What is Colitis?

Colitis is the term for inflammation of the colon. The hallmark of colitis is a gooey 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 urgency.

An example of a fatty food this dog should not eat

Vomiting can be a feature of this condition though the main 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. This is the last part of the intestine that leads to the anus. Food has already been digested and absorbed before reaching the colon. The primary function of the colon is to absorb water and store stool.

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. 

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 problems with obtaining nutrients from food. Diarrhea from the colon is less debilitating but still but still very uncomfortable. The following are characteristics of large intestinal diarrhea:

  • Large intestinal diarrhea is associated with straining to defecate. Often this straining is unproductive, leading pet owners to think their pet 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 straight forward given the above classic findings, though how to proceed depends on the signs. Is the problem acute (i.e., suddenly there) or chronic (has been happening for several weeks regularly) or episodic (happens then goes away then happens again)?

Acute Colitis

A pet that has sudden symptoms of colitis may have stress-related colitis (common after boarding, moving, severe weather or other change) or a dietary indiscretion-related colitis from eating an inappropriate food. 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.

Colitis Chronically or in Recurrent Episodes

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 work up 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 testing additional 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 should probably be given as these treatments are safe and inexpensive. 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 with a probiotic 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.

If it doesn’t look like the above treatments of deworming, antibiotics, or diarrhea medication is going to solve the problem, then diagnostics continue to the next level. The two major veterinary labs offer PCR (DNA amplification) for harder to detect organisms that may be causing the problem. 

Trichhomonas in cats, Cryptosporidium in dogs and cats, and more. This is testing uses a fecal sample.

If nothing is making the colitis better, a novel protein diet can be tried. Some individuals do have food allergies that can lead to soft stool. GI testing can also be conducted to look for things like exocrine pancreatic insufficiency (EPI) and other things. Most cases can be diagnosed and treated with the above steps.

The ones that can’t be require a colonoscopy with biopsies with the idea of examining colon tissue under the microscope to classify the inflammation. Treatment is then based on what is found.

Management Tips for Colitis

As diagnostics are pending or early on, your veterinarian might try to symptomatically manage your pet. The following are therapeutic medications and strategies that can be helpful in the treatment of colitis.

Metronidazole, Tylosin and Sulfasalazine

medications have anti-inflammatory properties

These medications have anti-inflammatory properties in the large intestine as well as ability to kill harmful organisms.

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. 


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, it’s best to use those made by well-established veterinary companies. Fortiflora and Visbiome are examples of two good products.

In summary, if your pet has symptoms of colitis, seeks veterinary attention. Hopefully, it will be straight forward but some cases require more diagnostics than others to make a diagnosis and effectively treat your animal companion.

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