The following is an outline of the approach I usually take to cases of diarrhoea in adult dogs or cats I see in general practice. It is my own personal approach and I am sure there are all sorts of variations possible (and I'm sure some would disagree especially 'proper' gastroenterologists!) but it usually works for me.
1. Always advise fasting for 24 hours and then feeding small, more frequent meals of a bland, low residue diet.
2. Ensure worming history is up to date and if in doubt treat with Drontal. If large bowel diarrhoea is present, do a faecal flotation if possible. If Trichuris is present, the patient will need to be wormed at two-monthly intervals indefinitely due to the robust nature of the eggs in the environment.
3. If the diarrhoea is very watery and smelly (small bowel) or very haemorrhagic (large bowel) I will usually prescribe metronidazole (10-20 mg/kg bid).
4. At this point the vast majority of cases have resolved. However ........
5. If small bowel diarrhoea persists after points 1-3, I will consider treating for antibiotic-responsive diarrhoea with oxytetracycline, tylosin, amoxycillin or occasionally enrofloxacin (used in people for ARD).
6. If large bowel diarrhoea persists I will request a faecal culture for Campylobacter, Salmonella and E. coli and if available request tests for Clostridial toxins.
7. And/or (depends on the severity of the diarrhoea, client factors such as finances etc.) recommend commencement of a hyposensitivity diet (commercial or homemade). Discussion with the owner will include the fact that dietary hypersensitivity is uncommon but worth checking for as (a) the next step will be gut biopsy and (b) it is the first management decision when some of the inflammatory bowel disorders are diagnosed by biopsy (lymphocytic-plasmacytic or eosinophilic enteritis/colitis). Diet will be recommended for four to six weeks but if there is absolutely no response after two weeks then the prognosis for a good response is reduced.
8. And/or (if the owner will not/cannot comply with the dietary trial and/or the patient has large bowel diarrhoea but with not a lot of fresh blood--just mucus and tenesmus) advise adding fibre to the diet (unprocessed bran or soluble). I personally don't combine a hypoallergenic diet with fibre addition because if the patient responds I don't know which component it has responded to--the diet or the fibre--which has significant implications for future dietary recommendations (not to mention the wallet of the client!).
9. If all the above fails or if the patient is hypoproteinaemic or if the patient has really bloody large bowel diarrhoea and severe tenesmus, or if there is any other clue that neoplasia may be the cause, I will recommend ultrasound by a specialist and/or biopsy--either by endoscopy, if equipment and expertise are available, or by exploratory laparotomy.