Maybe try the B vitamin? or liquid?
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Hypericum, that's what its called.
It sounds like mareks to me. Start her immediately on treatment. You could call someone at Cornell University Poultry Science Department. I think that there is an herbal remedy for Mareks that has shown very good results, I just can't remember the name of it.
If it is Mareks shouldnt it have gotten worse instead of stable? They have all been together (old and new) for over a month now and no one else is sick as well.