Just because your chickens are getting better after use of an antibiotic is beside the point. They still had CRD symptoms and that they improved after getting the chlortetracycline, sort of points to them having it. Most respiratory diseases including MG (CRD,) coryza, ILT, and ORT are chronic and the flock should be considered to be carrying the disease, though some may not have symptoms. Infectious bronchitis can make them carriers for up to a year. You cannot sell birds or hatching eggs ever as long as a single bird is on your property. Testing or a necropsy while symptomatic is the best way to identify what disease is affecting them. Many chickens can be infected by wild birds who are carriers.
