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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

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

Clinical Pathology

Intestine

Basic Anatomy and Physiology

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. 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 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

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 Livinstone, London, England, 2000.

Pathology

Developmental Anomalies

Segmental

Segmental anomalies of the intestines are common and range from incomplete occlusion (stenosis) to complete occlusion (atresia) of the lumen.

Atresia ani

• A common congenital defect of the lower gastrointestinal tract.
• Both the anus and rectum can be affected.
• Occurs in all species but is most often seen in calves and pigs.

Atresia coli

• Considered the most common segmental anomaly of the intestine in domestic animals.
• Can affect in particular the spiral colon of Holstein calves and the small colon of foals.

Atresia ilei

• It is less common to see atresia in the small intestine.
• Mostly seen in calves.

Persistent Meckel's diverticulum

• Meckel's diverticulum is a remnant of the omphalomesenteric duct of the yolk sac.
• Is usually an incidental finding though it can be associated with impaction and or intussusception.

Megacolon

• Occurs in cats, dogs, and pigs.
• May be due to a lack of myenteric plexuses that occurs when neuroblasts fail to migrate from the nerual crest to the colorectal myenteric plexuses.
• Congenital colonic agangliosis, a condition occurring in white foals, can lead to megacolon. It is thought to be an autosomal recessive condition.

References

Brown CC, Baker DC, Barker IK: Alimentary System. In: Jubb, Kennedy and Palmer's Pathology of Domestic Animals, ed. Maxie MG, 5th ed., pp. 1-296. Elsevier, Philadelphia, PA, 2007.

McGavin DM & Zachary, JF: Pathologic Basis of Veterinary Disease, 4th ed, pp. 301-393. Elsevier, St. Louis, Missouri, 2007.

Physical Disturbances

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).

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 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 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

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

• 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

Volvulus and Torsion

Ruminants

caecal dilatation and torsion

Equine

right and left dorsal displacement colonic torsion and volvulus

Vascular Disturbances

pathogenesis

Inflammation

pathogenesis

• enteritis
• typhlitis
• colitis
• proctitis

? about subheadings as many fit under specific pathogens notes subcategorising by types of enteritis???

Inflammatory bowel disease

description of

Colitis

Typhlocolitis

in dogs: pipe to species specific section includes histiocytic unlcerative colitis trichuris vulpis, pipe to parasites

Neoplastic and Proliferative Lesions

description

Diarrhoea

definition and pathogenesis characteristics of l.i. diarrhoea

Diseases due to pathogens

Bacteria, Viral, Mycotic brief explanation then pipe to different page

Bacterial

brief explanation

Escherichia coli

description

Salmonellosis

description Link to salmonellosis in specific species

Clostridial enteritis

description

Viral

description Bovine viral diarrhoea Rinderpest Peste des petits ruminants Malignant catarrhal fever Herpesvirus Adenovirus Coronavirus Rotavirus Parvovirus

Mycotic

description Intestinal phycomycosis and aspergillosis Candidiasis Intestinal histoplasmosis

Diseases in Animal Species

Avian

coccidiosis

Carnivores

Cats

Panleucopenia Feline infectious peritonitis Feline leukaemia Feline immunodeficiency virus

Dogs

Parvovirus 2 Distemper Salmonella Rotavirus Coronavirus Ascarids Hookworms Giardiasis Neoplasms Salmon poisoning Haemorrhagic enteritis

Horses

Rhodococcus equi Equine monocytic ehrlichiosis Equine granulomatous enteritis Clostridial enteritis Lawsonian associated Potomac horse fever Parasites Antibiotic associated diarrhoea

Ruminants

Bovine viral diarrhoea Rinderpest Malignant catarrhal fever Paratuberculosis Chlamydiosis Parasitism (list specifics to l.i.) Copper deficiency Salmonellosis Yersiniosis Coccidiosis Escherichia coli Poisoning Winter dysentery

Swine

Rotavirus Transmissible gastroenteritis Escherichia coli Salmonella Lawsonian intracellularis Swine dysentery Porcine circovirus 2 Intestinal spirochaetosis Ascaris suum

Parasites

Cryptosporidiosis Giaridasis Ascariasis Hookworm diseas Trichuriasis Strongyloidosis Pinworms Cestodes Trematodes