Note for Pet Owners:
If you have an animal with diarrhoea there are some important guidelines that you should follow:

  • If you haven't done so - Get your pet examined by your veterinarian.
  • Always give the medications that your veterinarian has prescribed at the correct dose and at the correct times.
  • Contact your veterinary practice if you are concerned that your animal is having a relapse, or if it appears to react abnormally following treatment.
  • Keep your animal on the strict diet that your veterinarian recommends and AVOID feeding high fat foods because fat intake can make the condition much worse. If your pet is a bin scrounger or likely to steal food - stop him/her . Many "snacks" are relatively high in fat - so cut them out !!

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The definition of diarrhoea is as follows:

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

The definition of diarrhoea does not cover the appearance of the stool - and it should be realised that not all forms of diarrhoea result in loose stools. The stool can still be formed, as it is with
steatorrhoea which results from the presence of undigested fat in the faeces.

Sometimes mucous will be seen, and sometimes fresh blood (see dysentery and haematochezia and melaena). 

The photograph below is of dysentery


The photograph below is of black, tarry melaena


The photograph below is of haematochezia

The picture below is of faeces, blood and mucus typical of colitis.

Sometimes blood can be present in a faecal stool without there being any obvious external change in appearance - this is called occult blood, and can be detected using sensitive laboratory tests.

The appearance of the diarrhoea can give a very good idea of the location of the gastrointestinal disorder:

Appearance of faecesLocation of disorder
Normal colour. Increased volume. Large, bulky soft stool.Small intestine
Very watery - no obvious bloodSmall intestine
Very watery stool with fresh blood (dysentery)Small intestine. Typical of canine parvovirus infection or Haemorrhagic gastroenteritis
Yellow, brown or green soft, bulky stoolSmall intestine
Pale, greasy, putty-coloured, soft, fatty foul smelling stoolSteatorrhoea
Black, tarry stools.Melaena
Very pale, white, crumbly stoolsLack of bile pigment
Small volume, but frequently passed stoolsColon
Formed faeces or watery faeces with mucusColon
Clear mucus or mucusColon
Watery faeces with fresh blood and mucusColon

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

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

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:
TESTExocrine Pancreatic InsufficiencyProximal Small Intestine Disease *Distal Small Intestine Disease *Bacterial Overgrowth
TLI TestLowNormalNormalNormal or Increased
Bile acidsNormalNormalLowLow
Faecal FatIncreasedNormalIncreased 
Absorption Test (e.g. xylose)DecreasedDecreasedNormalDecreased

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.


Last updated : October 2013