Barbara and Siegmar. I feel so horrible that you still have no concrete explanation as to why Tigris is ill.
You will most likely know when he is feeling poorly. That would be my guide for the time being. I wonder if you could contact the person(s) breeder where you got Tigris, to see if they have any insight. Sometimes breeders have problems passed down from cat to cat. I know my breeder would be happy to answer any questions I may have about my cats' health.
Gosh, you must be SO frustrated and worried.
Paws crossed here in Vermont, hoping you get some answers soon. And that the answers are ones you can work with and cure.
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