The first point to make is that if diarrhoea persists for more than 24 hours it is potentially serious and a veterinarian should be consulted without delay.
The question refers to diarrhoea continuing. This implies that it has continued for longer than a short duration of time. In cases of diarrhoea of short duration without excessive fluid loss treatment can take place at home. Otherwise a veterinarian should be consulted.
In cases of diarrhoea of short duration food should be withheld for 24 hours. Small amounts of water should be given periodically. As the cat recovers food can be introduced gradually by feeding three small meals a day. The food should be high in meat protein. Examples might be Hill’s Prescription Diet Feline i/d or for example strained baby food. High-carbohydrate foods and dry food should be avoided. This is because cats have a low tolerance for carbohydrates and high carb diets which can prolong the diarrhoea. There should be a gradual return to the usual food after the cat has recovered.
An alternative viewpoint was made by Jane in a comment re. withholding food for 24 hours:
“Our vets no longer recommend the 24hr starve as an initial response to diarrhoea. They suggest feeding much smaller amounts with added water. The rationale behind this change is that diarrhoea can cause & be complicated by dehydration, and, lack of nourishment will weaken the cat
In simple cases with no blood in the stool, they usually suggest a prebiotic/kaolin paste. Sometimes a lighter, low fat diet for a few days is recommended.
If the diarrhoea lasts for more than 24hrs or is happening several times over 12hrs, it’s straight around to the vets for my two. Both are healthy, but are FIV+, so we tend to jump on any symptoms quickly.
Risking a complication or serious infection is not worth it.”
In cases of prolonged diarrhoea you have to identify and remove the underlying cause. Let’s remember that diarrhoea is a symptom not a disease. It’s quite complicated and beyond the capabilities of a normal cat caretaker/guardian.
A common cause is overfeeding. Other causes due to the transit time in the gut being speeded up might include dead animals, garbage and rotten food, rich foods, indigestible items and intestinal parasites.
Diarrhoea can be caused by ingesting toxic substances but this does not happen very often. They might ingests something toxic when they lick their feet or their coat. Such substances might include gasoline, oil, cleaning fluid, refrigerants, insecticides, bleaches, plants, mushrooms, and building materials.
Some cats are more lactose intolerant than others which means that they are unable to digest milk and some milk byproducts. They lack the adequate amount of the enzyme lactase.
Some cats are not able to tolerate certain foods such as beef, pork, chicken, fish, eggs, spices, wheat, soy, corn which can be found in food for people and in commercial cat food.
An assessment as to the cause can be made by examining the colour, odour, consistency and frequency of the cat’s stools. You will need a veterinarian to do this.
I have many pages on the subject on this website but they are advisory and discussion points really. Please click on this link to see them. I must stress again that diarrhoea can be serious and therefore you shouldn’t really put off going to a veterinarian to diagnose and treat the illness properly.
Source: Cat Owner’s Home Veterinary Handbook 278-280.