Chronic large bowel diarrhea is a common problem in dogs. Diarrhea is often accompanied by hematochezia, excess fecal mucus, and tenesmus. The frequency of defecation is increased and the quantity of feces / defecation may be reduced. Weight loss is uncommon. Common causes include highly digestible diet responsive diarrhea, whipworms, Clostridium perfringens enterotoxicosis, irritable bowel syndrome, fiber-responsive large bowel diarrhea, and neoplasia. A thorough diagnostic plan should be followed to reach an accurate diagnosis efficiently. The author's diagnostic plan includes multiple fecal examinations, rectal cytology, elimination of dietary indiscretion, feeding a highly digestible diet for 3-4 weeks, treatment for whipworms, laboratory evaluation consisting of a CBC, biochemical profile, and UA, flexible colonoscopy with multiple mucosal biopsies.

After following the above diagnostic plan it is common to identify no abnormalities and hence make the diagnosis chronic idiopathic large bowel diarrhea. In a recent review of dogs with chronic large bowel diarrhea, idiopathic cases were the most common diagnosis and represented 25% of dogs that underwent colonoscopy. In the author's experience, these dogs have either irritable bowel syndrome, fiber-responsive large bowel diarrhea, or Clostridium perfringens enterotoxicosis in which rectal cytology has not identified an abnormal number of spores. The purpose of this presentation is to review these three conditions and demonstrate how they can be diagnosed, and successfully treated in practice.

Portions of the following discussion are reprinted from the author's chapter in Leib MS and Monroe WE (ed.) Practical Small Animal Internal Medicine. WB Saunders, Philadelphia, PA, 1997: 736-738.

Irritable bowel syndrome (IBS) is a commonly diagnosed but poorly described functional disorder of the intestines that occurs in dogs. Synonyms include spastic colon, nervous colitis, and mucus colitis. Colonic dysfunction exists in the absence of structural, biochemical, or microbiologic abnormalities.

Diarrhea is often intermittent and hematochezia is uncommon. Bloating, nausea, vomiting, and abdominal pain may occur. Often stressors can be identified that are associated with development of cyclic clinical signs. Dogs may be nervous, high-strung, or have abnormal personality traits.

The identification of abnormal personality traits or stressors that initiate episodes of diarrhea in a dog with chronic idiopathic large bowel diarrhea is strongly suggestive of IBS. If the dog responds to dietary fiber supplementation, I call the condition fiber-responsive large bowel diarrhea. If the dog does not respond to fiber supplementation, I treat in a trial and error fashion with antispasmodics, CNS sedatives, and opioids and have the owner attempt to reduce stress. The intermittent nature of clinical signs may make assessment of therapy difficult.

Pain can often be relieved by antispasmotic agents and the effects of stressors can be reduced by sedatives. Librax® (Roche) contains the sedative chlordiazepoxide (5 mg) and clidinium bromide (2.5 mg), an anticholinergic agent. A suggested dosage is 0.1-0.25 mg/kg of clidinium or 1-2 capsules PO BID-TID. The drug can be given when the owner first notices abdominal pain or diarrhea or when stressful conditions are encountered, and can usually be discontinued after a few days. Other anticholinergics such as propantheline (Pro-Banthine®, Searle), 0.25 mg/kg PO BID-TID, hyoscyamine (Levsin®, Schwarz Pharma Kremers Urban), 0.003-0.006 mg/kg PO BID-TID, or dicyclomine (Bentyl®, Lakeside Pharmaceuticals), 0.15 mg/kg PO BID-TID have been suggested. Anticholinergics can decrease or inhibit GI motility which may worsen diarrhea. In people, side effects include xerostomia, urinary retention, blurred vision, headache, psychosis, nervousness, and drowsiness.

The prognosis for cure of IBS in dogs is guarded. Affected dogs may have intermittent clinical signs for years. However, environmental and pharmacologic therapy may result in control or reduction of clinical signs. Dogs that respond to fiber supplementation have a very good to excellent prognosis.

Fiber-responsive large bowel diarrhea

The author routinely adds fiber to a highly digestible diet in dogs with chronic idiopathic large bowel diarrhea, even if irritable bowel syndrome has been diagnosed. In cases of fiber-responsive large bowel diarrhea (FRLBD), chronic intermittent or continuous large bowel diarrhea is usually accompanied by hematochezia, excess fecal mucus, and tenesmus. Abdominal pain and vomiting can occur in some dogs. Nervousness, abnormal personality factors, and stressors have been identified in approximately 40% of cases. However, in some of these cases, a temporal relationship to the diarrhea could not be established.