Idiopathic inflammatory bowel disease (IBD) is defined as gastrointestinal signs (vomiting, anorexia, diarrhea) greater than 3 weeks duration, with incomplete response to dietary trials and anthelmintics, biopsy findings of mucosal inflammation, and clinical response to immunomodulatory therapies. IBD is believed to be due to a loss of mucosal tolerance to intestinal antigens such as commensal enteric bacteria or dietary components. Important differential diagnoses for IBD include systemic diseases (hyperthyroidism, diabetes mellitus, liver disease, pancreatitis, renal failure), food sensitivity, gastrointestinal lymphoma, chronic parasitism (giardia, tritrichomonas), and infectious diseases. A therapeutic trial with fenbendazole (Panacur) 50 mg/kg/day for 5 days should be considered for treatment of occult giardia, prior to pursuing more in-depth diagnostics. Diagnosis of IBD requires intestinal biopsies. The cornerstone of treatment for IBD consists of immunomodulatory drug therapy (usually prednisolone), which should ideally not be given without biopsy confirmation of IBD, since infectious intestinal disease can mimic IBD and could become substantially worse on immunosuppressive therapy. Furthermore, a long duration of signs (>1 year) does not eliminate GI lymphoma as a potential differential diagnosis, where chemotherapy would be more appropriate. All of the other therapies described below are often used prior to intestinal biopsy or in conjunction with immunomodulatory therapy after biopsy confirmation.

A dietary trial is often the first step in managing cats with suspected or confirmed IBD. Cats may improve on novel protein (elimination) diets, because these diets avoid exposure to proteins to which the cat's GI system may have previously become sensitized. Commercial "hypoallergenic" diets contain a single protein source, and are usually milk-, corn-, and wheat-free. They are highly digestible with a moderate amount of soluble fiber. A complete dietary history should be taken, so that a diet with a novel protein (for that cat) can be selected. Hydrolyzed protein diets such as Hill's z/d Ultra Allergen Free or Royal Canin Feline Hypoallergenic diet are formulated to avoid the antigenicity of intact dietary proteins. Diarrhea or lack of palatability can be an issue with some cats. The prevalence of food sensitivity in cats with idiopathic chronic GI signs was 16/55 (29%) in one large study. An additional 11/55 (20%) had signs resolve on diet change but did not reoccur on rechallenge. Although a 4-6 week dietary trial has been recommend, in the above study, cats improved within 2-3 days after a diet change. A response to dietary therapy is more likely in cats with concurrent dermatologic signs (military dermatitis, pruritis of the head and neck, alopecia). Response to a dietary trial occurred in cats who did not necessarily have a peripheral or tissue eosinophilia on biopsies. GI signs may also improve after a change in diet, even if the cat does not have a true dietary allergy (immunologic event). Non-immunologic causes of an adverse reaction to food may occur because of disaccharides (such as lactose), food additives (coloring agents), pharmacologically active products (histamine), or food toxins. Many GI diets are highly digestible diets of moderate energy density, which do not contain preservatives or coloring agents, and are given in small frequent feeding (at least 3-4 meals/day). A high-fiber diet has been recommended in cats with colitis

Probiotics are live organisms (usually bacteria but sometimes yeasts) that promote a beneficial microbial balance in the intestinal tract. Examples include Lactobacillus species, Enterococcus faecium, Bifidobacterium species, and Saccharomyces species. Potential benefits of probiotics include inhibition of intestinal colonization by pathogenic bacteria, modulation of gut flora, and inhibition ofbacterial translocation across the intestinal wall. Purina fortiflora (encapsulated E faecium strain SF68) is reported by the manufacturer to decrease fecal concentrations of clostridium perfringens and diarrhea in kittens. Probiotis may be beneficial in the treatment of IBD, but it is important that the probiotic contain viable organisms that have been shown to colonize the intestinal tract of cats.