35 Introduction to the colon and colonic disorders

The large intestine comprises the caecum, colon, rectum and anus (Fig 35.1). The primary role of the colon is the resorption of water and electrolytes. Colonic bacteria break down fermentable fibre to produce short chain fatty acids – acetate, butyrate and proprionate. Butyrate is an essential energy source for colonocytes.

Fig 35.1 Illustration of the large intestine, which comprises the caecum, colon, rectum and anus.

Diarrhoea associated with the colon occurs when there is interference with water or electrolyte resorption, abnormal motility or presence of abnormal luminal substrate following pancreatic, hepatic or small intestinal disease.

Diseases of the colon include:

• Diarrhoea

• Constipation and megacolon

• Neoplasia

• Rectal disease; neoplasia, strictures, foreign bodies

• Colonic vascular dysplasia

• Typhilitis – rare

Causes of large bowel diarrhoea include:

• Obstructive disease

• intraluminal obstruction, e.g. tumour, constipation, foreign body, intussusception, stricture

• extraluminal obstruction (hernia, masses, adhesions)

• Ischaemic disease – infarct, intussusception

• Non-inflammatory disease

• motility disorders, e.g. irritable bowel syndrome (IBS)

• congenital malformations

• Systemic disorders

• toxaemias (e.g. pyometra, peritonitis)

• intolerance or allergy

• foreign material (bones, hair, plastic)

• Secondary to unabsorbed nutrients from small intestinal diarrhoea

Clinical signs of large intestinal disease include:

• Chronic or acute diarrhoea

• Intermittent vomiting (20% of cases)

Investigation of large intestinal disorders

An initial minimum data base, especially faecal testing, as for small intestinal diarrhoea (see page 116) may be helpful in chronic cases. In cats, a fresh saline faecal preparation should be examined for Tritrichomonas foetus under the microscope and a sample sent for PCR test.