My vet gave me medication for one of my cats who has having constant runs several times a day for weeks that was the concentrated bacteria found in yogurt, but that had no actual yogurt, just the bacteria. It kills all the existing bacteria in the stomach replacing it with this new good bacteria. This might be an option for very bad runs. It was in a liquid form packed in one syringe for oral use per package. It is supposed to be a cure all for diarrhea that doesnt react to traditional medication, but it is available by perscription only. Unfortunately, it only worked for 24 hours on our cat who was later labeled as having IBS and is now onpredisone.
I wish I could remember the name, but I can not. It might be worth describing to your vet and seeing if he/she has heard of it and what they think.