if my vet comfirms that my chickens do have crd and i am able to get them well with tylan. i know they will always be carriers. can i vaccinate any new birds before indroducing them to keep them from contracting crd? do i treat the chickens that are in my current flock that do not show any symptoms? i can not cull the complete flock its at least fifty left. i have culled the worst 25. i am getting about 15 eggs per day even though they are sick. what would you do?