What is inflammatory bowel disease?
Inflammatory bowel disease (IBD) is not a specific disease. Rather, it is a specific reaction that the stomach or intestines have to chronic irritation.
What are the clinical signs of IBD? 
If the stomach is involved, your cat will have chronic vomiting. This is the most common form. If the intestines are involved, chronic diarrhea will occur. In some cats, both parts of the digestive tract are involved, so both vomiting and diarrhea occur. If the disease occurs for several weeks to months, weight loss and poor appetite are common.
When does IBD generally occur? 
IBD is most common in middle-aged to older cats (generally between ages 5 and 12), but it can occur in younger cats as well.
How is IBD diagnosed?
The chronic irritation that causes IBD stimulates the body to send cells from the immune system to the affected area. The most commonly found cells are lymphocytes and plasmacytes. Thus, the disease is diagnosed when these cells are identified in abnormal levels in the tissue. A pathologist is responsible for this part of the diagnosis; his/her report usually calls the disease lymphoplasmacytic gastritis (relating to the stomach), lymphoplasmacytic enteritis (relating to the intestine) or lymphoplasmacytic colitis (relating to the colon).
In order to obtain these cells, a biopsy is required. In most cases, an endoscope is passed into the cat’s stomach or colon (while the cat is under anesthesia). A tiny biopsy instrument is passed through the endoscope and used to take small samples of the lining (mucosa) of the affected organ.
Is this the only test required for diagnosis?
The tissue reaction that occurs in the stomach or colon is diagnosed with a biopsy. However, determining what causes the tissue reaction to occur requires further testing. Tests or treatments should be performed to rule out stomach and intestinal parasites, cancer and infections. Diseases such as hyperthyroidism, diabetes and pancreatitis should also be considered, though these will be ruled out by blood testing before an endoscopic biopsy is recommended. In many cases, the cause cannot be determined. IBD can be present concomitantly with other diseases as well.
How is IBD treated?
The ideal way to treat this problem is to diagnose the underlying disease that is causing the reaction. Sometimes the above mentioned tests will do that and sometimes a cause cannot be found. In the latter situation, the disease is called idiopathic. This means that a disease is present, but there is no known cause. Many cases of IBD are considered idiopathic.
Some cats with IBD respond to a change in diet. A prescription food that contains a protein source that is new to the cat, such as liver or fish, may help. Unfortunately, a true food trial requires that the test diet be fed exclusively for four to six weeks. If dietary therapy is not successful or feasible, drugs are used to suppress the inflammatory reaction. The type of drug chosen is dependent on the severity of the clinical signs and the biopsy result. 
Does this mean that I will be medicating my cat for the rest of his/her life?
Long-term therapy is required for many cats. Generally, a cat is treated for a few months before the medication is weaned or discontinued. If the signs of vomiting or diarrhea recur, medication is resumed.
Could stomach infections be a cause of IBD?
Some spiral-shaped bacteria can cause vomiting in cats. The most common are Helicobacter pylori, which have been shown to be the cause of disease (including stomach ulcers) in humans and are also pathogens in cats. However, they are also found in many normal cats and humans. Therefore, finding spiral-shaped bacteria on biopsy is not always meaningful. It is considered a pathogen only if an associated inflammation is in the stomach mucosa.
Are these infections treatable?
Usually. When found in humans, successful treatment may require several medications or a combination of medications. Veterinarians are currently using drugs that are effective in humans to treat cats. This approach is successful in most cats, but we have quite a great deal to learn about the most effective means of treatment.
Can these bacteria affect me or my family?
This is a concern for all cat owners. It is known that many people carry these bacteria in the stomach for decades before disease occurs. Therefore, it is almost impossible to know the source of the bacteria. It is doubtful that cats are involved in the transmission process, but that has not been determined at this time.
What about hairballs?
Some cats are meticulous groomers and typically swallow hair every day. Since hair is not digestible, it could easily be a source of chronic irritation to the stomach or intestines. Frequent brushing and the use of hairball medications are recommended to cats with IBD, especially if they have a history of vomiting or passing hairballs.
What is the prognosis?
If a response to diet change occurs, the cat can be maintained on a this diet for the rest of his/her life (as long as it is balanced). If the cat responds to medication for stomach bacteria, a good prognosis is justified. If a response occurs to corticosteroids or drugs that change the motility of the stomach, the long-term prognosis is also good if administration of the drug is feasible. However, if there is no response to diet or corticosteroids, the prognosis is more guarded. At that point, further testing is suggested to see if an underlying disease can be found.
See Inflammatory Bowel Disease in Dogs