Although IBD is a common cause of chronic vomiting and diarrhea in dogs and cats, its cause remains unknown. Proposed mechanisms of disease include an abnormal response of the immune system (hyperactivity or hypersensitivity) to either normal bacterial contents of the gut or various components of ingested food, versus an appropriate immune response to a pathogen that causes the disease.

Dietary factors are believed to play a role in the disease process because many animals respond to dietary manipulation. Regardless of the cause, IBD results in vomiting and diarrhea secondary to an accumulation of white blood cells in the lining of the intestinal walls. This infiltration by white cells causes inflammation and inhibits normal digestion and absorption of food. Abnormalities of gastrointestinal motility, or movement of the gut due to muscular activity in the intestinal walls, may also cause some of the clinical signs seen in IBD.

Several different types of IBD exist. These are differentiated based upon the type of white cell that is involved in the infiltrate. The clinical signs that are noticed in your pet depends upon whether the inflammatory process is occurring in the upper or lower gastrointestinal tract. Animals with upper GI involvement tend to experience vomiting, diarrhea with a normal to increased volume of feces, weight loss if signs are chronic and severe, occasional dark, tarry stools, which may represent blood loss into the gut, and occasional gas and borborygmus (loud GI sounds). Animals with lower GI involvement may experience vomiting as well.

The character of the diarrhea is different with lower GI disease. This usually manifests as more frequent defecation of a smaller volume, with straining to defecate, fresh blood and/or mucus in the feces, and a greater urgency associated with defecation. Usually animals with lower GI disease do not show significant weight loss but some may vomit.

Upper and lower GI IBD is common in dogs but cats more commonly suffer from IBD involving the upper GI tract. Many diseases can cause vomiting and diarrhea and must be differentiated from IBD. These include:

Diseases involving other body systems that have gastrointestinal side effects. These might include diseases affecting the kidneys and/or urinary tract, liver, immune system and endocrine system (hormone producing glands).

Bacterial infection of the GI tract. Campylobacter, Salmonella and Clostridium are types of bacteria that can infect the GI tract and cause vomiting and diarrhea.

Parasitism. GI worms or Giardia (a protozoal organism) can also cause vomiting and diarrhea.

Food allergy or intolerance. Animals that have allergies to certain foods may develop vomiting and diarrhea.

Exocrine pancreatic insufficiency. The pancreas plays a vital role in food digestion. If the pancreas is not producing the necessary enzymes that aid in food digestion, this may result in diarrhea, and to a lesser extent, vomiting.

Fungal disease of the GI tract. Histoplasmosis is a fungal infection that can involve the GI tract and cause vomiting and diarrhea. This fungus is limited to the midwestern part of the United States.

Neoplasia (cancer). Cancers affecting the GI tract, most notably lymphosarcoma, can cause the same clinical signs as IBD. Lymhposarcoma is a cancer of the lymphoid tissues, of which there is an abundant supply in the GI tract.