What to Feed an Old Cat Who Has Diarrhea
What is diarrhea?
Diarrhea is unformed or loose bowel movements, usually with increased amount and frequency. It is a result of faster movement of fecal material through the intestine combined with decreased absorption of water, nutrients, and electrolytes. Diarrhea is not a disease, but rather is a sign of many different diseases. There are many causes of diarrhea. Diarrhea may occur as the only sign, as one of several signs of a more generalized disease problem, or with signs that result from prolonged or severe diarrhea.
How can I tell if my cat has diarrhea?
If your normally well-trained cat suddenly starts having accidents around the house and the stools are unformed to fluid, then diarrhea is obvious. However, if the cat is still using the litter box and covering up its feces or defecates outdoors, you may not initially notice diarrhea.
Staining and soiling of the fur around the back end in longhaired breeds is often associated with diarrhea. It is important to remember that some variation in the consistency of stools occurs in normal cats.
Changes in diet can lead to temporary changes in the stool. If frequent liquid or semi-liquid stools persist for more than two days, you should consult your veterinarian. If your cat is showing other signs of illness, do not wait; call your veterinarian immediately.
If you have more than one cat, then it is important to try to determine if more than one cat has diarrhea.
What are some causes of diarrhea?
Diarrhea is not a disease in itself but a clinical sign that may reflect one or more of many different problems. Most involve some degree of inflammation of one or more areas of the gastrointestinal (GI) tract. The GI tract is the continuous tube that moves food from mouth to anus. Infectious agents, such as bacteria, viruses, coccidia, and intestinal worms, or non-infectious irritants, such as chemical toxins or poisonous plants are some of the more common causes of inflammation. Food intolerances or allergies to specific ingredients of a diet can also be responsible for diarrhea.
How is the cause of diarrhea determined? 
It is important to provide your veterinarian with a detailed medical history. Ideally you should write this out in chronological order before you go to the clinic. Be as detailed as possible on the date you first noticed a problem, and the progression of the clinical signs. Was there any change in your cat's diet such as a new bag of food? Note what diets and treats your pet has eaten in the last 1-2 weeks. Note any changes in the normal routine of your cat or your household. How frequent are the stools? What is the color, consistency, and smell of the feces? Is the cat showing any other signs such as vomiting, loss of appetite, lethargy, or loss of weight? To help you put this history together, please see the handout "Diarrhea Questionnaire and Checklist for Cats".
Your veterinarian will usually ask you to bring a representative sample of fresh fecal material with you to your appointment. Besides a thorough clinical exam, your veterinarian may recommend additional diagnostic tests. In mild cases of diarrhea, these tests may be deferred unless initial treatment fails or the condition worsens. In-depth diagnostic tests may include blood work, stool, and rectal swab samples for parasite examination, DNA testing and culture, radiographs (X-rays), and endoscope exam.
How is diarrhea treated?
Initially, and often in advance of an in-depth work-up, a non-specific approach may be adopted. For otherwise healthy adult cats, your veterinarian may advise you to withhold food for 24 hours or to feed small quantities of a light, easily digestible diet. Water should be offered at all times. The best diets are often veterinary supplied diets specifically formulated with a balance of fibers that feed the good bacteria found in your cat's intestine. In some cases, a bland, home-prepared diet such as boiled rice or pasta with boiled skinless chicken may be recommended.
Antidiarrheal agents, dewormers, and/or probiotics (bacteria that support intestinal health) may be prescribed in some cases. Metronidazole (brand name Flagyl®) and tylosin (brand name Tylan®) are commonly prescribed anti-diarrheal agents that decrease the intestinal inflammation that often leads to diarrhea. Dewormers commonly used include Profender®, Panacur®, and Drontal®.
"Antidiarrheal agents, dewormers, and/or probiotics (bacteria that support intestinal health) may be prescribed in some cases."
Many probiotics are available that may be helpful in cats who have diarrhea. As the quality and effectiveness of probiotics and supplements are not always known, it is always recommended to ask your veterinarian before giving your cat anything of this nature. Many cases of acute diarrhea respond quite readily to this conservative treatment, allowing the body's healing mechanisms to correct the problem, without the initial cause ever being established. As the stools return to normal, the cat's regular diet can gradually be reintroduced, mixed initially with the veterinary recommended diet.
If there is little or no improvement over two or three days, if the cat is not drinking any water, or if the cat's health worsens, then your veterinarian should be notified at once. Your cat may require more aggressive treatment based on the results of an in-depth clinical work-up as outlined above. Blood and urine tests may be added to the clinical work-up to look for underlying organ dysfunction. Loss of fluid is one of the most serious aspects of severe or prolonged diarrhea, and if vomiting is present, dehydration can rapidly escalate. Correcting the dehydration may require intravenous or subcutaneous fluids (fluids that are administered under the skin, usually over the shoulders).
Can I use anti-diarrheal medications from the human pharmacy?
Some of the preparations recommended for people are very dangerous for cats so never use a medication without consulting your veterinarian first. Products containing acetylsalicylic acid (Aspirin®) or acetaminophen (the active ingredient in Tylenol®) are extremely toxic in cats.
My cat has chronic diarrhea. Will it get better?
Chronic diarrhea that has been present longer than two to three weeks may prove more difficult to diagnose and treat effectively. Even extensive work-up does not always provide a definitive answer to the problem. However, in most cases a thorough clinical work-up, including food trials, can result in successful management of the diarrhea.
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Source: https://vcacanada.com/know-your-pet/diarrhea-in-cats