Diarrhea is the passing of loose or liquid stool, more often than normal.
Diarrhea can be caused by diseases of the small intestine, large intestine or by diseases of organs other than the intestinal tract. Your ability to answer questions about your pet's diet, habits, environment and specific details about the diarrhea can help the veterinarian narrow the list of possible causes, and to plan for specific tests to determine the cause of diarrhea. (Anatomy of the digestive system: dog / cat)
Small intestinal and large intestinal diarrhea have different causes, require different tests to diagnose and are treated differently. Small intestinal diseases result in a larger amount of stool passed with a mild increase in frequency; about 3 to 5 bowel movements per day. The pet doesn't strain or have difficulty passing stool. Animals with small intestinal disease may also vomit and lose weight. Excess gas production is sometimes seen and you may hear the rumbling of gas in the belly. If there is blood in the stool it is digested and black in color.
Disease of the large intestine including the colon and rectum cause the pet to pass small amounts of loose stool very often, usually more than 5 times daily. The pet strains to pass stool. If there is blood in the stool, it is red in color. The stool may be slimy with mucus. The pet does not usually vomit or lose weight with large bowel diarrhea. A sudden onset of small intestinal diarrhea may be caused by viruses including canine distemper, canine parvovirus, canine coronavirus, feline panleukopenia virus or feline coronavirus, in young, poorly vaccinated pets. Small intestinal diarrhea can be caused by bacteria such as salmonella, clostridia or campylobacter although these same bacteria can be found in the stool of normal dogs and cats.