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Inflammatory Bowel Disease in Dogs and Cats
Revised: March 25, 2023
Published: January 01, 2001

What is Inflammatory Bowel Disease? 

The membranes and layers of the normal GI tract are delicate and highly organized. There is a muscle layer responsible for the proper movement and mixing of digestive enzymes/nutrients, plus there is a high-surface area lining responsible for absorbing nutrients. Inflammatory bowel disease refers to the condition that results when cells involved in inflammation and immune response are called into the delicate layers of the GI tract. These cells disrupt both nutrient movement and absorption leading to weight loss, diarrhea, vomiting, or any combination thereof.

Chronic vomiting results if the infiltration is in the stomach or higher areas of the small intestine. Watery diarrhea with weight loss results if the infiltration is in the lower small intestine. Mucous diarrhea with fresh blood (colitis) results if the infiltration occurs in the large intestine. Of course, the entire tract from top to bottom may be involved. Many people confuse inflammatory bowel disease with irritable bowel syndrome, a stress-related diarrhea problem. Treatment for IBS is aimed at diet and stress management; IBS is a completely different condition from IBD.

Why Does this Happen?

Infiltration of the bowel with inflammatory cells occurs when something inflammatory (or, in other words, stimulating to the immune system) is ongoing within the intestinal tract. The cause of this inflammation could be parasites, toxic materials produced by bacteria living in the bowel, the actual bacteria themselves, or even digested food proteins. Many experts believe that the fundamental problem is a defect in the barrier function of the bowel such that the offending materials listed above gain entry to the bowel tissue in a way that would not occur in normal bowel. The diagnosis of IBD presumes that a tangible cause of inflammation has not been found despite extensive testing. You may hear the phrase "diagnosis of exclusion" in relation to IBD. This means that all the tangible diseases have been ruled out or excluded, so the answer must be IBD.

Why Would the Veterinarian Think My Pet Might Have IBD?

A little vomiting or diarrhea here and there seems to be pretty standard for pet dogs and cats. After all, cats groom themselves and get hairballs. Dogs eat all sorts of ridiculous things they aren’t supposed to. Still, many owners notice that their pets seem to have vomiting or diarrhea a bit more often than it seems they should. It might be subtle where you notice that you are cleaning up a hairball or vomit pile rather more frequently than with previous pets or it could be the realization that you have not seen the pet have a normal stool in weeks or months. Typically, the animal doesn’t seem obviously sick beyond GI signs. Maybe there has been weight loss over time but nothing acute. There is simply a chronic problem with vomiting, diarrhea, or both. Once it is clear that a smoldering problem is occurring, a medical workup is appropriate. Chronic GI disease has many causes so before the IBD conclusion can be drawn, many conditions must be explored first.

If vomiting occurs weekly or more, this is a reason to see the veterinarian for an evaluation.

How is IBD Diagnosed?

Before we get to IBD, we need to get through the step-by-step testing sequence that explores other causes of GI disease, because, as we said, to diagnose IBD the tangible causes must be ruled out.

The first step in pursuing any chronic problem is a metabolic database. This means running a basic blood panel and urinalysis to rule out biochemically widespread problems, such as liver disease or kidney disease, pancreatitis, or hyperthyroidism in cats that could be responsible for the signs. Since IBD is localized to the GI tract, such a database is usually normal but might express a general inflammatory response in the blood or a loss of blood proteins as often there is a leak of albumin, an important blood protein, from the intestine into the bowel contents. The database not only serves to rule out metabolic causes for the patient's symptoms but also assesses other areas, potentially turning up unanticipated problems and identifying factors that could change what medications are used.

Fecal testing and broad-spectrum deworming is often performed at this time to rule out parasites as a cause of chronic inflammation. If the patient is young or has been housed with multiple animals, more obscure parasites may be afoot, and often special fecal testing can be sent to the laboratory for PCR testing. Typical organisms screened by this kind of testing include Giardia, Cryptosporidium, Salmonella, Tritrichomonas, and Clostridium perfringens.

In dogs, a condition called Addison's disease is able to create chronic waxing and waning intestinal disease, among numerous other possible manifestations. This condition, more correctly termed hypoadrenocorticism, is often referred to as "the Great Imitator" as it can mimic many other diseases besides IBD. This condition revolves around a deficiency in cortisol, a crucial hormone in adaptation to stress. Treatment is relatively straightforward so it is important to screen for it. This is done with a screening test called baseline cortisol blood level or with a longer test called an ACTH stimulation test, which is a more definitive test that requires an hour or two in the hospital.

In both cats and dogs, a trypsin-like immunoreactivity (TLI) test would be performed to rule out pancreatic exocrine insufficiency, a deficiency of digestive enzymes. This condition is relatively easy to treat but, like Addison's disease, cannot be diagnosed without a specific test. Typically this test is run in combination with a vitamin B-12 level and a folate level. When intestinal bacterial populations alter (we used to say "overgrow" but that is not technically accurate), folate levels rise and B-12 levels drop. Antibiotics are likely indicated in this situation as well as vitamin B-12 injections.

Veterinarian performing abdominal ultrasound
Photo by MarVistaVet

Somewhere in the course of this workup, an ultrasound of the abdomen is generally recommended. Ultrasound is able to image and enable sampling of areas within the belly that cannot be accessed by endoscopy. The texture of the liver and pancreas are evaluated and the size of the mesenteric lymph nodes, which serve the bowel, are examined. The layering of the bowel is also evaluated to see if it is thickened, as is more typical of IBD, or more disrupted as with intestinal cancer such as lymphoma. Since cancer is a consideration for most adult animals with chronic intestinal disease, this kind of imaging is particularly valuable and if any unusual textures or even masses are discovered, they may be needle aspirated for analysis.

After all the Testing for Other Diseases

So, let's say we've come to the end of an extensive testing sequence with no conclusion. At this point, we can probably feel comfortable making the diagnosis of inflammatory bowel disease. What to do next is going to depend on our patient.


The Stable Patient
If our patient has a normal cobalamin (vitamin B-12) level, is not losing significant weight, has a normal appetite, normal blood protein levels, and good energy level, then we have time to try some treatment approaches and see if one of them works.

There are three types of IBD: food-responsive, antibiotic-responsive, and steroid-responsive. They are generally explored in that order as long as the patient is stable enough for the process.

Food Responsive IBD

Recent studies have shown that patients with normal albumin levels and without vitamin B-12 deficiencies have a 50:50 chance of responding to diet alone (no drugs needed). What sort of diet? The diets that have shown the most consistent success are the hydrolyzed protein diets.

Hydrolyzed proteins are "predigested" to create protein segments that are too small to stimulate the immune system. Further, they typically are made with medium-chain fatty acids (easier to absorb than the more customary long-chain fats) and favorable omega 3 to omega 6 fatty acid ratios.  Often special nutrients, called "prebiotics," are included to promote a healthier bowel bacterial population.  In other words, there is more to these diets than just their predigested proteins. Another approach is the use of novel protein diets. The idea here is that the patient cannot have an immunological reaction to a protein source he or she has never experienced. (It takes a long time exposure to a protein before the immune system will respond against it so a new protein should be safe). This means using an unusual protein such as rabbit, venison, fish (for dogs), or duck (so long as the patient has not been fed these foods before). 

As mentioned, it takes about a month to expect a good response, but there should be at least some response within the first 2 weeks of feeding the test diet. If a good response is seen after the first couple of weeks, the diet should be continued for a full 12 weeks, and after that, there is a fair chance that the patient can return to his or her original diet without consequences.

If there has been no substantial improvement, the next step will be an antibiotic trial.

Antibiotic-Responsive IBD

Antibiotics can solve the IBD problem for a number of patients. If the diet has not brought meaningful improvement, a two-week trial of either metronidazole or tylosin would be a good next step. If the patient shows good improvement within this two-week period, the treatment is continued for a total of 4 weeks. If the symptoms recur when the medication course is completed, the patient may need indefinite treatment. Metronidazole has some issues with side effects when used long-term but tylosin is used widely in this way and many animals cannot live a normal life without it.

If there has been no meaningful response after 2 weeks of antibiotics and no meaningful response to diet, then suppression of the immune system is probably going to be needed.

Steroid-Responsive IBD

The cornerstone of treatment for inflammatory bowel disease is the suppression of inflammation. When diet and antibiotics have not provided results, a trial course of corticosteroids (such as prednisolone or dexamethasone) is needed. IBD most commonly involves lymphocyte infiltration into the delicate bowel tissues, and corticosteroids will kill these lymphocytes and hopefully restore the bowel’s function. Corticosteroids should work on inflammatory bowel disease in any area of the intestinal tract. A month of this type of medication would be the next trial. If results are still underwhelming, then stronger immune suppression (as with cyclosporine or chlorambucil) is needed.

Long-term use of immune suppression should be accompanied by appropriate periodic monitoring tests.

In cases where it is particularly important to spare the patients from the side effects of long-term steroids a medication called budesonide can be used. This medication is not readily absorbed from the GI tract and serves as a topical treatment for the lining of the intestine.

The protocol described above where all three forms of IBD are explored could easily take 2 months or longer to run through. If the patient is not comfortable enough for this kind of treatment testing, it may be better to seek an intestinal biopsy right off the bat.

The Unstable Patient
If our patient is not feeling well, is losing significant weight, has a poor appetite, has vitamin B-12 deficiency, or has low blood protein levels, then we need to confirm IBD with a biopsy and look for complicating additional conditions. Biopsy samples are obtained either by surgery or by endoscopy.


Surgery involves actually opening the patient's abdomen and harvesting samples from different areas of the intestine and stomach. Other organs can be visualized and sampled as well. Obviously, this is an invasive procedure, and the patient may not be in the best condition to go through it. On the other hand, it is something most veterinary hospitals can perform without referral to a specialty center, and specialized equipment is not necessary. 


Alternatively, endoscopy involves the use of a skinny tubular instrument (an endoscope) that has a tiny fiber optic or video camera at the end. The endoscope is inserted down the throat, into the stomach, and into the small intestine, and small pinches of tissue are obtained via tiny biting forceps. If the large intestine is to be viewed, a series of enemas is needed before the procedure as well as a relatively long fast. The endoscope is inserted rectally and again tissue samples are harvested. The advantage of this procedure over surgery is that it is not as invasive as surgery. Patients typically go home the same day. 

Disadvantages are expense (often referral to a specialist is necessary) and the fact that the rest of the abdomen cannot be viewed. Growths that are seen via endoscopy cannot be removed at that time and a second procedure typically must be planned whereas, if surgical exploration is used to obtain the biopsy, any growths can also be excised at that time.

Is it at all Reasonable to just Try Treatment and Skip the Expensive Biopsy?


As mentioned, the stable comfortable patient has time to try different treatments. In cats, where intestinal lymphoma is common, there is a great deal of overlap in treatment between IBD and intestinal lymphoma. Both conditions are called infiltrative bowel diseases and it can be hard to distinguish them. Many people choose a protocol to cover both conditions.

Certainly, with IBD the diagnostic tests tend to be much more costly than the treatment. The problem is making sure there is enough confidence in the diagnosis of IBD that there will be no harm in skipping diagnostics. It is not unusual to take the work-up all the way through ultrasound and decide on treatment based on the information gained up to that point.

Junior DeLunior
Junior DeLunior - white cat belonging to Dr. Wendy Brooks, that had colitis due to IBS.
Photo by MarVistaVet

If immunosuppressive drugs are used, it may be difficult to go back later and attempt a biopsy as the true diagnosis may become obscured by prior treatment. 

IBD continues to be a common cause of chronic intestinal distress in both humans and animals. Research for less invasive tests and for newer treatments is ongoing.

This article is dedicated to Junior DeLunior, my cat from 1984-2003, who had colitis due to inflammatory bowel disease and Helicobacter overgrowth. Assist-fed for three years.

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