The only way you are going to know for certain if she has it is to do a necropsy. If she has it then it is likely present throughout your flock. I would treat her with a heavy vitamin regimen and hope that she is just suffering from a deficiency. You are too late to save her and the flock from being exposed to Marek's if that is indeed what she has, but since it is a disease that is fatal in the long run anyways I would give her a chance to see if she may be suffering from some other problem. She will have at least a good year before the Marek's becomes an issue unless she is struck down completely. Then, if she dies, you can send her in for necropsy.
Just in case of death, I would do the research now to establish where she will need to go, how she will need to be shipped, cost, etc.
Sorry you are going through this.
