A cat owner on the mumsnet.com website is asking the question in the title. I’m going to respond to her directly. I have posted most of this article on the mumsnet.com website in an effort to help. But the vet should never be ignored. We know that.
You have made the presumption that the reason for the diarrhoea is the food that they have eaten at a boarding cattery. That’s a fair assumption but the first step is to identify and remove the underlying cause so you’ve got to really be sure about the cause.
If it is of a short duration, it can be treated at home provided excessive fluid loss is not present. If diarrhoea persists for more than 24 hours it is potentially serious and you should consult a veterinarian without delay.
Veterinarians (the ones I listen to in their excellent manual 😎) say that you should withhold all food for 24 hours and periodically give them very small amounts of water. The cat will recover and then you introduce food gradually feeding three or four small meals a day.
You can begin with a diet high in meat protein or even strained meat baby food. Hill’s Prescription Diet Feline i/d is a good example of a food for a cat that has suffered from food intolerance. However, it may be hard to find this product. Perhaps the vet stocks it. It is a wet cat food product.
You should avoid high carbohydrate foods and dry cat foods. Foods high in carbohydrates (dry foods are high in carbs) can prolong diarrhoea because cats have a low tolerance for carbohydrates. You then gradually return to the usual food when the cat has recovered.
WITH VET ADVICE ONLY, you may want to prescribe loperamide (Imodium) but if a cat has caught an infection which causes the diarrhoea this suggestion may be contraindicated because it can keep the infectious organism in the bowel longer because it may slow the body’s efforts to expand it. Also, this drug is apparently controversial and it may cause excitement in some cats.
Hope this helps a bit and good luck.
P.S. (1) see pumpkin above and (2) a shorter answer would be to contact the vet for a commercially available bland (hypoallergenic) food while your cat recovers. There are a lot of options. It depends really on the cause and the severity. It sounds as if it is mild and transient in which a hypoallergenic type commercial food should do. Although I think you have to be careful. Diarrhea can be serious. Perhaps it is a case of hypoallergenic food plus watch and wait to see how it develops.