This is what the best authority on animal diseases says:Hi @Auroradream26 , thanks again for your advice with our Wheaten Marans pullet. I gave her a new sling to sit in last night because it's easier for her to eat that way now.
I called Dr. Sheryl Davidson at the poultry pathology place in Kennett Square today and they said I can bring her in anytime. I explained the symptoms and she conferred it sounds like Marek's. I'm to bring her in alive, they will euthanize her and then do an necropsy to confirm Marek's. She said if it is Marek's when I come in tomorrow she will give me the number of a place that will euthanize the rest of the flock and dispose of them responsibly.
http://www.merckvetmanual.com/poultry/neoplasms/marek’s-disease-in-poultry
This is the most disturbing quote:
Marek’s disease is one of the most ubiquitous avian infections; it is identified in chicken flocks worldwide. Every flock, except for those maintained under strict pathogen-free conditions, is presumed to be infected. Although clinical disease is not always apparent in infected flocks, a subclinical decrease in growth rate and egg production may be economically important.