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Colitis-Related Diarrhea in Dogs and Cats
Revised: February 28, 2024
Published: January 01, 2001

Drawing of a dog next to an overturned trash can
Graphic by MarVistaVet

What is Colitis?

In brief, colitis is the term for inflammation of the colon. The chief feature of colitis is a gooey, frequently splattery diarrhea featuring mucus, fresh blood, or both. The stool may start normal then finish soft or may seem gooey throughout. There is often accompanying cramping, gas, and a sense of immediate urgency (the sudden need to run for a bathroom). Vomiting can be a feature of this condition though the characteristic diarrhea is the hallmark. Colitis may be acute (lasting only a few days) or chronic (lasting weeks or months on end). Even in chronic cases, weight loss is usually not a feature of this condition.

What and Where is the Colon?

The colon is another term for the large or lower intestine. For those who do not know the lower intestine from the upper intestine, the colon/lower intestine is basically the last segment of the digestive tract. Before reaching the colon, food has been extensively processed. It has already been chewed up in the mouth, acid-treated in the stomach, and enzyme-treated in the small intestine. Most of the nutrients have already been absorbed by the time the undigestible leftovers have passed down to this last part of the GI tract.

But the digestive tract is not through with the food remnants quite yet and this is the where the business of the colon comes in. The colon has three functions: absorbing water, storing stool, and further digesting unabsorbed nutrients. The bacterial population in the colon is about 10 times more dense than that of the small intestine. These bacteria, often referred to as  good or helpful bacteria, take fibers that were undigestible to the host and actually process them into biochemicals that provide nourishment to the colon cells. The bacteria that live in the colon not only feed the cells of the colon, but they also control the pH of the colon environment so that excreted toxins are trapped in the stool and will be pooped away rather than reabsorbed back into the body. They also produce flatulent gases and pigments that lead to the ultimate color of stool. The relationship we have with our colon bacteria is mutually beneficial and is an excellent example of symbiosis.

Diarrhea Can Come from Either the Small Intestine or the Large Intestine and the Approach is Different in Either Case

In classifying diarrhea, it is important to determine whether the problem originates from the small intestine, the colon, or possibly both.  Small intestinal diarrheas tend to be more serious as they involve fundamental problems with obtaining nutrients from food. Diarrheas of the colon are less debilitating but still uncomfortable. The following are characteristics of large intestinal diarrhea:

  • Large intestinal diarrheas are generally not associated with weight loss, and patients generally have normal energy levels and normal appetites.
  • Large intestinal diarrhea is associated with straining to defecate. Often, this straining is unproductive, leading to the erroneous conclusion that the patient is constipated.
  • There is cramping, flatulence, and a sudden sense of urgency. The pet may not be able to get to an appropriate area before the diarrhea erupts.
  • There is often mucus or slime in the stool
  • There is often fresh blood in the stool.
  • The stool may begin looking normal and formed but finish as a puddle.

A diagnosis of colitis is generally straightforward given the above classic findings, though how to proceed depends on the signs. Is the problem acute (i.e., suddenly there) chronic (has been happening for several weeks regularly), or episodic (happens then goes away, then happens again)?

Colitis Suddenly (Acute Colitis)

A pet that has sudden symptoms of colitis probably has stress-related colitis (common after boarding, moving, severe weather, or other change) or dietary indiscretion-related colitis (usually involves treats or raiding the garbage). These episodes are generally minor and can be cleared with a short course of medication such as metronidazole or sulfasalazine and/or dietary therapy. Parasites, especially Giardia and whipworms, can also cause colitis and the pet may be tested for those to rule them out or be dewormed. In general, a few days of medication and a bland diet should resolve the problem and the pet will be back to normal quickly. During recovery, it is common for the pet to have no stool at all for a couple of days. This is normal and not a sign of constipation. If, however, the pet's diarrhea is not clearly improved in two to three days, contact the veterinarian to see if further testing is needed.

Diagnosing the Cause of Colitis

If the symptoms of colitis have been going on for a month or more or if they keep recurring and resolving over and over, then a medical workup is needed. It is important to make sure simple causes of colitis have been ruled out, so parasite testing becomes especially important.

A good fecal examination for worms and coccidia plus additional testing for giardia should be performed. If any of these tests are positive, then obviously the parasite in question can be addressed; it is always best to identify the cause of the colitis if it is possible to do so. That said, even if these tests are negative, it is still a good idea to include a broad spectrum de-worming and coccidia treatment, which should probably be given as these treatments are safe and inexpensive. Whipworms, in particular, are difficult to detect and commonly cause colitis symptoms. We want to be sure we have ruled out the simple causes of colitis before more advanced diagnostics begin. Similarly, a week or so of metronidazole, sulfasalazine, or tylosin may effectively treat a toxin-producing Clostridium perfringens infection and potentially solve the entire problem. Of course, a basic blood panel and urinalysis are in order, as they are with any chronic disease, to assess the patient's general health.

After all the preliminaries above are covered, the patient is assessed for results. If it doesn't look like one-time treatment of deworming, antibiotics, or diarrhea medication is going to solve the problem, then diagnostics continue to the next level. Most reference labs now have the capability to do PCR (DNA) testing for more obscure organisms such as Tritrichomonas in cats, Cryptosporidium in dogs and cats, and more. This is testing uses a fecal sample and can be used to non-invasively rule out unique infections.

The last step in colitis diagnostics is a colonoscopy with biopsies with the idea of examining colon tissue under a microscope to classify the inflammation. Depending on the type of cells infiltrating the colon lining, the colitis can be classified as lymphocytic/plasmacytic (a form of inflammatory bowel disease), histiocytic (which tends to stem from a type of E. coli infection,) or not truly colitis at all and the entire problem may be the psychosomatic condition known as irritable bowel syndrome. All these conditions have different treatments.

A colonoscopy requires a period of fasting (usually a couple of days) and enemas or some other kind of fluid to clear the colon of residual stool so the naked tissue of the colon can be viewed and sampled. Referral to a specialty hospital is likely needed. Colonoscopy is performed under general anesthesia and not every patient is a candidate for that, plus expense may be a concern. These disadvantages must be weighed against the quality and quantity of information that can be obtained by evaluating a tissue sample from the colon.

Depending on your pet's specific case and the cause of colitis, your veterinarian may or may not recommend antibiotics. 

Management Tips for Colitis

Colitis is best managed when its cause is known and specific therapy can be instituted. When this is not possible, symptomatic management is often attempted. The following are therapeutic medications and strategies that can be helpful in the treatment of colitis.

Metronidazole and Tylosin

These medications are antibiotics with anti-inflammatory properties in the large intestine and the ability to kill harmful organisms.


This medication consists of a sulfa antibiotic bound to a salicylate anti-inflammatory. The sulfa bond protects the anti-inflammatory medication until it gets to the large intestine, thus saving the anti-inflammatory effect for the disease of the large intestine. This is an effective medication but is typically given three times a day, which is an inconvenience. Cats are sensitive to salicylates, thus this medication is primarily used in dogs.

Dietary Fiber

The role of fiber in colitis is confusing as there is an assortment of fiber preparations (soluble fibers, insoluble fibers, and mixtures). In general, colitis is felt to be a fiber-responsive disease, but there are so many combinations of fiber types that it is hard to know what the patient may be responding to. Insoluble fibers, like cellulose, bulk up the stool and are stimulating to the colon lining. This may not be what is in order if the colon is already irritated although giving some structure to diarrhea may be a good thing. Soluble fibers, like psyllium, are fermented by the colon bacteria into nutrients for the colon cells, which helps them heal. Prescription high fiber diets often have a mixture of soluble and insoluble fibers that have been scientifically determined to help colitis patients. Alternatively, a low residue diet (one of high digestibility) could be used and soluble fiber added to it. The idea with this strategy is to have maximum intestinal absorption of nutrients in the small intestine so less material enters the colon, but once the material gets there the soluble fibers added help the colon cells to heal.

Prebiotics such as Fructooligosaccharides (FOS)

Prebiotics are basically food for beneficial colon bacteria. Feeding a diet rich in prebiotics promotes a healthy colon bacterial population, which in turn helps resolve diarrhea.

Fructooligosaccharides are carbohydrates involving fructose (fruit sugar) units attached to glucose (starch sugar) units. Regular dietary carbohydrates are digested by the bacteria of the small intestine, leaving only the undigested fibers and other dregs for the teeming masses of the large intestine. FOSs are not fibers but they are digested in the large intestine (not the small intestine) in the same way that fibers are, yielding the same biochemicals that fibers do. Why is this good? Tests in healthy animals indicate that this will help remove pathogenic bacteria from the large intestine and promote growing helpful bacteria. Think of it as an anti-crime program in the New York City of bacteria. Diets that contain FOSs or other prebiotics may be helpful in managing colitis. Several such diets are available.


A probiotic is a protected culture of live helpful bacteria that can colonize the patient's intestine. The bacteria must be protected from the acid of the stomach so as to survive to the lower intestine. Once there, the bacteria make a home and make by-products that are nourishing to the intestinal and local immune system cells. There are numerous products on the market for both humans and animals; the problem has been that since these products are not regulated as drugs by the FDA, they are required only to be safe, not necessarily effective. In fact, a recent study found that most such products do not actually contain the live cultures they are advertised to contain. If you want to add a probiotic to a pet's regimen, we recommend sticking to well-established veterinary companies. That said, probiotics are mainly made to colonize the small intestine, not the large intestine, so their benefit in colitis is still somewhat speculative.

Elimination Diet

Colitis can result from a food intolerance (an example would be lactose intolerance from which numerous people suffer). Intolerances can result from dyes, preservatives, contaminants or even natural proteins in the food. Similarly, colitis can result from an actual food allergy. The solution for these intolerances is the feeding a "pure" diet, ideally a home-cooked food made with carbohydrates and proteins that are novel or new to the patient. An 8 to 10 week diet course is typically needed and no other chews or treats can be eaten during the trial. Food allergy cannot be diagnosed by blood test or skin test. At this time, response to an elimination diet is the only test for food allergy or intolerance.  Most people are not in a position to home cook an appropriate food for their pet. Fortunately, several novel protein diets and hydrolyzed protein diets have been developed. For more details on using an elimination diet, see our food allergy page.

Prednisone is the cornerstone of treatment for inflammatory bowel disease, which must be diagnosed by biopsy. Sometimes a trial course of this medication is suggested for colitis but it is important to keep in mind that such trials can interfere with future diagnostics and can create some degree of immune suppression. Prednisolone could be disastrous in the event of an E. coli-related histolytic colitis, for example. Many patients with inflammatory bowel disease are never able to fully discontinue prednisolone, so be sure to discuss the pros and cons of attempting this therapy without a biopsy.

Histiocytic Ulcerative Colitis: A Specific Form of Colitis

This condition is also called Boxer colitis because the Boxer breed seems predisposed. This form of colitis is particularly ulcerative and involves infiltration of the tender colon lining with cells called histiocytes. These cells are the cells that are normally called into the scene of inflammation relatively late so that they can absorb the dead cells and debris that have been created by the inflammatory event.

Dogs with this condition typically show symptoms at an early age (less than two years) and become more debilitated than typical colitis patients. It is currently believed that this condition results from an inappropriate immune response against the common bacteria of the colon, especially E. coli. Dogs with this form of colitis do not respond well to the usual remedies listed above but instead seem to show an excellent response to the antibiotic enrofloxacin. This antibiotic is particularly effective against gram negative bacteria (so classified because their cell wall's staining properties when tested). It has thus been inferred that these bacteria are at the root of the problem.

Histiocytic ulcerative colitis can be confirmed by biopsy, although signs of colitis in a young Boxer are highly suggestive of this condition.

In Summary:

  •   The colon is another term for the large or lower intestine.
  •   The colon has three functions: absorbing water, storing stool, and further digesting unabsorbed nutrients. 
  •   Inflammation of the colon is called colitis. Diarrhea is a characteristic sign; the condition may be accompanied by cramping, gas, and vomiting. Weight loss is usually not a feature.
  •   Colitis may be acute (lasting only a few days) or chronic (lasting weeks or months).
  •   There are “good’, or “helpful” bacteria that live in the colon and small intestine.
  •   It is important to find whether diarrhea originates from the small intestine, the colon, or possibly both.
  •   Small intestinal diarrhea tends to be more serious as it prevents food nutrients from being absorbed.
  •   Diarrhea of the colon are less debilitating but are still uncomfortable.
  •   Acute (or sudden) colitis can be stress-related or dietary, is usually mild, and can be cleared up with a short course of medication or a change in diet.
  •   Chronic, or recurrent, episodes of colitis mean the signs have been going on for a month or more, and this means a medical work up will probably be recommended by your veterinarian.
  •   Colitis is best managed when the cause is known and specific therapy can be instituted.
  •   Many different parasites can cause colitis, including coccidia and giardia.
  •   Food intolerance from dyes, preservatives, contaminants, or even natural proteins in the food can cause colitis.
  •   If other reasons have not been found, the last step in colitis diagnostics is a colonoscopy with biopsies to exam colon tissue under the microscope to classify the inflammation.
  •   Antibiotics that have anti-inflammatory properties in the large intestine as well as the ability to kill harmful organisms are often prescribed.
  •   An elimination diet to narrow down foods that may not be tolerated by your pet can be helpful.  
  •   Dietary fibers and pro- and pre-biotics may be suggested, as soluble fibers ferment by colon bacteria into nutrients, which help the colon heal. Prescription high-fiber diets often have a mixture of soluble and insoluble fibers that have been scientifically determined to help colitis patients.
  •   Histiocytic Ulcerative Colitis, also called Boxer colitis, is a condition in which the Boxer breed seems predisposed. This form of colitis is particularly ulcerative and involves infiltration of the tender colon lining with cells called histiocytes.Back to top

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