In brief, colitis is the term for inflammation of the colon. The chief feature 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 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.

In classifying diarrhea, it is important to determine whether the problem relates to the small intestine (diarrheas originating here are more serious) or large intestine. Diarrheas of the large intestine have the following common characteristics:

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)?

A pet that has sudden symptoms of colitis probably has a stress-related colitis (common after boarding, moving, severe weather or other change) or a 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 metronidazoleor 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. 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 2 to 3 days, contact the veterinarian to see if further testing is needed.

If your pet has had symptoms of colitis for one month or more, a more complete search for the actual cause should be taken. The first step is to run a basic database. This should include blood chemistry, a white and red cell profile (a CBC), and at least one fecal test for parasites. Cats should have their viral status (feline leukemia virus and feline immunodeficiency virus) confirmed. If the cat has a history of living in a crowded household, cattery, or shelter, testing for Tritrichomonas foetus infection may be in order. A test for pancreatic ability to produce digestive enzymes may also be in order. (Exocrine pancreatic insufficiency is a disease of the small intestine that produces a diarrhea very heavy in fat, which can be confused with the mucous diarrhea of colitis.) A fecal smear or cytology test where the bacteria of the stool sample (as opposed to worm content) may be examined microscopically can help rule out pathogenic bacteria that can cause colitis (especially Clostridial organisms).

In dogs, whipworms are difficult to confirm by fecal floatation testing (this test detects worm eggs and whipworms only periodically release their eggs). It may be prudent to deworm the dog for whipworms and see if the problem resolves. In addition or instead of deworming, a course of metronidazole, sulfasalazine, or tylosinmay be prescribed and/or a new diet may be recommended as a trial.

If response to a short course of simple treatment is short-lived and if blood testing reveals no explanation, then colonoscopy with biopsies will probably be necessary to reach a diagnosis. At this point, common underlying causes may include infiltration of the lining of the colon with inflammatory cells as in inflammatory bowel disease or the entire problem may turn out to be more psychosomatic as in irritable bowel syndrome. The colon biopsy readily distinguishes these conditions by showing the inflammatory infiltrate in the former and normal tissue in the latter. It should be emphasized that the abbreviations IBD and IBS are often incorrectly used interchangeably. In fact, these are two completely different conditions.

 
Information gathered from Veterinary Information Network.