
If you haven't done so, offer some poultry vitamins to her. It looks like in that sweet photo with the little boy that she likes eggs

I'm so sorry. I wish we had better answers, there is no way to know for sure without some testing. The only other thing I can think of would be to have a fecal sample tested for Cocci and worm overload and a gram stain for bacterial infection. This would rule those out, but, I do agree with Barbara that it does look like Marek's.
Marek's testing is done through necropsy, so if she doesn't make it you may want to consider sending her to your state diagnostics lab - I know this would be hard, but it would give you answers.