Description
The definition of diarrhoea is as follows:

Normal bowel movements are altered resulting in an increase in any one of the following:

Cause
Diarrhoea is a very common clinical sign which can be associated with a whole variety of causes including:

FUNCTIONAL CAUSE	REASON	UNDERLYING CAUSE
Osmotic movement of water into the faeces	Excessive food intake, overloading the system with retention in the intestine lumen.	Can be just excessive intake of one nutrient e.g. high fat content
	Failure to digest nutrients properly resulting in retention in the lumen of the intestine	Exocrine pancreatic insufficiency, bacterial overgrowth, enzyme deficiency (e.g. lactase deficiency), bile salt deficiency
	Dietary intolerance
	True dietary allergy (rare)
	Diseases of the mucosal lining of the small intestine	Inflammation, neoplasia (cancer), gluten enteropathy (Irish Setters)
	Diseases of the wall of the intestine	Chronic enteritis, chronic cellular infiltration, lymphangiectasia, cancer (e.g. lymphosarcoma), canine sprue
Secretory diarrhoea due excessive fluid secretion into the faeces	Secretory stimulants	Endotoxins produced by bacteria in the intestine
		Hydroxylated bile acids in the intestine lumen
	Inflammation of the intestine	Bacteria,Parasites, Viruses
Increased permeability of the intestine wall		Lymphangiectasia , protein-losing enteropathy
Motility disorders of the intestine	Reduced intestinal transit time	Low fibre diets, partial obstruction of the intestine
	Rapid intestinal transit time	Many diseases of the intestine

Diagnosis
The primary cause of a diarrhoea is not always possible to detect - particularly in acute diarrhoea when eating rubbish, a high fat snack or an unusual foodstuff might not have been noticed by the owners.

In chronic diarrhoea the appearance of the stool is very helpful in identifying the likely site of the problem, and the following tests can help with identifying an underlying cause:

TEST	Exocrine Pancreatic Insufficiency	Proximal Small Intestine Disease *	Distal Small Intestine Disease *	Bacterial Overgrowth
TLI Test	Low	Normal	Normal	Normal or Increased
Cobalamin	Normal	Normal	Low	Low
Folate	Normal	Low	Normal	High
Bile acids	Normal	Normal	Low	Low
Faecal Fat	Increased	Normal	Increased
Absorption Test (e.g. xylose)	Decreased	Decreased	Normal	Decreased

Consequences
A patient can lose large amounts of water in diarrhoea and this causes dehydration which, in severe cases such as canine parvovirus disease, can be life-threatening.