history, physical examination, and fecal characteristics see Table: Differentiation of Small-Intestinal from Large-Intestinal Diarrhea Differentiation of Small-Intestinal from Large-Intestinal Diarrhea. The distinction is important because it narrows the differential diagnoses and determines the direction of further investigation.