Thank you all for your replies and suggestions. Sassy declined rapidly to the point of not being able to stand or walk at all, and her head/neck was droopy and weak. She continued to eat the starter/grower feed that I made into a mash, but refused to drink. I gave her vitamins in water early on, when she was drinking, and also some egg for selenium. Her eyes looked normal, and she continued to cluck and chatter. That is Blukote on her neck where the other two were pecking at her. I did separate her to eat, but let her sleep with the others. She became defenseless when she could no longer walk. I euthanized her on Monday. It's so sad to watch how this disease hits and so rapidly progresses. Now I'm hoping the other two don't succumb to it also. They are just a few weeks from laying age, and I understand that that can be a stressor too which activates the disease process, if it is Marek's. It's discouraging.

Maybe I'm too old to take on chicken-keeping(just turned 71). I just wasn't expecting such deadly problems in my little flock of 4. One sudden death, one nearly died from Coccidiosis(and I fear she has severe intestinal damage, poops still watery with little solid material in them), and now having to euthanize the third. If starting out with vaccinated chicks is the way to go, I would advocate for that. I started with unvaccinated 7-wk old pullets. I certainly have learned a lot since then, and have so appreciated the support and help from you all at BYC!! Thank you so much!