Old post, but dont always assume mareks, get fecals done, check for mites and start vitamins. My flock is positive and i vac any incoming since there is ancedotal evidence that it limits tumor grown. My deaths have dropped considerably. Check here for live testing...
Well I wouldn't recommend a metal shed, yes I've never had predators other than snakes, but oklahoma summers make it an oven. So you'd need a way to cool it.