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Hi Mikala, Many thanks for your advice - yes, the vet did the tests necessary (hair and skin biopsy) and all came back negative. I had been worried about ringworm, because some of my other cats had it this year and were on Itrafungol, but there was no sign of fungal infection on this boy. On the day I posted this question, I started him on a diet of poached chicked breast and fresh fish (steamed) and the scratching stopped immediately. His wounds began to heal and scabbed over. Foolishly, three days ago I tried him with some tinned sardines, and he scratched for about twelve hours, so back to square one with the healing process. He also scratched when he ate a tiny bit of fresh mince - so it seems that his allergies may include red meat and any processed food. It is two steps forward, one step back all the time, and the FIV complicates matters by slowing down the healing, but so far he seems to be ok on the chicken and fish, and his wounds are healing.
I suppose it will be a matter of healing his wounds, then trial and error to find out what he can and cannot eat. He is a very affectionate, loving boy, and is only about ten months old so he is well worth the effort.
Thanks again