Logical Approach to Diarrhoea
World Small Animal Veterinary Association World Congress Proceedings, 2009
Jill Maddison, BVSc, DipVetClinStud, PhD, FACVSc, MRCVS
The Royal Veterinary College, North Mymms, UK

Identify the Problem

Diarrhoea is defined as an alteration in the normal pattern of defecation resulting in the passage of soft, unformed stools with increased faecal water content and/or increased frequency of defecation.

It is important to consider the animal's previous defecation pattern, as the frequency of defecation and the nature of faeces varies between individuals.

Identify the Body System

Diarrhoea can be due to disorders of the small bowel and/or large bowel or to other systemic disorders such as hepatic disease, pancreatic insufficiency, pancreatitis, hyperthyroidism or hypoadrenocorticism.

Diarrhoea associated with secondary gastrointestinal disease most frequently has the characteristics of small bowel disease. However, in secondary gastrointestinal disease, with the exception of pancreatic insufficiency, diarrhoea is not usually the primary presenting complaint.

Classification of Diarrhoea

A rational diagnostic and therapeutic approach to chronic diarrhoea in the dog and cat is dependent on a basic understanding of the function of the gut and the classification of the type of diarrhoea that is present.

Identify the Anatomical Location

While symptomatic therapy is appropriate for the majority of animals with acute diarrhoea, chronic diarrhoea will often present the veterinarian with a diagnostic challenge where the more routine laboratory aids are not useful.

Avoid Frustration!

Failure to elicit sufficient information from the client as to the characteristics of the diarrhoea to allow appropriate localisation of the pathology within the gastrointestinal tract will result in inappropriate diagnostic procedures with increased expense to the client and frustration of the veterinarian, client and patient.

The diagnostic work-up and therapy for small and large bowel diarrhoea differ. Therefore, it is of the utmost importance that prior to embarking on diagnostic procedures or therapy, an assessment is made as to whether the diarrhoea is:

 Acute or chronic

 Due to primary gastrointestinal or secondary gastrointestinal disease of small bowel or large bowel origin or mixed

Diarrhoea may have features of both small and large bowel which indicates either primary small bowel with secondary effects on the lower bowel, or diffuse disease involving both.

A thorough history is essential to differentiate small from large bowel disease. It is important to carefully question the owner as to the character of the faeces and to elicit information regarding consistency, colour, frequency, presence of blood or mucus. Related abnormalities should also be assessed, such as whether there has been significant weight loss, loss of appetite or vomiting.

Table 1.

Small Bowel Diarrhoea


Increased faecal bulk and/or increased water content


May be projectile and does not involve tenesmus


If blood is present it is usually digested (melaena)


Colour may be grey if large amounts of undigested fat are present or if the diarrhoea is due to lactose intolerance. A yellow-green colouration is common and due to malabsorbed bile salts.

Weight loss?

Chronic small bowel diarrhoea is often but not always associated with weight loss


Vomiting may also be present (but need not be) when present it is usually related to eating

Borborygmus and flatulence

Commonly occurs with small bowel diarrhoea as malabsorbed carbohydrates are fermented by colonic bacteria producing CO2 and H2


Appetite may be variable depending on the underlying aetiology

Water balance

If severe, the animal may be dehydrated or if very watery the patient may be polydipsic

Physical examination?

Physical examination is often unrewarding. Always do rectal examination for melaena or large bowel signs such as mucous and fresh blood that the owner may not be aware of.

Large Bowel Diarrhoea


The passage of small amounts of faecal material frequently


Mucous is often present either on the surface(indicating the lesion is in the lower colon or rectum) or throughout the faeces (indicating a lesion in the higher colon)

Fresh blood

If blood is present it will be undigested.


Tenesmus is often present, particularly if the lower involved colon or the rectum are

Weight loss?

Usually there is no weight loss


Usually the appetite is unaffected


Vomiting is infrequent and is unrelated to eating

Physical examination?

Physical examination is often unremarkable but it is imperative to do a rectal examination to check for strictures, masses or thickened mucosa

Identify the Lesion

Table 2. Causes of acute small bowel diarrhoea in dogs and cats.

Diet related

Overeating (especially pups)

Dietary change

Spoiled food

Ingestion of garbage




Most commonly ascarids, also hookworms



Giardia sp.

Coccidia sp. e.g., Cystoisospora spp. (formerly called Isospora)



Viral enteritis





Bacterial enteritis



E. coli

Campylobacter sp.








Table 3. Causes of chronic small bowel diarrhoea in dogs and cats.

Diet related

Lactose intolerance
Dietary hypersensitivity
Gluten intolerance


Intestinal parasites (as above)
Giardia sp.


Antibiotic responsive enteropathy


Eosinophilic enteritis
Lymphocytic-plasmacytic enteritis
Diffuse lymphosarcoma
Mast cell tumour (feline)

Miscellaneous and
secondary GI causes

Lymphangiectasia (primary or secondary)
Brush border enzyme biochemical defects
Motility disorders e.g., hyperthyroidism, lead toxicity, dysautonomia
Exocrine pancreatic insufficiency

Table 4. Causes of acute and chronic large bowel diarrhoea in dogs and cats.


Trichuris vulpi
Ancylostoma caninum
sp. (more commonly small bowel),
Entamoeba sp


Campylobacter sp.
Clostridia sp.
Salmonella sp.,
Yersinia enterocolitica
FIP Antibiotic responsive enteropathy

Diet related

Dietary hypersensitivity
Passing foreign material


Idiopathic ulcerative (plasmacytic-lymphocytic)
Histiocytic (Boxers)
Pseudomembranous (antibiotic associated)


Diffuse or discrete mass, e.g., lymphosarcoma




Scar or neoplastic (adenocarcinoma)

Miscellaneous (secondary GI)
(diarrhoea will not be the
major presenting sign)


Speaker Information
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Jill Maddison, BVSc, DipVetClinStud, PhD, FACVSc, MRCVS
The Royal Veterinary College
North Mymms, UK