Could be a food allergy? Could be something like Trtrichomonas foetus which is extremely difficult for test for and requires more than 1 sample. Most vets don't even know what it is let alone understand the testing procedure for it.

Have you done the usual things i.e. fast for 24 hours and then feed solely chicken/white fish for a few weeks? Goats milk probiotic yoghurt added to the food? Protexin? a 3 day Panacur worming course?

Am just thinking of things as they spring to mind.

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Purrs and best wishes
Kat
http://www.schimmelorientals.com