Selected Diseases of the Large Intestine Causing Vomiting and/or Diarrhea

by Chick Newman

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General Information...Irritable Bowel Syndrome...Fiber-Responsive Large-Bowel Diarrhea...Diet-Responsive Large Bowel Diarrhea...Constipation...Clostridium/Campylobacter-Related Large-Bowel Diarrhea...Idiopathic Colitis

General Information

[If not already examined, it is suggested that the reader consult some of the basic information about the anatomy and functionality of the gastrointestinal tract here.]

In order to deal with the diagnosis and treatment of large-intestine related diarrhea, it is essential to determine that the animal's clinical signs indicate the problem is primarily large intestine in origin. The table here is helpful in making that determination.

The hallmarks differentiating diarrhea primarily of large or small intestine origin are as follows:

Differentiation of Small from Large Intestinal Diarrhea

Small Intestine

Large Intestine

Mucus Rarely present Frequently present
Hematochezzia Absent except in Hemorrhagic Gastroenteritis  Syndrome May be present; appears as streaks of bright red blood on stool surface or mixed with loose stool
Volume Increased Normal or decreased
Quality of Stool Varies from nearly formed to watery, malodorous Loose to nearly formed; mucus may be present and may constitute the entire stool volume
Shape Variable, depending on the water content Normal or reduced in diameter
Steatorrhea Present with maldigestion/malabsorption disorders Absent
Melena May be present; appears as black, "tarry" stool Absent
Color Considerable variation, from tan or dark brown to black (not always melena); may be altered by medications Variable, brown to clear (with increased mucus) or red-tinged (with hematochezzia)
Frequency Usually increased (2-4 times per day) but may remain normal in some patients Increased...3-10 times per day (with normal or decreased volume)
Dyschezia Absent Frequent in dogs; less common in cats
Tenesmus Absent Frequent in dogs; less common in cats
Urgency Present in cases of acute and severe enteritis with rapid transit of large volumes of fluid-filled feces; otherwise normal Frequent
Associated Signs
Weight Loss Occurs with chronicity with both malabsorptive and maldigestive disease Unusual; may occur with severe colitis, diffuse neoplasia or histoplasmosis or if both small and large intestine diseases are present
Vomiting Common in patients with inflammatory bowel problems and acute infectious disorders May occur in 30-35% of patients with acute colitis...sometimes even BEFORE the onset of abnormal stools
Appetite Normal or decreased (or increased in some dogs with inflammatory bowel disease, e.g. Sharpei); may be cyclic, decreasing with flare ups of clinical signs. In cats may be increased with inflammatory bowel disease or lymphoma Usually normal; decreased if there is severe disease (neoplasia or histoplasmosis)
Halitosis May be associated with severe maldigestive or malabsorptive disease Absent
Borborygmus May be present Absent
Flatulence May be present Absent
Fecal incontinence Rare...only when associated with rapid transit of large volumes of water-logged feces May be present
Scooting or chewing at perianal area Absent Occasionally present

It is also important to realize that some diseases of the large bowel, primarily those resulting from a motility disorder, can also result in vomiting as a clinical sign.

Irritable Bowel Syndrome

General Information

This is a poorly understood condition, mostly in dogs, in which the colonic motility is aberrant in the absence of mechanical, microbiological or structural anomalies.( With chronicity, the health and natural turnover of colonocytes are affected and the structure of the colonic villi are then also compromised, further impeding the ability of colonocytes to function as they normally do to absorb water.) Other names include "spastic colon", "nervous colitis" and "mucus colitis". Stressors in the environment have been implicated, especially if the animal is nervous or high-strung or has other behavioral "quirks". Bloating, nausea, vomiting and abdominal pain may occur. Diarrhea is intermittent, hematochezzia is uncommon.


History and ruling out other causes of large-bowel diarrhea and vomiting, including ruling out ingestion of foreign materials (e.g. plants) that can cause an abrasive colits, parasites (whipworms, giardia, hookworms, strongyloides, cryptosporidia, other protozoans,), viral (parvovirus, corona virus, calici virus, feline leukemia virus, feline aids virus (FIV)) and bacterial disease (Clostridium or Campylobacter, Salmonella, pathogenic Escherichia coli, Yersinia), fungal disease (Histoplasma, Aspergillus), Algae (Prototheca) dietary intolerance or allergies, infiltrative diseases (inflammatory bowel, suppurative or eosinophilic colitis, granulomatous colitis, neoplasias), pancreatitis-associated colitis, intussusception, volvulus, strictures, polyps or other masses is the basis of assessing the problem. A work up includes:

[*if the animal responds to fiber, then the condition is referred to as fiber-responsive large bowel diarrhea (see below)]


Once a diagnosis of irritable bowel is ascertained--colonic dysfunction in the absence of structural, biochemical or microbial abnormalities-- treatment may include any of the above dietary modifications, empirical treatment for common parasites (even in the face of a negative fecal exam for parasites or other infectious agents) since negative results do not always rule out the presence of parasites; attempts to identify the sources of then reduce stress in a nervous animal should be undertaken in earnest.

Several authors** believe that drugs which are motility-altering and modify behavior and stress should be given serious consideration. These include antispasmodics, sedatives and opioids. The antispasmodics and the opioids may also offer relief from pain that accompanies spastic colon. The use of anticholinergic-antispasmodic drugs (which inhibit normal gastrointestinal motility) for diarrhea is controversial, however. There are arguments that by inhibiting intestinal muscularis function (and motility) these perpetuate an environment for bacterial overgrowth (which can result in small intestinal diarrhea) and permit accumulation of toxins that are potentially irritating, as well as interfering with normal mixing of ingesta and therefore with digestion of nutrients and absorption of water.

**"Hematochezzia, fecal mucus, tenesmus often characterize large-bowel diarrhea", Johnny D. Hoskins, DVM Diplomat ACVIM, in DVM Newsmagazine October 2004 pp 8s

**"Chronic Idiopathic Large Bowel Diarrhea in Dogs", Michael S. Leib DVM, MS, Diplomat ACVIM, in ACVIM Proceedings, 1999 pp522

Having said that, there are the above authors who argue strongly that the use of these drugs is beneficial in many of these animals. are some of their recommendations:

0.1-0.25 mg/kg of clinidium (or 1-2 capsules) two-three times daily as needed for perceived abdominal pain or diarrhea or stress Contains clordiazepoxide (a sedative) and clonidium bromide, an anticholinergic (decreases intestinal motility)
Propantheline bromide (ProPanthine®)
0.25 mg/kg two-three times daily Is anticholinergic
Hycosamine (Levsin®)
0.003-0.006 mg/kg two-three times daily Is anticholinergic
Dicyclomine (Bentyl®)
0.25 mg/kg two-three times daily Is anticholinergic
NOTE:  Anticholinergics can worsen diarrhea!


The probability that patients with irritable bowel syndrome will be cured is guarded; however elimination of environmental and behavioral issues that predispose animals to this condition can reduce incidences to intermittent occurrences, rather than continuous and chronic. In some cases, dietary modification can be helpful, if the animal has fiber-responsive diarrhea or dietary hypersensitivity-intolerance issues, rather than straight irritable bowel problems.

Fiber Responsive Large Bowel Diarrhea

General Information

Clinical signs in animals with this problem are similar to those with irritable bowel syndrome. Abdominal pain, tenesmus and in some cases vomiting may be present. Unlike irritable bowl syndrome, hematochezzia is more likely to be present in animals with fiber-responsive diarrhea than with irritable bowel syndrome. Some patients may be sensitive to stressors and/or have nervous or other predisposing personality traits. But in many patients, few or no other concurrent risk factors are identified.


Diagnosis is exactly as described for irritable bowel syndrome...eliminating all other causes of large bowel diarrhea first. This is followed by dietary elimination trial (see above) including observation for a positive response to the addition of fiber to the diet (see Treatment).


The addition of a primarily soluble (with a small amount of insoluble) fiber to the diet, such as psyllium (Metamucil®, Proctor and Gamble), or other soluble fibers (pectin, guar gum, mucilages and hemicelluloses, soy fiber) or combination soluble-insoluble fiber source (oat bran, beans, potato or pumpkin) can result in significant improvement in many dogs.

Several mechanisms for the beneficial effects of fiber have been proposed.

An example of a protocol is as follows:

Psyllium (Metamucil®), approximately 2 TBS daily added to highly digestible diet, will produce excellent responses in about 80% of dogs with chronic, large bowel diarrhea. Some dogs will require more...some less psyllium to achieve a desired result. In some instances, when the animal responds to fiber, fiber amount can be gradually reduced and even eliminated. And in a percentage of these animals, the diet can also be changed to a traditional or even a grocery store brand without recurrence of the diarrhea.

Diet-Responsive Large Bowel Diarrhea

General Information

In addition to fiber-responsive forms of large bowel diarrhea, some animals may respond to the form and composition of the diet. Allergies or intolerances to one or more nutrients may trigger inflammatory allergic reactions anywhere along the intestinal tract, including the large intestine. Additionally, dietary intolerance or maldigestion of certain nutrients may allow the offending ingredient to reach the large intestine, and trigger an inflammatory reaction. Inflammation of colonocytes affects their health and functionality (mostly their ability to absorb water).


After eliminating the most common causes of large intestinal dysfunction, a dietary elimination trial is initiated. For this purpose, the animal's diet is hypoallergenic, meaning is consists of ingredients to which the patient has not been heretofore exposed (and therefore are unlikely to trigger an allergic inflammatory reaction). The elimination trial is maintained for at least 10 weeks and the effect of the diet on the incidence of undesirable clinical signs is assessed. During the trial period, the animal may not be offered any other foods, treats, scraps, etc.; this provision must be strictly enforced if the test is to provide the needed information about dietary causes of the problem(s).

As an alternative to a hypoallergenic problem, some animals may respond to a highly digestible, low-residue diet. In this case, there is no restriction on the source of the diet (an animal may have eaten the ingredients at some time in its life); the requirement is only that the diet be easy to digest. In general, digestible diets contain simple proteins (or partially or completely digested proteins), and are low in fat and fiber (though some animals may respond favorably to the addition of fiber). The same adherence to the rules of compliance stated in the preceding paragraph for a hypoallergenic dietary elimination trial apply here, as well.


A favorable response to a dietary elimination trial usually results in a permanent dietary change to the more acceptable diet. In some instances, the addition of anti-inflammatory medications is included in the diet treatment regimen, particularly when diet results in only partial improvement. There are many anti-inflammatory medications, but the most common drugs are metronidazole and an oral corticosteroid.


General Information

Constipation is characterized by infrequent, difficult or absent defecations, resulting in the retension of feces in the colon and rectum. Prolonged contact of feces with the colonic mucosa results in the continued absorption of water and furthers the drying (and hardening) of feces, perpetuating the cycle of constipation and straining. In cats, there is a syndrome known as idiopathic megacolon; the colon dilates and is poorly motile or, worse, without any ability to propel fecal material. This is likely the result of an unknown defect(s) in the colonic smooth muscle layers or, less likely, in the intrinsic neural pathways including the myenteric plexi (see general anatomy) within the colon.

Stretch receptors within the serosa in the colon (or even serosal surfaces of other abdominal viscera) respond to distension and transmit signals to the vomit center of the brain inducing the vomit reflex; constipation, therefore, can be associated with vomiting, though this cause of vomiting is often overlooked.


There are numerous potential causes of constipation, many of which are listed separately. In general, factors potentiating the development of constipation include dietary, behavioral, pain, recto-colonic obstructions, neuromuscular problems, electrolyte imbalances (which affect muscularis function) and certain drugs and classes of drugs which are motility modifiers. Additionally, in some instances constipation is idiopathic, meaning the cause is unknown. An example is idiopathic megacolon in cats, mentioned above.

Clinical Presentation

In general, there is a history of failure to defecate over several days or longer and the animal may have been seen squatting repeatedly, sometimes crying out. Some patients maintain a hunched-up posture (due to abdominal discomfort) and dyschezia and in some instances hematochezia (indicating mucosal irritation or anorectal problems) may also be apparent. Frequently, animals are anorectic and some vomit. Due to irritation of the colonic mucosa, there may be excessive secretion of mucus that is expelled from the anus in the absence of actual fecal matter. Additionally, if there is damage to the innervation of the anus (the anal sphincter muscle), fecal incontinence may also be a concommitant presentation.


The history (above) and palpation of the abdomen, revealing a colon full of firm feces, are usually enough to establish the preliminary diagnosis. Determining the underlying etiology of the problem requires additional information, such as dietary history, environmental changes (litter box, other cats and stressors), medications. Routine blood work and urinalysis is essential (looking for underlying diseases and also for the evaluation of the metabolic consequences of prolonged constipation, such associated vomiting and dehydration). Rectal palpation for anocolorectal anomalies or masses and for assessment of the size and conformation of the prostate (if applicable) is an extremely important component to a thorough patient evaluation.

Abdominal radiographs are useful to confirm the diagnosis and also to evaluate the extent of fecal impaction including whether the colon is distended (extreme dilation of the colon is often associated with feline idiopathic megacolon), whether there is foreign material (e.g. bone chips) and whether there are radiographic signs associated with joints, vertebrae or other bones (pelvis) suggesting the presence of pain or neurologic impairment.

Stool in colon...

There is much stool in the colon (and much urine in the bladder) of this constipated animal

Additionally, the size and position of the prostate gland can also be determined radiographically and via rectal palpation. Prostamegaly (enlargement of the prostate gland) due to benign prostatic hyperplasia (BPH) is a common cause of constipation in middle aged or older intact male dogs. Other prostatic pathologies, including paraprostatic cysts or prostatic tumors can occur in any males; the enlarged prostate or associated cysts compress/obstruct the neighboring colon, reducing the lumen diameter and thus impeding the free passage of feces.

Other diagnostic modalities are sometimes necessary. If intraluminal obstruction is suspected, then contrast radiography and/or colonoscopy is useful to delineate the size and location of the obstruction.  If neurologic impairment at the level of the spinal cord segments innervating the colon is suspected, then contrast radiography, MRI or electrodiagnostic studies of suspected portions of the verterbrae-spinal cord are recommended. If prostamegaly is ascertained, then further evaluation of the prostate is required, including ultrasonographic studies with or without biopsy or cytology and microbial culture, as well as assessment via radiographic contrast studies (a "contrast urethrogram").


The goals of treatment are to promote normal colon motility, eliminate the underlying cause(s) and to prevent recurrence, if possible.

When constipation is mild and the animal is minimally uncomfortable without other systemic signs and without local complicating factors such as obstructions, then providing adequate hydration coupled with the use of oral cathartics may be beneficial. These include bulk-forming fiber, various types of laxatives, and certain promotility agents. Some of these are listed here.

Enema and suppositories may also be administered to soften hardened feces, if deemed necessary. Enemas consist of warmed liquids of varying formulations. Suppositories are long-acting solids retained in the rectum that either stimulate colonic motility, lubricate or soften impacted feces over a period of time.

When impaction is stubborn and unresponsive to the above simple procedures, a combination of warm enema with mechanical manipulation of the hardened fecal matter is undertaken, usually requiring general anesthesia and a gentle finger along with abominal massage in order to facilitate evacuation of the colon. To avoid excessive trauma to the intestine the procedure may require staging over a period of days during which careful manipulation and accompanying enema is repeated at regular intervals. Multiple anesthesias are then required.


Prevention initially means eliminating underlying predisposing conditions. Ingestion of potential obstructions, or irritants (such as hair) must be stopped. In cats who groom excessively, this means brushing regularly and removing any potential factors that may instigate excessive grooming (allergens, parasites and stressors) . Additionally, provide clean litter and placing litter boxes where cats will use them to encourage frequent defecations. Dogs should be exercised and provided frequent opportunities to defecate.

Medications that may promote retention of feces may require changes in dose or frequency. Consult with your veterinarian before making changes, however.

In some instances, changes in diet or modification of the current diet may be required. Stool softeners and periodic use of a laxative may also be considered.

When a serious underlying cause is identified during the diagnostic work up, then this must be addressed and, hopefully resolved (the individual diseases and their treatments are not specifically addressed here). In some instances, such as idiopathic megacolon in cats, subtotal colectomy (removal of the diseased, non-functional portion of the colon) is the only solution. Diarrhea and/or frequent defecation may result, though in most animals there is a positive outcome by four weeks following the surgery. Fecal incontinence occurs on occasion, but is rare.

Clostridium/Campylobacter Enterotoxicosis

General Information

Campylobacter and Clostridium are bacterial genera often implicated as causes of large-bowel diarrhea. The exact role of these organisms in perpetuating intestinal disease is not fully clear since some clinically assymptomatic dogs and cats...up to 50% in certain settings, such as animal shelters... are also shedding these organism in their feces.

Clinical Presentations

Campylobacter jejuni is a gram-negative curved bacterium which may also be a significant human pathogen. In some animals shedding campylobacter organisms, clinical signs are associated with infection and include watery, mucoid diarrhea that occasionally contains blood, that may also be associated with vomiting and tenesmus. In most cases the symptoms resolve after 5 to 15 days. These signs are attributable to an ulcerative enterocolitis resulting from the enteroxins secreted by the organism into bowel. These damage the integrity and functionality of the villi, mediated by a generalized inflammatory reaction affecting the mucosa and deeper layers.

Clostridium difficile is a pathogen of humans, causing a severe form of colitis, referred to as pseudomembranous colitis. Some dogs and cats have been shown to shed this organism, but there is no evidence the organism is responsible for colonic disease in either veterinary species.

Clostridium perfringens is a gram-positive rod-shaped, spore-forming bacterium. While also part of the normal flora of many animals, the organism has nevertheless been incriminated as the causative agent of the following disease variations: acute, necrotizing, hemorrhagic enterocolitis, acute nocosomial diarrhea and chronic diarrhea. These decriptions represent variations in the severity of the manifested clinical signs and extent of damage to the intestinal tract. In general, clostidial diarrhea is characterized as watery to soft with or without blood, mucus and tenesmus. Most cases are self-limiting,with symptoms resolving over a few days with antibiotic treatment. However, some severe infections, such as the acute necrotizing form, can be fatal.


A presumptive diagnosis of Campylobacteriosis is made by observing the morphologically characteristic "W-shaped" bacteria in stained preparations or spiral, "S-shaped" bacteria via wet-mounted darkfield microsopy or phase-contrast microscopy of fecal material. The diagnosis is presumptive because as mentioned earlier, some clinically normal animals also shed this organism. A more definitive diagnosis is based upon the isolation/culture of large numbers of organisms, using special selective growth media. The organism is, however, fastidious in its requirements and can be difficult to grow.

Presumptive diagnosis of Clostridium via stained fecal smears is made by looking for the characteristic "safety-pin" appearance of the organism, with the "bend" of the "pin" actually the result of the presence of a refractile bacterial spore. Most samples also contain a large number of white blood cells in the feces. A more definitive diagnosis can be obtained by assaying for the clostridial enterotoxin in the feces.


For treating Campylobacter infection, the antibiotic of choice is erythromycin, 10-15 mg/kg three times daily for a week. Unfortunately, erythromycin itself has resulted in vomiting or anorexia in many animals. Alternative treatment options are Enrofloxacin, 5-10 mg/kg once daily or Neomycin, 10-20 mg/kg three times daily. There is anecdotal evidence that chloramphenical and doxycycline may have activity against campylobacter. Some veterinarians have reported that bismuth subsalicylate (the active ingredient in Pepto Bismol) also has inhibitory effects on Campylobacter. Most infections are cleared by seven days of treatment.

Treatment of Clostridium infection can be accomplished with most of the penicillin drugs, metronidazole, tylosin, clindamycin or chloramphenicol at standard doses for 5-7 days. Tylosin has been a mainstay of treatment when for whatever reasons, infections are recurrent. Once daily treatments for a week or so, as needed, usually control this problem reasonably well.

Chronic Idiopathic Colitis (aka Inflammatory Bowel Disease)

General Information

The subject of inflammatory bowel disease (IBD) in animals is extremely complex. Basically the term is rather a description of the histological appearance of the affected portion of the intestine; it says nothing about the underlying cause(s). Any portion of the gastrointestinal tract can be affected . When the large bowel is the principle affected portion of the intestine, the syndrome is referred to as inflammatory colitis (which seems redundant), or idiopathic colitis. (This discussion will focus on the manifestations of large-bowel IBD, though the features may be similar histiologically to small-bowel IBD and sometimes gastgric-IBD; in fact, these latter conditions may occur concurrently and/or may be clinically more prevalent than large-bowel IBD.) IBD is classified by the preponderance of one or more types of inflammatory cells in histologic preparations of the diseased intestinal mucosa and deeper layers: lymphocytes, plasma cells, neutrophils, eosinophils and histiocytes. In general, all forms of large bowel IBD share similar clinical manifestions: large bowel diarrhea of varying severity. Current hypotheses regarding the initiation of IBD include genetics, dietary stimuli, allergy (food or otherwise), unexplained alterations in mucosal permeability ( ...presently there is speculation that there are abnormal responses of mucosa lymphoid tissue to intestinal bacterial populations due to alterations in the normal intestinal mucosal barrier that would otherwise prevent such interaction(s).) and psychological factors. The damage from inflammation affects the integrity of the intestinal barriers to irritants and toxins, further perpetuating continued secondary damage and more inflammation. The disease is diagnosed by excluding other causes of colitis, including those described here and elsewhere, such as parasites, fungal and bacterial causes.

There is a syndrome in Boxers and some other breeds (documented in a Doberman, Alaskan Malamute and a Mastiff) called "Histiocytic Ulcerative Colitis". Like other dogs with large bowel IBD, these animals show the signs of large bowel diarrhea. Histological appearance of mucosal biopsies reveals a preponderance of histiocytes, in comparison to the other types of inflammatory cells, listed above. This form of colitis has a poorer prognosis than when other inflammatory cells predominate.


Diagnosis of any IBD is usually by exclusion of other gastrointestinal diseases and non-gastrointestinal causes of diarrhea and by intestinal biopsy. Exhaustive work ups include blood panels, including tests for pancreatic diseases (pancreatitis, pancreatic insufficiency), urinalysis, fecal parasite checks, fecal cultures. Sometimes, abdominal ultrasound is in order. If more than the colon is involved (mixed large and small bowel signs), more specialized tests to localize affected intestinal segment(s) are done, such as B12 and folate measurements, a-protease inhibitor (if animal is losing weight). Biopsy can be done via endoscopy or my exploratory surgery.


Various treatment modalities may be tried, assuming no other definitive diagnosis is obtained, and biopsy results are consistent with IBD. A good starting place is to try a dietary elimination trial, to rule out a dietary hypersensitivity (or intolerance). This requires a novel protein or non-allergenic ingredients (e.g. hydrolyzed protein source) for up to 10 weeks; no other foods or treats are permitted during this period, and results are evaluated by evidence of clinical improvement and recurrence of clinical signs after challenge with the original diet/treats.

Antibiotics are ordinarily reservered for small intestinal bacterial overgrowth; however pathogens are a possible cause of large bowel disease (and should be suspected based upon the diagnostic workup, listed above). Metronidazole is both antibacterial and antiinflammatory and so might be beneficial due to both of those properties. Metronidazole is also an efficaceous agent to treat some instances of giardiasis. Tylosin is an efficaceous antibacterial and is anecdotally reputed to have efficacy agains cryptosporidia (more about cryptosporidia will be published here, later). Enrofloxacin has been touted as beneficial in some cases of histiocytic ulcerative colitis (J Vet Intern Med 2004; 18(4):499-504). Drugs with mesalamine, such as sulfasalazine or olsalazine are often beneficial in cases large bowel inflammation.

Antiinflammatory therapy is sometimes a required adjuvant treatment. Corticosteroids such as prednisone and budesonide at anti-inlfammatory or even higher (immune-suppressive) doses may be needed. Lower doses may be efficaceous in many cases or when these are combined with other immune-suppressive medications. Treatment with corticosteroids can result in some or all the symptoms of Iatrogenic Cushings Disease which may be problematic.

Other immune-suppressive drugs may be needed, either alone or in combination with corticosteroids; the use of drugs such as azathioprine or chlorambucil, may allow decreasing the corticosteroid dose (and side effects); however these agents often have lag times of days or weeks before their benefits are appreciated. Cyclosporine is another immune-suppressive medication that is ris being investigated to treat IBD. It is currently approved to treat atopic dermatits in dogs (Atopica® (Novartis)), and some preliminary work (ACVIM Proceedings, 2005, Allenspach et al) suggests that this drug is useful in some cases of steroid-refractory severe IBD. More work with this and other forms of treatment are needed.

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