There is a great debate going on whether or not to destroy chickens that survive CRD because they are carriers for life. But I have found that most chicken owners I know kept theirs. Wild birds carry these diseases so if you free range your chickens, your chickens will always be exposed to them. Getting rid of a whole flock of chickens and getting new ones doesn't mean your new chickens won't get sick. According to my local feed store who sells the Tylan 50, it is very common for chickens to get CRD.
