Diarrhea is one of the most common reasons that pets are seen by their veterinarians. Although most causes of diarrhea are related to disease of the gastrointestinal tract, occasionally it is a manifestation of disease processes arising in organs such as the liver and pancreas.

The first step in the diagnostic work-up of diarrhea is to determine if the diarrhea is acute or sudden in onset, or is of longer duration (chronic). Chronic diarrhea is defined as loose stool that persists for greater than 1 month, or is intermittently seen for several months. The second step in determining the cause of the diarrhea is to decide if the stool has characteristics suggesting origin in the small intestine or large intestine. These first two steps then determine the remaining work up.

Small bowel diarrhea is characterized by large volumes of watery to semi-liquid stool with little or no discomfort or straining associated with the bowel movement. Occasionally the pet may have some urgency associated with the defecation. Weight loss may be present if the diarrhea has been persistent.

Large bowel diarrhea is characterized by straining and urgency to defecate. There is often fresh blood or mucous in the stool. Weight loss is rare with purely large bowel diarrhea unless food intake is decreased. Some pets have diseases of both the large and small intestine, resulting in combinations of symptoms.

A minimum data base-consisting of a complete blood count, biochemical profile, urinalysis, and fecal examination-are obtained in almost every case of diarrhea. Occasionally, other tests, such as x-rays and ultrasounds, are performed. Fiber-optic endoscopy of the gastrointestinal tract often is used in chronic cases, or in instances of foreign body ingestion. A surgical exploration is performed when there is an obstruction of the bowel, or if the problem is beyond non-surgical correction.