I am so sorry you are going through this. You obviously love your birds and have been raising them as family pets. Know that they are lucky to have had someone so loving to them in their life.From everything I have read and everything I have observed. I just have to wait it out... see who survives and breed resistance in those birds. if I chose to keep chickens anymore. If we lose too many of our favorites I may just euthanized the rest... canning the whole idea of chickens. Additionally I know my chickens will never be safe as Marek's because they already have a compromised immune system.
I had one bird die maybe three years ago that I suspected had mareks but then none of my other birds had any problems, even the babies. So it was likely something else (second possible diagnosis was that she ate a mushroom in the yard).. I hope it gets better soon. 