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Diarrhea is an intestinal disturbance characterised by the passage of abnormally loose or watery stools (feces). It is not a disease in itself, but a symptom of an underlying disease or disorder. It can affect the small intestine, the large intestine or both. It may be acute (sudden onset), chronic (over a long period of time) or come and go.
Acute diarrhea has a rapid onset and lasts less than 2 - 3 weeks.
Chronic diarrhea lasts longer than 2 - 3 weeks.  Blood and or mucous may or may not be present in the feces.
Feces may also be yellow and frothy in appearance, be mixed with blood (known as 'dysentery') and/or mucus.
Diarrhea is a relatively common occurrence in cats and has a number of causes, some of which include:
- Blockage: Hairball or foreign object.
- Colitis: Inflammation of the colon.
- Diet: There are several diet-related possibilities. A sudden switch in your cat's food can cause diarrhea. Food allergies
- Medications such as antibiotics.
- Exocrine pancreatic insufficiency: Failure of the pancreas to secrete appropriate levels of pancreatic enzymes which are necessary for the digestion of food.
- Hyperthyroidism: Usually caused by a benign tumour of the thyroid gland.
- Heinz body anemia.
- Bacterial infection: Salmonellosis, campylobacter, E. Coli, tuberculosis, tularemia.
- Viral infection: FIV, FeLV, panleukopenia, rotavirus, pseudorabies.
- Protozoa infection: giardia, cryptosporidium.
- Parasitic infection: Liver flukes, parasitic worms.
- Inflammatory Bowel Disease: A group of conditions in which different types of inflammatory cell invade the intestines.
- Kidney disease.
- Liver disease.
- Certain cancers.
- Pancreatitis: Inflammation of the pancreas.
- Poisoning: Zinc, Ibuprofen, uremic, other.
- Insect or spider bites or sting.
Small intestine diarrhea: Volume is increased, frequency 2-3 times normal, no mucus, urgency may be normal to mildly increased. 
Large intestine diarrhea: Volume is normal to decreased, mucus and blood may be present, the urgency is increased and frequency is more than 5 times normal. 
Other symptoms may also occur depending on the underlying cause of diarrhea and may include:
- Loss of appetite (anorexia).
- Increased thirst (due to fluid loss).
- Flatulence (farting).
- Weight loss.
- Increased urgency to defecate and/or defecating outside the litter tray.
Mild cases of diarrhea, lasting 24 hours or less, where your cat seems to be otherwise well can be watched carefully at home. As a precaution, take away your cat's food for 12-24 hours to see if the problem resolves. Water should be left out during this time.
Your cat should see a veterinarian for diarrhea if the following occurs:
If vomiting is also present.
If diarrhea contains blood.
If the diarrhea is black and/or tarry.
If he also has a fever.
If he is lethargic.
If he appears to be in pain (hunched over, pain when touched).
If he is a kitten under 12 months of age.
If he has pale gums.
Your veterinarian will perform a physical examination of your cat and ask you some questions to determine if the diarrhea is acute or chronic? If there have been any changes to your cat's diet, possible exposure to toxins, other symptoms you may have noticed. The type and colour of diarrhea, along with accompanying symptoms can help your veterinarian narrow down a cause.
Tests will vary depending on other symptoms your cat is displaying, some tests your veterinarian may wish to perform include:
- Complete blood count to check for anemia, infection, inflammation.
- Biochemical profile to check for liver disease, kidney disease, pancreatic function, hyperthyroidism etc.
- Urinalysis to check kidney function and level of hydration.
- Multiple fecal examination tests (flotation, cytology, smear, zinc sulfate) to determine if the cause is parasitic, bacterial, protozoal.
- FIV and FeLV tests.
- Thyroid test to evaluate for hyperthyroidism in middle-aged to older cats.
- X-Rays to check for blockage, foreign body or tumour and assess the internal organs.
- Ultrasound to evaluate for cancer or intestinal blockage and assess the internal organs.
- Endoscopy and biopsy. A thin flexible tube with a light and camera is passed into the stomach and small intestine to view these structures. A biopsy may be taken at this time.
- Colonoscopy and biopsy. Similar to the endoscopy, a thin flexible tube is passed into the rectum and colon to view the structures and take a biopsy if necessary.
Treatment naturally depends on the cause of diarrhea.
If the diarrhea is acute, and the cat seems otherwise fit and well, your veterinarian may choose to withhold food for a day or so. Water is still to be provided. After the fasting period, food may be re-introduced but this will usually need to be bland for a few more days.
- Blockage: Surgery to remove the blockage or laxatives to help it pass.
- Colitis: Eliminate the cause where possible, switch to a highly digestible diet.
- Diet: Avoid switching types of food suddenly. If an allergy is the cause, gradually replacing his diet to a hypoallergenic type. Avoid dairy if lactose tolerance is suspected.
- Drugs or toxins (plants, poisons etc).
- Exocrine pancreatic insufficiency: Replacing enzymes with a pancreatic enzyme extract, feeding a highly digestible diet, antibiotics if necessary.
- Hyperthyroidism: Surgical removal of the tumour or radioactive iodine treatment to kill the tumour.
- Infection: Antibiotics for bacterial infection, anti-parasitic medications for worms, supportive care for viral infections.
- Inflammatory Bowel Disease: Switching to a highly digestible diet, corticosteroids, immunosuppressive drugs.
- Kidney disease: Dietary changes to reduce protein and phosphorous binders.
- Liver disease: Supportive care such as nutritional care and IV fluids, surgery to treat portosystemic shunt.
- Neoplasia (lymphoma, carcinoma, and others): Chemotherapy.
- Pancreatitis: Supportive care such as painkillers, anti-nausea medications and antibiotics if necessary.
- Poisoning is usually treated by removing the toxin (where possible) by inducing vomiting or pumping the stomach, supportive care is commonly necessary.
Your veterinarian may recommend your cat be fed Hills I/D for a short period. Hills I/D is low in fat, highly digestible and is specifically designed for cats with gastrointestinal disorders.
Other supportive care may include IV fluids to treat dehydration and anti-diarrhea medications.
If the diarrhea is mild and your cat otherwise seems well, you may wish to try treating it at home for a day or so, if symptoms persist or you notice other symptoms, seek veterinary attention.
Never give anti-diarrheal medications to your cat unless your veterinarian has told you it is safe to do so as many human medications are extremely toxic to cats.
Feeding a bland diet of chicken and rice (mix 1 cup cooked chicken breast with 1/2 cup cooked brown rice) or baby food (make sure it contains no onion or garlic, which is toxic to cats) for several days can help give the unsettled digestive tract a chance to rest and recover.
Lactobacillus milk or plain yoghurt:
Lactobacillus, a type of "friendly" bacteria residing in the intestinal tract of mammals (including cats). These bacteria protect the body against harmful bacteria which can cause disease. This can be of help to cats with diarrhea, especially if they have been or are on a course of antibiotics which don't discriminate against good and bad bacteria. Giving lactobacillus milk can replace lost helpful bacteria.
Either cooked (steamed or boiled) or canned (not the pie filler type) may be of help in relieving diarrhea. Add 1 tablespoon to food, or plain, if your cat will eat it.
   The Feline Patient - Gary D. Norsworthy, Mitchell A. Crystal, Sharon K. Fooshee and Larry P. Tilley.