What can cause chronic diarrhea? The first step in answering this question is to determine if the chronic diarrhea is due to disease in the gastrointestinal tract, or secondary to disease in another organ system (for example, some cats with the endocrine disease hyperthyroidism exhibit diarrhea as a major sign).
To determine if the chronic diarrhea is secondary to another disease process, a comprehensive history, physical exam and preliminary screening labwork (CBC, biochemistry profile, urinalysis, fecal evaluation) is done. In some cases, additional specialized blood tests or diagnostic procedures may also be necessary to completely answer the question.
If the diarrhea is not associated with disease in another organ system, the next step is to determine if the diarrhea is due to disease of the small bowel (small intestine) or disease of the large bowel (large intestine). Disease of the small bowel (duodenum, jejunum, ileum) usually has clinical signs that are distinctive and different from disease of the large bowel (descending, transverse and ascending colon). In some animals, the disease process will extend to involve both the small and large bowel.
Patients with small bowel diarrhea often exhibit weight loss and poor body condition because the major job of the small bowel is to facilitate digestion and absorption of nutrients. Without adequate digestion and absorption, the patient eats, but continues to lose weight. The undigested and unabsorbed nutrients appear in the stool, and this gives rise to increased volumes of soft stool.
In contrast, patients with large bowel diarrhea are usually in good body condition, as digestion and absorption of nutrients is not a major function of the large bowel. Rather, the major function of the large bowel is to absorb water, produce mucus and act as a storage receptacle. Clinical signs noted most often in patients with large bowel diarrhea include good overall body condition, increased frequency of defecation (but smaller amounts of stool), mucus in the stool, occasionally fresh red flecks of blood in the stool, and sometimes straining to defecate.
Other clinical signs that may be present in patients with both small and large bowel diarrhea include intermittent vomiting, increased loudness of intestinal sounds (borborygmi) and flatulence. In cats, the inflammatory process in the intestinal tract may also concurrently involve the pancreas and/or the liver.
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