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Diarrhea in a dog or cat is characterized by abnormally frequent, watery stools. Clinical signs associated with diarrhea include frequency of evacuation of loose feces which results in an increase in the volume of feces passed. The diarrheal feces contains not only increased amounts of water and electrolytes, but also may contain mucous, blood, fat or undigested food. Diarrhea can originate from the small intestine or the large intestine (colitis) and is further sub-classified as acute (sudden onset of brief duration) or chronic (long-term).

Acute Diarrhea—Small Intestine

Acute diarrhea originating in the small intestine usually lasts less than 48 hours. The feces seldom contains mucus, but it is not uncommon to find blood in the feces. The animal usually loses its appetite or is anorexic. The feces is brown or reddish-brown in color. The animal exhibits a sense of urgency to defecate as well as an increased frequency and may continue straining after defecation.

Chronic Diarrhea—Large Intestine

Chronic diarrhea originating in the small intestine lasts 7-10 days or longer. The animal passes a large volume of watery feces and has bowel movements two or three times as often as its normal frequency. The feces is brown in color unless there is blood in the stool in which case it will have a black tarry appearance. Little or no mucous is present in the feces (as opposed to Colitis which may have much mucous in the diarrhea).