Diarrhea is an intestinal disturbance characterised by the rapid movement of abnormally loose or watery stools (feces). It is not a disease in itself, but a symptom of an underlying disease or disorder.
Diarrhea can affect the small intestine, the large intestine or both. It may be acute (sudden onset), chronic (over a long period of time) or intermittent (come and go). There may be an increase in the number of bowel movements, an increased amount of feces or watery feces. Feces may also be yellow and frothy in appearance, be mixed with blood (dysentery) and/or mucus.
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.
What causes diarrhea in cats?
Food allergy to grains or protein.
Food intolerance, such as lactose in milk.
Dietary indiscretion (eating something he shouldn’t have, such as garbage).
Parasitic infection: Liver flukes, parasitic worms such as roundworm and hookworm.
Histoplasmosis (disseminated): A rare fungal infection which usually affects the lungs, in some cases the infection can spread. throughout the body causing a number of symptoms including diarrhea.
Colitis: Inflammation of the colon.
Inflammatory bowel disease: A group of conditions in which different types of inflammatory cells invade the intestines.
Pancreatitis: Inflammation of the pancreas.
Hyperthyroidism: Caused by a benign hormone-secreting tumour of the thyroid gland.
Kidney disease: Chronic which is slow and progressive and seen most often in middle-aged to senior cats or acute which is sudden onset.
Liver disease: Which may be due to ingestion of toxins, congenital disorders, infection or inflammation.
Complications of diabetes.
Blockage: Hairball or foreign object.
Certain medications such as antibiotics, chemotherapy, antihistamines, steroids.
Exocrine pancreatic insufficiency: Failure of the pancreas to secrete appropriate levels of pancreatic enzymes which are necessary for the digestion of food.
Heinz body anemia: A type of anemia (reduced number of red blood cells) characterised by the presence of Heinz bodies on the red blood cells which leads to their destruction by the cat’s own immune system.
Malabsorption is an inability of the digestive system to absorb nutrients as it should, there are several underlying causes of this including inflammatory bowel disease, exocrine pancreatic insufficiency, blockage, bacterial overgrowth, parasites, certain cancers.
Symptoms can vary depending on the underlying cause and if the small or large intestine are involved.
Small intestine diarrhea: Volume is increased, frequency 2-3 times normal, no mucus, urgency may be normal to mildly increased.  Cats with chronic small intestinal diarrhea lose weight and body condition as they are not absorbing nutrients.
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:
Does a cat with diarrhea need to see a veterinarian?
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. Leave water out at this time. See a veterinarian if the following symptoms are present:
Diarrhea which contains blood
If he appears to be in pain, such as hunched over or a tucked up belly
Kittens under 12 months of age, or senior cats
Identifying a cat with diarrhea in a multi-cat household:
When there’s more than one cat in the home, it can be difficult to identify the cat with diarrhea, unless you watch them in the litter tray. An easier way to determine which has diarrhea is to grab a box of crayons (Crayola claim theirs are non-toxic to humans, so I would give this brand a try if you can), and using a pencil sharpener or cheese grater, make some crayon shavings, each cat has his own colour. Add 1 teaspoon of shavings to each cat’s food. Obviously, this means that the cats must be fed separately and any uneaten food removed. The shavings will pass harmlessly through the cat and out via the feces.
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, or other symptoms you may have noticed.
The type and colour of diarrhea, along with accompanying symptoms as well as your cat’s age can help your veterinarian narrow down a cause. For example, a kitten is more likely to have eaten something he shouldn’t, a senior cat is at greater risk of hyperthyroidism or cancer.
Tests will vary depending on other symptoms your cat is displaying and may include the following:
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, culture, and sensitivity) 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 to evaluate the small intestine and stomach. An endoscope is a thin flexible tube with a light and camera which enables your veterinarian to see the small intestines and stomach.
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. Your veterinarian may recommend that you withhold food if your cat seems otherwise fit and well, water should still be provided. Food may be re-introduced after a day, this will usually be bland to rest the gastrointestinal tract.
Gastrointestinal blockage: Surgery to remove the blockage or laxatives to help it pass.
Colitis: Eliminate the cause where possible, such as switching your cat to a highly digestible diet.
Diet: Avoid switching types of food suddenly. If an allergy is a cause, gradually replacing his diet to a hypoallergenic type. Lactose intolerant cats should not have dairy.
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 and protozoan infection, 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): Surgery to remove the tumour if possible and/or chemotherapy to shrink it.
Pancreatitis: Supportive care such as painkillers, anti-nausea medications, and antibiotics.
Poisoning is usually treated by removing the toxin (where possible) by inducing vomiting or pumping the stomach, supportive care is commonly necessary.
Heinz body anemia: Treating the underlying cause, blood transfusions may be necessary for severely anemic cats as well as fluid therapy and supportive care.
Anaphylaxis: This is a medical emergency. Administration of epinephrine (adrenaline) will be necessary, which counteracts the body’s immune response. Supportive care will be necessary while your cat recovers.
Diabetes complications: Treating the underlying cause and managing diabetes correctly with diet, proper insulin administration and regular monitoring of your cat’s blood glucose levels.
Histoplasmosis: Mild cases may not require treatment, a severe infection will require anti-fungal antibiotics as well as supportive care.
Malabsorption: Finding and treating the underlying cause is necessary such as pancreatic enzyme supplements for cats with exocrine pancreatic deficiency. Supportive care may also be necessary.
Your veterinarian may recommend your cat be fed Hills I/D while he recuperates, designed for gastrointestinal disorders, this food is highly digestible and low fat. Other supportive care may include IV fluids to treat dehydration and anti-diarrhea medications.
Other supportive care may include IV fluids to treat dehydration, nutritional support, 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. Diet:
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 be of help. 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.
Pumpkin: 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.