Mareks is shed in the feather dander and spread by inhalation. So cleaning the environment thoroughly is your best bet as well as allowing the area to sit without host, I have read that it can survive up to 65 days under normal temperature. My thoughts would be that a good hard winter kills off many organisms that are dangerous to chickens. I had a few Mareck birds about ten years ago from so various sources, I culled when they became obvious and I haven't seen it since in my other birds, so I'm not sure it's always the panic that it appears, especially in a climate like mine where we get cold winters.
