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If diarrhoea persists despite symptomatic treatment or is chronic, severe and associated with significant weight loss and/or evidence of hypoproteinaemia, dehydration or systemic illness, then a more detailed investigation is indicated. Only a small proportion of diarrhoea cases require investigation as chronic disorders. An animal presenting with intermittent but repetitive episodes of diarrhoea over many months should also be considered for investigation unless there is a proven cause (e.g. regular garbage bin raiding).

The following is a general outline that can be used to approach stable cases of diarrhoea in adult dogs and cats in general practice. More severely affected animals and very young animals will require investigation and/or more intensive supportive treatment earlier in the course of their disease. Every case should be considered individually, and client factors such as budget must be taken into account.

1 Take a detailed history to assess for trigger factors (bin raiding, extra table scraps from the roast dinner, inappropriate diet or snacks, hunting etc). Perform a thorough physical examination to look for relevant abnormalities (e.g. palpable bowel lesion, dehydration, weight loss).

2 Ensure anthelmintic history is up to date, and if in doubt treat with an appropriate anthelmintic (usually more appropriate in young animals).

3 Recommend highly digestible food until diarrhoea resolves. If animal is not eating, monitor the situation in case intervention is needed (e.g. recheck in appropriate time period between 24 and 72 hours).

4 Antibiotic treatment is rarely indicated in acute diarrhoea unless the animal is systemically unwell, and even then it may not be warranted.

5 Consider faecal culture:If the animal is systemically unwellWhen diarrhoea is acute and haemorrhagicIf the diarrhoea is very severeIn multiple animals in a crowded environment such as a kennel environmentIf the owner or the pet is immunocompromised or if the owner is also affected with diarrhoea.

Overall, most dogs and cats with chronic diarrhoea do not require faecal culture; it rarely adds to the clinical picture, and it increases the overall cost of investigation.

Treat with antibiotics if bacterial cause is strongly suspected but culture is not possible. Select antibiotics following appropriate rational antibiotic therapy guidelines.

1 The majority of acute diarrhoea cases will resolve or improve within 24–48 hours. However, if the problem becomes chronic…

2 Consider the merits of faecal parasitology and Giardia testing depending on age of animal and previous treatments/response to fenbendazole treatment. Note that some dogs with Giardia infection will respond to fenbendazole but relapse afterwards.

3 Recommend a diet trial (commercial or homemade, novel protein source or hydrolysed).Use the animal’s diet history to choose a ‘novel’ protein‐source diet or choose a hydrolysed diet if many different foods have been given.The diet must be fed for 2–3 weeks initially by which time some response is expected and up to 12 weeks if successful before other foods are introduced.No other foods, chews, supplements or drinks other than water should be given. Diet trials are important as food‐responsive chronic enteropathy is common, and the next diagnostic step is often invasive and costly (gut biopsy).If the diarrhoea is large bowel in origin in a dog, diets can be similar to those mentioned above or a fibre‐enriched diet can be trialled – commercial high‐fibre diets or psyllium/bran added to a balanced diet can be used. Bran should not be given if gluten sensitivity is suspected. However, gluten sensitivity in dogs is very uncommon and has only been identified in a handful of breeds e.g. Irish Setters, Wheaten terriers and Border terries.Care must be used if fibre is given to underweight animals as overall calorie intake tends to become reduced especially if appetite is low.Diet trial alone is rarely appropriate in significantly underweight, anorexic, hypoproteinaemic or systemically unwell animals.

4 Before any invasive, costly or extensive diagnostic plans, ensure that seconordary GI disease is not present, for example, exocrine pancreatic insufficiency (EPI), hyperthyroidism in cats, hepatic disease hypoadrenocorticism in dogs, by performing routine haematology and biochemistry +/‐ total T4 (cats), basal cortisol/ACTH stimulation test (dogs), trypsin‐like immunoreactivity (TLI). Secondary disease is more likely if diarrhoea is small bowel or mixed in origin.Even if the electrolytes are normal, if a dog is systemically unwell and does not have a stress leukogram, consider doing a basal cortisol/ACTH stimulation test to rule out hypoadrenocorticism before any invasive tests, for example, biopsy.Measurement of serum cobalamin is routinely done at this point to assess whether supplementation is needed. Low serum cobalamin is seen with distal SI disease but normal levels don’t rule it out.

5 Test for Tritrichomonas with PCR if it is a cat with large bowel signs (may do this earlier in the process if it is a purebred cat or from a multi‐cat household or shelter).

6 Perform an abdominal ultrasound to assess for structural bowel lesions that may occur with chronic diarrhoea (e.g. hyperechoic mucosal speckles/striations; intestinal wall thickening, intestinal masses, lymph node enlargement in neoplasia) and for other disorders that may cause diarrhoea, for example, abdominal neoplasia, intussusception. Ultrasound is indicated sooner if there is a palpable abnormality in the abdomen or if diarrhoea is severe and accompanied by weight loss or severe vomiting.

7 Assuming that secondary GI disease has been ruled out, if small bowel diarrhoea persists after points 1–10 for more than a month or so and, depending on the severity, species and breed (German shepherd/young large breed dog), the animal’s clinical condition and owner concerns, consider treating for antibiotic‐responsive diarrhoea with metronidazole (10 mg/kg PO BID) or tylosin (5–10 mg/kg PO SID).Four to six weeks of therapy is usually the recommended treatment. The most appropriate drug has not been determined by controlled clinical trials.Relapse following the treatment course is common.The role of pre/probiotics in cases of idiopathic ARD has yet to be determined.Relapses may be reduced or less severe in patients fed a high‐fibre diet.

8 If all the above fails or if the patient is hypoproteinaemic or significantly underweight or if neoplasia is suspected on physical examination or imaging, biopsy is indicated.Endoscopic biopsy is preferred if appropriate expertise and facilities are available.Whilst there are some limitations with this technique, surgical biopsy may be overly invasive for many cats and dogs with chronic diarrhoea.Exploratory laparotomy may be required:If endoscopic equipment and expertise are not availableDiagnostics have indicated that the disease is a focal lesion that may be unreachable with an endoscope, for example, a jejunal mass.If possible, consult a medicine specialist prior to biopsy if you are unsure.

9 If biopsy is not an option for cost reasons or lack of access to appropriate facilities/expertise, following careful client discussion of the risks of misdiagnosis and inappropriate treatment…Treat with prednisolone 2‐3 mg/kg once daily and observe response.Large dogs should have no more than 40 mg/m2 once daily.If good response is seen, taper the prednisolone dose by 20–25% every 4 weeks.Consider a second immunosuppressive drug (e.g. azathioprine, ciclosporin, chlorambucil, mycophenolate or leflunomide) if steroid side effects are moderate to severe to allow more rapid tapering of prednisolone. Chlorambucil or ciclosporin are the most commonly used second‐line drugs in cats.