In the dog, diarrhoea represents a significant proportion of all visits to veterinary clinics. Successful approach to chronic diarrhoea is usually more challenging, and a systematic approach is required to narrow down the list of possible differential diagnoses. Additionally, close cooperation between the veterinarian and the dog's owner is an essential prerequisite for success.

Diagnostic Approach

The causes of chronic diarrhoea in dogs are multiple, and numerous factors are involved. Importantly, the prevalence of different diseases can vary depending on the geographical location.

Parasite Infestation

It is essential to rule out the presence of intestinal parasites. Faecal shedding of parasite ova or cysts is not continuous, therefore a single negative parasitological faecal analysis may not truly rule out parasite infestation. Moreover, several protozoan parasites may be difficult to detect (e.g., Giardia spp. for which enzyme-linked immunosorbent assay (ELISA) testing is more sensitive than direct faecal smears or faecal floatation). However, negative results in three successive faecal samples reliably confirm the absence of intestinal parasites. A 'practical' alternative to multiple faecal examinations is systematically administering a broad-spectrum anthelminthic drug to treat locally prevalent endoparasites (e.g., fenbendazole 50 mg/kg orally daily for 5 days).

Diet

When parasites have been ruled out, dietary problems are probably the most frequent cause of chronic diarrhoea in dogs. In a recent clinical study including 70 adult dogs referred for further work-up of chronic diarrhoea, clinical signs resolved in 39 (55%) patients following a dietary elimination trial of 10 days (consisting of exclusive feeding of a novel protein diet). Excellent compliance on the part of the dogs' owners was probably an essential component for the success of dietary therapy.

Food can elicit gastrointestinal inflammation in several ways: in genuine food allergy an immunological reaction against one of the dietary components (allergen) is at the origin of the problem. In such cases, it is recommended to feed the dog for several weeks with a diet consisting of nutrients with which it has had no previous contact (individually tailored hypoallergenic diet). To confirm the diagnosis and find out what exactly the dog is allergic to, it is necessary to perform a dietary challenge with different allergens which were part of the original diet (e.g., beef, chicken, lamb, etc.) and to wait for the recurrence of clinical signs. Most of the dogs in the Bern study that responded to dietary elimination probably suffered from food intolerance. Their clinical signs disappeared within 7-10 days of receiving only the hypoallergenic diet. However, dietary challenge did not elicit recurrence of clinical signs in most of those which underwent that test. Food intolerance is not due to an immunological phenomenon. It could be caused by the inability to adequately digest some of the dietary components or to substances added to the diet during industrial food processing.

It is therefore recommended to prescribe a hypoallergenic diet to all dogs which did not respond to treatment for gastrointestinal parasites. Strict compliance from the owners is essential. In our experience, most dogs respond within 7-10 days with a significant improvement of stool consistency and clinical status (although some of them may require more time). The diet change must occur progressively. Many suitable hypoallergenic diets are available from the main pet food manufacturers. At this time there is no published evidence that diets based on hydrolysed proteins are superior to novel protein diets. In our recent study 31 of 39 dogs (79%) with food-responsive disease could be returned to a commercial diet after 14 weeks of elimination diet. The remaining 21% had to be maintained on a strict diet to avoid recurrences.

What To Do If Parasiticide Treatment and Elimination Diet Fail

In cases of large bowel diarrhoea (colitis with typical clinical presentation), a therapeutic trial can be initiated with metronidazole (10-20 mg/kg orally q12h for 5-10 days) and addition of fibre to the diet (e.g., psyllium, depending on dog's size between 1 teaspoon and 2 tablespoons mixed with food every 12 hours). In refractory cases, performance of mucosal biopsies prior to further treatment is the best course of action.

A number of dogs with chronic intestinal disease respond well to antimicrobial treatment (antibiotic-responsive diarrhoea or ARD). The aetiology of ARD is unknown and a bacterial infection with unidentified bacteria cannot be ruled out at this time. German Shepherd Dogs may be predisposed to that disease due to insufficient production of immunoglobulin A (IgA). Clinical signs may vary considerably: chronic, recurring, mostly small intestinal diarrhoea is frequent (although large intestinal signs may also occur). Additional signs include borborygmus, flatulence, decreased appetite and weight loss. Once parasite treatment followed by 2-4 weeks' elimination diet have been attempted without success, many small animal gastroenterologists consider ARD a possible diagnosis and recommend antimicrobial treatment with oral metronidazole (10-20 mg/kg q12h), tylosin (10-20 mg/kg q12h) or tetracycline (20-25 mg/kg q8h). Interestingly all three substances may also exert immunomodulating or even antiinflammatory effects on the intestinal mucosa in addition to their antimicrobial properties.

In dogs presented with chronic diarrhoea and systemic signs, a global and more aggressive approach must be adopted. A detailed clinical examination followed by complete blood count (CBC) and chemistry panel with all the usual parameters, including total protein and albumin, and urinalysis (particularly checking for proteinuria) are necessary. The possibility of exocrine pancreatic insufficiency must be ruled out (serum trypsin-like immunoreactivity (TLI), serum folate and cobalamin concentrations). Rectal cytology is a simple test that may be very useful for the diagnosis of intestinal infections (especially fungal infections). Abdominal ultrasonography is recommended as it may show disruption of the typical architecture of the intestinal wall. However, ultrasonographic findings, which may also include intestinal wall thickening and enlargement of mesenteric lymph nodes, are not specific for a particular disease, with the possible exception of lymphangiectasia. They may just confirm the presence of intestinal inflammation. If no clear diagnosis can be made at that time, performance of mucosal biopsies is generally necessary. Gastrointestinal endoscopy is the least invasive method; however, it only allows sampling of specific segments of the intestine, and the biopsy sites may be quite superficial. This is why a more invasive, surgical approach with collection of transmural biopsy specimens may be preferred in some cases. Several studies have shown that proper interpretation of intestinal biopsy specimens is not easy and requires good communication between clinician and pathologist.

Specific Laboratory Tests of Interest in Dogs with Chronic Diarrhoea

 Serum albumin concentration: although serum albumin may be mildly to moderately decreased with many intestinal diseases involving the small intestine, severe decreases (<20 g/l) are usually due to protein-losing enteropathy and are often associated with panhypoproteinaemia. Possible causes include moderate to severe inflammatory bowel disease (IBD) or lymphangiectasia. Patients with severe hypoproteinaemia are usually severely affected and may present with ascites, hydrothorax and/or subcutaneous oedema. Hypoalbuminaemia was previously associated with refractoriness to treatment and recently was found to be a negative prognostic factor (serum albumin <20 g/l) in dogs with chronic enteropathies.

 Serum acute phase proteins: C-reactive protein (CRP) serum concentration has been shown to increase in parallel to clinical activity of disease and may be a useful parameter to monitor the evolution of disease and/or assess response to treatment in difficult patients. However, it lacks specificity as inflammatory disorders from most organ systems may elicit an increase in CRP.

 Serum cobalamin: decreased serum concentration of cobalamin may be present in distal small intestinal dysfunction. Serum cobalamin concentrations of <200 ng/l were found to have negative prognostic value in dogs with chronic enteropathies, although albumin and cobalamin concentrations were strongly correlated in these dogs.

 Faecal alpha1-proteinase inhibitor: can be a useful early marker of intestinal protein loss. In patients with protein-losing enteropathy, it may also serve as a practical tool for monitoring the evolution of disease and/or assess response to treatment.

 Faecal culture: this can be useful if specific bacteria are suspected (e.g., Campylobacter spp.), but can be difficult to interpret (is the identified bacterium at the origin of the problem or does it just represent an opportunistic growth in the inflamed intestine?).

Conclusion

Due to the multiple causes of chronic diarrhoea in dogs, a systematic approach is necessary to narrow down the list of differential diagnoses and initiate a successful treatment. While the initial efforts are non-invasive, analysis of mucosal biopsy specimens is necessary in some cases. In the vast majority of cases, this approach will be successful.

References

1.  Allenspach K, Wieland B, et al. Chronic enteropathies in dogs: evaluation of risk factors for negative outcome. Journal of Veterinary Internal Medicine 2007; 21: 700-708.