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Chronic Large Bowel Diarrhea in a Young Irish Setter

Tammy J. Owens, DVM, MS, DACVN, University of Saskatchewan, Saskatoon, Saskatchewan, Canada


|November/December 2021|Peer Reviewed|Web-Exclusive

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Clinical History & Signalment

Edward, a 3.5-year-old, 75.2-lb (34.1-kg), intact male Irish setter was presented with recurrent large bowel diarrhea (Table) of 3 months’ duration during the winter months. He lived in a consistently subfreezing area. The diarrhea was initially managed by his owner (a clinician) and an internal medicine service. Three months after diarrhea was observed by his owner, Edward was presented to a nutrition service.

Episodes often started with vomiting, decreased interest in food, and mildly decreased energy, and then progressed to diarrhea with hematochezia, mucus, and tenesmus that continued even as initial clinical signs improved. The episodes only sometimes coincided with a change in routine; the 2 episodes prior to presentation developed precipitously after consuming a few pieces of buttered popcorn or a bite of buttered toast. Fluctuations in body weight were minor, with small decreases (<2%-5% body weight) during acute episodes that quickly returned to baseline when Edward was feeling well.

A trial of fenbendazole (50 mg/kg PO every 24 hours for 3 days; 2 doses repeated 2 months apart), 2 trials of metronidazole (11 mg/kg PO every 12 hours for 7 days; only one trial was completed, the second was stopped after a few days due to lack of response and apparent nausea with administration), and a trial of tylosin (12 mg/kg every 12 hours for 14 days) were completed without resolution or consistent improvement. Edward’s owner had also administered probiotic kaolin-pectin paste and probiotic capsules (per package directions). An elimination food trial using a dry, extruded, hydrolyzed soy protein diet fed for 4 weeks elicited no clinical response. 

Edward was up to date on routine preventive care and had no significant medical history other than mild separation anxiety. His owner characterized him as a high-energy dog.



Clinical Sign Small Bowel Large Bowel
Constipation or tenesmus Rare Common
Frequency Normal or increased (2-3× per day) Increased (>3× per day)
Urgency Not common Common
Volume Increased Decreased
Mucus Rare Common
Hematochezia Not common Common
Weight loss Common Not common

Physical Examination

Physical examination was unremarkable. Edward was bright, alert, and responsive, and vital signs were normal. BCS was 4/9, and muscle condition was normal. No masses or irregularities were palpated. Rectal examination revealed a smooth, normal-feeling prostate; no enlargement was appreciated by palpation.


Differential diagnoses for chronic large bowel diarrhea include infectious diseases (eg, parasites), chronic inflammatory enteropathy, food-responsive enteropathy, functional disorders (eg, stress colitis, idiopathic diarrhea), and neoplasia.1-3

CBC, serum chemistry profile, and urinalysis were performed 3 weeks prior to presentation at the nutrition service; results were normal. Fecal flotation and Giardia spp and Cryptosporidium spp immunofluorescence assays were negative. ACTH stimulation test did not support a diagnosis of hypoadrenocorticism. Abdominal radiography and ultrasonography were unremarkable except for mild prostatic enlargement consistent with the patient’s intact status. PCR tests for Escherichia coli, Salmonella spp, Campylobacter spp, Cryptosporidium spp, Clostridium perfringens enterotoxin, Clostridium difficile toxins A and B, canine distemper virus, canine circovirus, enteric coronavirus, parvovirus, and Giardia spp were negative. Consultation with a veterinary nutritionist was pursued prior to intestinal and colonic biopsy. 

Chronic inflammatory enteropathy, food-responsive enteropathy, and chronic idiopathic large bowel diarrhea were the top differentials based on diagnostic results. The etiology of chronic idiopathic large bowel diarrhea is not completely understood, but some dogs appear to respond to a change in dietary fiber, whereas other dogs benefit from behavioral therapy or drugs for treatment of stress; some dogs require both. Need for concurrent administration of medications (eg, loperamide) for control of clinical signs is variable.



Treatment & Management

Studies of dogs with chronic diarrhea have shown that many conditions respond to simple therapeutic trials (eg, parasiticides, diet trials).1,4-6 A variety of interventions and less-invasive diagnostics may therefore be beneficial prior to colonoscopy and other advanced diagnostics.7

A subset of dogs with chronic idiopathic large bowel diarrhea respond positively to some changes in dietary fiber, and fiber has the potential to more broadly support gut health. A fiber supplementation trial was therefore administered by adding unflavored psyllium husk (0.3 g/kg every 24 hours divided between meals, then slowly increased over a few weeks to slightly <1 g/kg every 24 hours) to Edward’s current hydrolyzed diet. Bowel movements were monitored, and his owner was instructed to return to the previous tolerated dose if adverse effects (eg, overly soft stool) occurred with the increasing dose. The dose was re-evaluated at follow-up appointments based on clinical signs. Treatment with psyllium fiber was deemed successful based on no recurrence of large bowel diarrhea. 

The published median effective dose for coarse psyllium husk is 1.33 g/kg every 24 hours5; however, a range (0.31-4.9 g/kg) has been effective.5 Patients with no change after receiving doses at the upper end of the published effective range (ie, 4.9 g/kg every 24 hours) are considered nonresponsive.5


  • A fiber trial should be used in patients with large bowel diarrhea that are stable and unresponsive to other initial interventions.
  • Psyllium fiber has a unique combination of attributes (see Properties of Psyllium Fiber) and has been beneficial in treatment of multiple diseases in several species.
  • The estimated median effective dosage of coarse psyllium fiber is 1.33 g/kg (range, 0.31-4.9 g/kg) every 24 hours5 and should be titrated to effect. The author typically starts at the lower end of the range for large bowel diarrhea (dividing the dose over daily meals) and increases the dose slowly as needed. Higher doses may be needed in some patients.
  • The effective dose may change depending on base diet, types and amounts of fiber already present, variabilities in individual patient gut microbiome, and other factors. For example, although total dietary fiber concentrations may be known for some pet foods, the amounts of individual fiber types and their total physiologic effects on the gut are typically unknown. Providing a diet that contains a special fiber blend and adding supplemental fiber is often based on trial and error and titration to effect.
  • A large subset of dogs with chronic idiopathic large bowel diarrhea responds to fiber.1,3,5 Further diagnostics or additional treatment (eg, behavioral therapy, antispasmodic medications) may be needed. Inflammatory bowel disease may need to be ruled out in patients that do not respond to routine interventions (eg, fenbendazole, metronidazole, tylosin, fiber). 

Prognosis & Outcome

Edward’s stool quality was consistently good (ie, formed, moist to semimoist, left little to no residue when picked up) while he was being fed the hydrolyzed soy protein diet with added psyllium; however, his willingness to consume the diet decreased. Water and baked, skinless, boneless chicken breast were added to his diet without apparent negative effect. Long-term continuation of feeding chicken breast could have unbalanced his diet if it exceeded 10% of total daily kilocalories in combination with the psyllium supplement, as treats and other foods may unbalance a diet when they exceed 10% of kilocalorie intake. 

Given Edward’s apparent tolerance for chicken, it is possible he did not need a hydrolyzed protein diet; however, there is not always perfect correlation between tolerated commercially processed ingredients and those prepared in the home. His owner continued to avoid butter as a possible trigger, but other dairy products were successfully reintroduced several weeks later without issue.  

After several weeks, no further episodes occurred. Based on Edward’s tolerance of a nonhydrolyzed, chicken-based diet and the need to feed at least 90% of total daily kilocalories from a complete and balanced food, he was transitioned to a therapeutic GI diet containing a prebiotic fiber blend with psyllium. His owner was instructed this was to be a long-term diet or an attempt should be made to transition back to the original diet.

The prognosis for most dogs with fiber-responsive large bowel diarrhea is good to excellent. Success in reducing or eliminating psyllium supplementation or using over-the-counter diets was only 20% to 50% in previous studies3,4; however, these were referral cases, which may underrepresent success in the general population.


Psyllium, which is derived from seeds of Plantago spp plants, provides predominantly soluble dietary fiber that can increase digesta viscosity or form mucilaginous gels in water at sufficient concentration.11 Psyllium is less rapidly fermented in the colon compared with other soluble dietary fiber, which benefits the distal colon by allowing fermentation to occur there. 

Psyllium increases concentrations of short-chain fatty acids, particularly n-butyrate, after fermentation by gut microflora. Short-chain fatty acids are beneficial in restoring microbial balance8 and acidifying colonic contents. N-butyrate is preferentially oxidized by colonocytes, increasing colonocyte proliferation, mucosal weight, brush border enzymes, and mucin secretion. Intestinal function is enhanced by increased absorption of water and electrolytes, improvement of the mucosal barrier and immune function, and inhibition of pathogenic bacteria.10,11 

Psyllium also promotes peristalsis, cleansing, and viscosity or softening; slows transit time; and binds certain toxins.11 Psyllium has demonstrated success in patients with chronic idiopathic large bowel diarrhea and possesses unique attributes as a single fiber source in the author’s experience.


Large bowel diarrhea develops when the colon’s capacity to resorb water or store feces decreases. Mucosal damage and inflammation can lead to hematochezia and mucus in the stool. Multiple disease processes can result in large bowel diarrhea, some of which can be challenging to definitively diagnose. Despite extensive testing, many diagnoses are based on response to treatment.4 Following an evidence-based treatment protocol may circumvent advanced diagnostics and increased costs.7 

Large bowel diarrhea can respond to parasiticides, antibiotics, or diet changes. Trials of highly digestible GI diets or diets with novel or hydrolyzed protein can be given safely in many cases. Overall, many enteropathies are food responsive.4,6,7 Younger dogs with large bowel diarrhea are likely to respond to diet change (eg, highly digestible diets, hydrolyzed diets, high-fiber diets) alone.4 Dogs that do not respond to highly digestible or novel-protein diets may benefit from a high-fiber diet or psyllium fiber supplementation. 

Fiber-responsive large bowel diarrhea is a subset of chronic idiopathic large bowel diarrhea (a diagnosis of exclusion) that may overlap with irritable bowel syndrome or other stress-related issues.1-3,5,8 Chronic idiopathic large bowel diarrhea may account for 20% to 25% of referral cases,1,5 but prevalence in the general population is less understood. Clinical signs may be potentiated by stress, certain dietary triggers, or other factors not currently understood, but specific signs are not predictive of response to fiber.

Dietary fiber is classified by structure, solubility, viscosity, fermentability, ability to adsorb substances, and effect on GI physiology. Viscosity and fermentability are especially pertinent to digestive health. Viscous gels slow gastric emptying, increase small intestine transit time, and slow absorption or reduce digestibility of some nutrients.9,10 Fermentation by gut microflora has the potential to be highly beneficial but can be detrimental if excessive. 

Patients with chronic idiopathic large bowel diarrhea that respond well to fiber supplementation or a high-fiber diet generally have a positive prognosis. Fiber-responsive large bowel diarrhea can affect dogs of any age, weight, or breed, and clinical signs are commonly intermittent.1,5 Despite primarily affecting the large bowel, reduced appetite, abdominal pain, slight weight loss, and frequent vomiting are possible. 

Success of psyllium or other fiber supplementation with a desirable insoluble:soluble fiber ratio and fermentability in cases of fiber-responsive large bowel diarrhea is likely multifactorial. Duration of treatment and ability to decrease or discontinue fiber supplementation in patients with fiber-responsive large bowel diarrhea is variable.1,5 In one study, up to 50% of dogs appeared to tolerate a decrease or discontinuation of fiber supplementation after 2 to 3 months.3 Some patients benefit further from additional treatments (eg, behavioral therapy, probiotics, medical management).1,5,8


  • Chronic idiopathic large bowel diarrhea is a diagnosis of exclusion, but the index of suspicion may be increased in some cases (eg, history of stressful events or possible anxiety-related behavior, absence of major abnormalities on physical examination or initial diagnostics, absence of significant or progressive weight loss, minimal or inconsistent response to standard empiric therapies). 
  • Evidence-based protocols (eg, parasiticides, antibiotics, diet trials) prior to advanced diagnostics may avoid unnecessary diagnostics and reduce cost in patients with chronic diarrhea.
  • A highly digestible GI diet or elimination diet trials can be safely given prior to definitive diagnosis. If signs persist, transitioning to a high-fiber diet or supplementing with psyllium fiber may result in a positive response. 
  • Patients with chronic large bowel diarrhea that respond to diet change with fiber supplementation have a good to excellent prognosis.
  • Titrating the fiber dose to effect is important. Administering a dose that is too low may prevent a positive response.
  • Long-term diet therapy and/or fiber supplementation may be needed.


For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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