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<big><center>[[Alimentary - includes liver and pancreas|'''BACK TO ALIMENTARY''']]</center></big>
<big><center>[[Alimentary System - Pathology|'''BACK TO ALIMENTARY''']]</center></big>
==Introduction to the Intestines==
=== Basic Anatomy and Physiology ===
The intestines are a portion of the alimentary tract extending from the pylorus to the anus. In mammals, the intestines are divided into two main segments: the small intestine and the large intestine. The small intestine, consisting of the duodenum, jejunum and ileum, acts as the main site of absorption for digestive products in the gastrointestinal tract. The large intestine, consisting of the caecum and colon, functions to extract water, electrolytes and nutrients as well as to concentrate and propulse faecal material to the rectum for defaecation.
Functional differences between the small and large intestine are reflected in their anatomic and histologic organisation (Figure 1). The small intestinal mucosa in many species is highly folded and made up of numerous villi that greatly increase the absorptive surface area. Enterocytes, the main absorptive cells, are the most numerous and characteristic cells of the small intestinal epithelium. Others include goblet cells, paneth cells, neuroendocrine cells, stem cells, and intraepithelial lymphocytes. Another important feature of the small intestine is the presence of Peyer's patches within the lamina propria. They contribute to immune function and generation of immune responses within the mucosa.
In contrast to the small intestine, the large intestinal mucosa has no villi and consists of 2 main cell types: absorptive cells and mucus secreting goblet cells. A distinguishing histologic and anatomic characteristic is the thick muscularis mucosa layer that aids in propulsion of faeces to the rectum. Another important feature is the presence of commensal bacteria which plays different roles depending upon the type of animal (Dyce et. al, 2002; Young & Heath, 2000).
=== Defense Mechanisms ===
'''Secretions'''
*Mucus - inhibits contact and protects the mucosal surface
*Digestive enzymes - nonspecifically target bacteria and viruses
*Bile - kills some bacteria and viruses
'''Epithelium'''
*"Tight junctions" between epithelial cells prevent entry of macromolecules and pathogens into the intestinal tract.
*Epithelial cells have a very high turnover rate thus preventing pathogens with a longer life cycle from successfully colonising.
'''Commensal flora'''
*Commensal flora competitively inhibit attachment of pathogens to enterocytes in addition to competing for nutrition and substrates. Many also produce inhibitory growth substances that are toxic to other bacteria (McGavin and Zachary, 2007).
'''Movement'''
*Continuous peristalsis discourages persistence of toxins and aids in their elimination from the gut.
'''Cell-mediated and humoural defences'''
*The lamina propria contains macrophages, B and T lymphocytes, plasma cells, and mast cells.
*Lymphoid aggregates known as Peyer's patches within the small intestine aid in immunity.
*Secretory IgA and IgM provide humoural immunity and help prevent attachment of pathogens to the intestinal epithelium.
*Lysozyme from Paneth cells inhibits bacterial growth (McGavin & Zachary, 2007).
=== Comparative ===
The natural diets of different animal species largely govern the structure and function of the gastrointestinal tract.
Broadly, animals can be classified as carnivores, herbivores or omnivores according to diet. Digestion and absorption of nutrients in carnivorous animals such as the dog and cat takes place largely in the small intestine. Because only a small amount of fermentation occurs in the large intestine of carnivores, they have a minimally developed caecum in comparison to nonruminating herbivores such as the horse which relies heavily on the end products of microbial digestion from the large intestine. The pig, an omnivorous animal has both a relatively long small intestine and an expanded portion of the colon that aids in fermentation of the fibrous material in the diet. Ruminants are an exception as fermentation largely occurs in the stomach (Reece, 2005).
'''References''' <br /br>
Dyce KM, Sack WO, Wensing, CJG: Textbook of Veterinary Anatomy, 3rd ed, p. 129. Saunders, London, England, 2002.
McGavin DM & Zachary, JF: Pathologic Basis of Veterinary Disease, 4th ed, pp. 301-393. Elsevier, St. Louis, Missouri, 2007.
Reece, WO: Functional Anatomy and Physiology of Domestic Animals, 3rd ed., pp. 312-368. Lippincott Williams & Wilkins, London, England, 2005.
Young B, Heath, JW: Wheater's Functional Histology: A Text and Colour Atlas, 4th ed, pp. 249-274. Churchill Livingstone, London, England, 2000.
==Contents==
===[[Intestines Developmental - Pathology|Developmental Anomalies]]===
===[[Physical Disturbances]]===
===[[Intestines Vascular Disturbances - Pathology|Vascular Disturbances]]===
===[[Intestines Proliferative - Pathology|Proliferative Pathology]]===
===[[Intestines Inflammation - Pathology|Inflammatory Pathology]]===
===[[Diarrhoea]]===
===[[Parasites]]===
===[[Diseases due to pathogens]]===
===[[Diseases in animal species]]===
===[[Clinical Pathology - Pathology|Clinical Pathology]]===
'''Intestine'''
'''Intestine'''
== Basic Anatomy and Physiology ==
== Basic Anatomy and Physiology ==
=== Introduction ===
=== Introduction ===
The intestines are a portion of the alimentary tract extending from the pylorus to the anus. In mammals, the intestines are divided into two main segments: the small intestine and the large intestine. The small intestine, consisting of the duodenum, jejunum and ileum, acts as the main site of absorption for digestive products in the gastrointestinal tract. The large intestine, consisting of the caecum and colon, functions to extract water, electrolytes and nutrients as well as to concentrate and propulse faecal material to the rectum for defaecation. (Dyce et. al, 2002; Young & Heath, 2000).
The intestines are a portion of the alimentary tract extending from the pylorus to the anus. In mammals, the intestines are divided into two main segments: the small intestine and the large intestine. The small intestine, consisting of the duodenum, jejunum and ileum, acts as the main site of absorption for digestive products in the gastrointestinal tract. The large intestine, consisting of the caecum and colon, functions to extract water, electrolytes and nutrients as well as to concentrate and propulse faecal material to the rectum for defaecation.
Functional differences between the small and large intestine are reflected in their anatomic and histologic organisation (Figure 1). The small intestinal mucosa in many species is highly folded and made up of numerous villi that greatly increase the absorptive surface area. Enterocytes, the main absorptive cells, are the most numerous and characteristic cells of the small intestinal epithelium. Others include goblet cells, paneth cells, neuroendocrine cells, stem cells, and intraepithelial lymphocytes. Another important feature of the small intestine is the presence of Peyer's patches within the lamina propria. They contribute to immune function and generation of immune responses within the mucosa.
Functional differences between the small and large intestine are reflected in their anatomic and histologic organisation (Figure 1). The small intestinal mucosa in many species is highly folded and made up of numerous villi that greatly increase the absorptive surface area. Enterocytes, which are important in nutrient absorption, are the most numerous and characteristic cells of the small intestinal epithelium. Others include goblet cells, paneth cells, neuroendocrine cells, stem cells, and intraepithelial lymphocytes. Another important feature of the small intestine is the presence of Peyer's patches within the lamina propria. They contribute to immune function and generation of immune responses within the mucosa.
In contrast to the small intestine, the large intestinal mucosa has no villi and consists of 2 main cell types: absorptive cells and mucus secreting goblet cells. A distinguishing histologic and anatomic characteristic is the thick muscularis mucosa layer that aids in propulsion of faeces to the rectum. Another important feature is the presence of commensal bacteria which plays different roles depending upon the type of animal species.
In contrast to the small intestine, the large intestinal mucosa has no villi and consists of 2 main cell types: absorptive cells and mucus secreting goblet cells. A distinguishing histologic and anatomic characteristic is the thick muscularis mucosa layer that aids in propulsion of faeces to the rectum. Another important feature is the presence of commensal bacteria which plays different roles depending upon the type of animal (Dyce et. al, 2002; Young & Heath, 2000).
=== Defense Mechanisms ===
=== Defense Mechanisms ===
==== Secretions ====
==== Secretions ====
*Mucus - inhibits contact and protects the mucosal surface
*Mucus - inhibits contact and protects the mucosal surface
*Digestive enzymes -
*Digestive enzymes - nonspecifically target bacteria and viruses
*Bile - kills some bacteria and viruses
*Bile - kills some bacteria and viruses
==== Epithelium ====
==== Epithelium ====
*"Tight junctions" between epithelial cells prevent entry of macromolecules and pathogens into the intestinal tract.
*"Tight junctions" between epithelial cells prevent entry of macromolecules and pathogens into the intestinal tract.
*Lysozyme from Paneth cells inhibits bacterial growth (McGavin & Zachary, 2007).
*Lysozyme from Paneth cells inhibits bacterial growth (McGavin & Zachary, 2007).
=== Comparative ===
=== Comparative ===
carnivores, ruminants,horse, swine, rabbits [Figure]
The natural diets of different animal species largely govern the structure and function of the gastrointestinal tract.
Broadly, animals can be classified as carnivores, herbivores or omnivores according to diet. Digestion and absorption of nutrients in carnivorous animals such as the dog and cat takes place largely in the small intestine. Because only a samll amount of fermentation occurs in the large intestine of carnivores, they have a minimally developed caecum in comparison to nonruminating herbivores such as the horse which reies heavily on the end products of microbial digestion that occur in the large intestine. The pig, an omnivorous animal has both a relatively long small intestine and an expanded portion of the colon that aids in fermentation of the fibrous material in the diet. Ruminants are an exception as fermentation largely occurs in the stomach. The following figures illustrate the differences between the gastrointestinal tracts of the main domestic animal species (Reece, 2005). [Figures!!]
'''References''' <br /br>
'''References''' <br /br>
McGavin DM & Zachary, JF: Pathologic Basis of Veterinary Disease, 4th ed, pp. 301-393. Elsevier, St. Louis, Missouri, 2007.
McGavin DM & Zachary, JF: Pathologic Basis of Veterinary Disease, 4th ed, pp. 301-393. Elsevier, St. Louis, Missouri, 2007.
Reece, WO: Functional Anatomy and Physiology of Domestic Animals, 3rd ed., pp. 312-368. Lippincott Williams & Wilkins, London, England, 2005.
Young B, Heath, JW: Wheater's Functional Histology: A Text and Colour Atlas, 4th ed, pp. 249-274. Churchill Livinstone, London, England, 2000.
Young B, Heath, JW: Wheater's Functional Histology: A Text and Colour Atlas, 4th ed, pp. 249-274. Churchill Livinstone, London, England, 2000.
==== Obstruction ====
==== Obstruction ====
Intestinal obstruction can be the sequel to either mechanical or functional causes. Mechanical obstruction occurs due to physical blockage of the intestinal lumen whereas functional obstruction results from a decrease or inhibition of intestinal motility due to loss of smooth muscle contraction. (Brown et. al, 2007).
Intestinal obstruction can be the sequel to either mechanical or functional causes. Mechanical obstruction occurs due to physical blockage of the intestinal lumen whereas functional obstruction results from a decrease or inhibition of intestinal motility due to loss of smooth muscle contraction. (Brown et. al, 2007).
===== Physical =====
===== Mechanical =====
Mechanical obstruction of the intestine, either acute or chronic, can occur in all species of animals. Acute obstruction usually involves the upper or middle small intestine whereas chronic obstruction typically occurs in the distal small intestine or large intestine. Causes of obstruction can be divided into three main categories: intraluminal, intramural, or extrinsic. Intraluminal causes include food impaction and foreign bodies, intramural causes include neoplasia and extrinsic causes include adhesions, neoplasia and prostate enlargement. The following figure illustrates the pathogenesis occurring in intestinal obstruction [Figure].
====== Foreign Bodies ======
====== Foreign Bodies ======
*Foreign bodies of all types can be found in the intestines. While some may pass through posing no problems, others can cause acute obstruction, pressure necrosis and eventually perforation. Foreign bodies can also be chronic, remaining for long periods of time without causing disturbance.
*Enteroliths, stones consisting of magnesium ammonium phosphate around a central nidus (often a metallic foreign body) occur mostly in horses greater than 4 years of age. Typically, they lodge at the pelvic flexure or the transverse colon.
====== Impaction ======
====== Impaction ======
Impaction of the colon can occur in all species.
*In the dog and cat, dehydrated faecal material is the main cause of impaction.
*In the horse, faeces, digesta, sand, or fibrous material can all contribute to impaction Predisposing factors include poor dentition, water deprivation, a high roughage diet and general debility.
*Antihelminthic administration or large parasite burdens can also lead to impaction.
====== Extrinsic ======
====== Extrinsic ======
Neoplasia, Adhesions, Prostate enlargement
Obstruction of the intestine due to external factors such as tumours, abscesses, and fibrous adhesions is a common occurrence.
*Neoplasia in structures adjacent to the intestines can spread and cause external compression. Pancreatic tumours in particular can extend and impinge on the duodenum.
*Inflammatory adhesions, following gut perforation, peritonitis or surgery, consist of fibrous tissue bands that may restrict intestinal motility and cause kinks in the mesentery.
*Pedicles of tumours such as lipomas in horses can become wound in loops of intestine leading to obstruction and possible strangulation.
*Prostatic enlargement in the dog can lead to compression of the rectum.
===== Functional =====
===== Functional =====
*Paralytic ileus is a common condition that can occur following trauma or abdominal surgery. Intestinal stasis leads to distension with gas and fluid as well as a flaccid intestinal wall.
*Dysautonomia is a condition that most notably affects horses and cats.
**Equine dysautonomia (grass sickness) occurs mostly in the UK and western Europe. ''Clostridium botulinum'' is thought to be implicated although the exact cause is still unknown. Affected animals are dull, restless, show signs of colic, and in acute cases become severly tympanic, avoid swallowing and salivate excessively. Degenerative lesions are seen in the autonomic nerve ganglia, including enteric plexuses.
**Feline dysautonomia (Key-Gaskell syndrome) occurs mostly in the UK and continental Europe. It is also of unknown aetiology although environmental toxins, infectious agents and botulinum toxins have been suggested as causative factors. Clinical signs include anorexia, depression, bradycardia, decreased lacrimation, altered pupillary dilataion, megaoesphagus and constipation. As in equine dysautonomia, degenerative lesions of autonomic nerve ganglia can be seen.
==== Displacement ====
==== Displacement ====