I have a hen that is nearly 6 months. When she was 12 weeks old she was lost in over 100 degrees for a day and when we found her she was dehydrated and her feet were curled. I treated her with vitamins. It progressed to the point that she could only stumble backward and could not eat/drink unless food/water was elevated. After about 6 weeks of isolation and treatment she recovered to the point that she was happy with her sister, but never completely got better. She had a tremble in her legs. About a week ago the leg issues came back and today I found her in the coop with her legs splayed and her wing out. It's very characteristic to what I've read about Marek's disease. I think the poor little thing has suffered enough. I think it's time to cull her. How do I do that? I cannot chop her head or wring her neck. I have euthanized rodents with dry ice fumes in a closed container. Would this work on a bird as well? It's so very sad, but suffering is even worse. I'd like to get new chicks in the spring, but am very concerned about introducing them to an infected flock. No other hens have shown signs of illness. Only one other is her age. Any advice would be greatly appreciated.