Ok, I currently own a small flock of only four chickens, but I have lost three chickens to mareks in the past.-here is how it went:
Cheeky was a common brown and was top of the pecking order until I started noticing changes in her, she ate less, she didn't run over to me as the others did at feeding time, she hadn't really done that much pecking lately, and then one day, she didn't leave the coop in the morning, and I had to lift her out. Her comb was floppy, and I could tell from looking at her that she wasn't well. We took her inside and phoned a vet. The vet was not trained to deal with chickens or anything, but happened to have chickens herself, and she said that what we were describing sounded like mareks. We nursed Cheeky in a towel inside because she was paralysed and could no longer walk, and fed her until she stopped eating.Cheeky slipped away peacefully in my arms one evening, I was holding her, my mum was there to, her head flopped down and just like that she was gone. I don't think she was ever in any pain.
That same day, just two hours before Cheeky died, we had rescued battery hens delivered to us. I think that if we had known then what we know now about mareks, we would have phoned up to say give them to another home, but at the time we weren't as aware of mareks, and so two little new common browns joined us. We named them Dafty and Blondie, and I must admit that at first it was hard to look at them because they reminded me of Cheeky, but over time I grew to love them for themselves. Six months after Cheeky died, Blondie fell ill. I recognised the symptoms immediately as the same as Cheeky. It felt like a stab in the heart to know that we were losing another of our girls, and I think it wasn't as painful when Cheeky died, because we didn't know as much about what was happening to her. We had planned to bathe Blondie before she died. we even bought Epsom salts, but we never had the chance and Blondie died alone while we were out. A few months later Dafty caught the disease and died as-well. None of our birdies lasted longer than a week after we took them inside, but I don't think any of them suffered.
The advice I would give to anyone with a sick bird would be to just care for them, and if they won't drink from a container, use a pipette to drop water on the birds beak.