In the dog, diarrhea represents a significant proportion of all visits to veterinary clinics. Acute-onset diarrhea usually responds well to treatment against intestinal parasites, dietary modification and/or symptomatic therapy, however successful approach of chronic diarrhea is usually more challenging. Affected dogs often receive treatment with different diets, antimicrobials or anti-inflammatory drugs without success. A systematic approach is required to narrow down the list of possible differential diagnoses. Finally, close cooperation between the veterinarian and the dog's owners is a pre-requisite to optimize treatment success.
Diagnostic Approach
The causes of chronic diarrhea in dogs are multiple, multiple factors are involved, and the prevalence of different diseases can vary depending on the geographical location. Some diseases can generally be ruled out using relatively simple exams (e.g., intestinal parasites). However, the diagnostic approach other diseases may require therapeutic trials (e.g., adverse reaction to food) or more detailed investigations. In severely sick animals (with obvious systemic signs), it is preferable to immediately adopt a more aggressive, global approach involving blood tests, diagnostic imaging, and possible endoscopy or laparotomy with sampling of mucosal biopsies.
Parasite infestation: It is essential to rule out the presence of intestinal parasites. Fecal shedding of parasite ova or cysts is not continuous, therefore a single negative parasitological fecal analysis may not truly rule out the possibility of parasite infestation. Moreover, several protozoan parasites may be difficult to detect (e.g., Giardia spp. for which ELISA testing is more sensitive than direct fecal smears or fecal floatation). However, after analyzing 3 successive fecal samples, negative results confirm the absence of intestinal parasites. A "practical" alternative to multiple fecal exams consists in systematically administering a broad spectrum anthelminthic drug to treat locally prevalent endoparasites (e.g., fenbendazole 50 mg/kg p.o. daily during 5 days).
Diet: When parasites have been ruled out, dietary problems are probably the most frequent cause of chronic diarrhea in dogs. In a recent clinical study including 65 adult dogs referred to the University of Bern, Switzerland for further workup of chronic diarrhea, clinical signs resolved in 2/3 of patients following a dietary elimination trial of 7-10 days (consisting of exclusive feeding of a novel protein diet). Excellent compliance of the dogs' owners was likely an essential component for the success of dietary therapy. Many of them had decided that they would participate in the study as a last resort before giving up on controlling their dog's problem, and were therefore very motivated.
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 during several weeks with a diet consisting of nutrients to which it 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 with 7-10 days after they were fed exclusively 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 immulogical 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 recommendable to prescribe a hypoallergic diet to all dogs which did not respond to treatment against GI 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 hydrolyzed proteins are superior to novel protein diets. When the clinical signs have subsided, it is advisable to maintain a strict diet to avoid recurrences.
What to do if Parasiticide Treatment and Elimination Diet Fail?
In cases of large bowel diarrhea (colitis with typical clinical presentation), a therapeutic trial can be initiated with metronidazole (20-25 mg/kg p.o. BID during 5-10 days), and addition of fiber to the diet. However, sampling of mucosal biopsies prior to further treatment may be the best course of action.
The existence of the syndrome of idiopathic small intestinal bacterial overgrowth (SIBO) is currently questioned by most veterinary gastroenterologists. However, all agree that a number of dogs with chronic intestinal disease respond well to antimicrobial treatment (antibiotic responsive diarrhea or ARD). The etiology 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 immunoglobulins A (IgA). Clinical signs may vary considerably: chronic, recurring, mostly small intestinal diarrhea is frequent (although large intestinal signs may also occur). Additional signs include borborygmi, flatulence, decreased appetite and weight loss. Once parasite treatment followed by 3-4 weeks elimination diet have been attempted without success, many small animal gastroenterologists consider ARD a possible diagnosis and recommend oral antimicrobial treatment with metronidazole (10-20 mg/kg BID), tylosine (10-20 mg/kg once daily or BID) or tetracycline (10-20 mg/kg TID). Interestingly all 3 substances may also exert immunomodulating or even anti-inflammatory effects on the intestinal mucosa in addition to their antimicrobial properties.
In dogs presented with chronic diarrhea and systemic signs including lethargy, a global approach must be adopted, and the approach must be more aggressive. A detailed clinical exam followed by 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 or TLI). 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, sampling 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 biopsies may be quite superficial. This is why a more invasive, surgical approach with collection of transmural biopsies may be preferred in some cases. Several studies have shown that proper interpretation of intestinal biopsies is not easy, and requires good communication between clinician and pathologist.
Specific Laboratory Tests of Interest in Dogs with Chronic Diarrhea
Serum albumin concentration: Although serum albumin may be mildly to moderately decreased with many intestinal diseases with involvement of the small intestine, severe decreases (< 20 g/l) are usually associated with a syndrome called protein-losing enteropathy, and are often associated with panhypoproteinemia. Possible causes include IBD or lymphangiectasia. Patients with severe hypoproteinemia are usually severely affected and may present with ascites, hydrothorax, and/or subcutaneous edema. They usually require more aggressive and intensive treatment.
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 for intestinal diseases 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, however its sensitivity is low.
Fecal alpha1-proteinase inhibitor can be a useful early marker of intestinal protein loss. In patients with protein-losing enteropathy, it may also be used for monitoring the evolution of disease and/or assess response to treatment.
Fecal culture: It 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 diarrhea in dogs, a systematic approach is necessary to narrow down the list of differential diagnoses and initiate a successful treatment. In spite of these efforts, some complicated cases may not show the expected response to treatment and lead to frustrations for dog owner and veterinarian. However, such cases are the exception and not the rule...
References
References are available upon request.