Your birds can't get IB and then have it turn into MG/CRD--they are 2 different diseases--or they could have had another respiratory disease. Most birds that get secondary infections with IB, usually develop pneumonia or get e.coli that can spread all over the body. It is hard to culture IB unless you do it within the first week of symptoms. With MG it can come back whenever they are stressed, such as a cold spell or other stresses. Treating with antibiotics will help your birds to live and lay eggs for you, but will not cure them. Once MG/CRD is in your flock, they will all be carriers. And all eggs can have MG--they are fine to eat but not to sell for hatching. Here are 2 links with good information on MG:I already have Tylan 50 injectable and have administered it. I also have Tylan 100 powder. So, let me make sure I have this right, after reading a lot on the subject. Because I am very very intelligent.
My birds most likely had gotten a bad case of Infectious Bronchitis....which is ok, because it happens a lot. However, since I didn't give them antibiotics for this at the time they had it, and I waited too long....some of my birds are now going through a secondary infection which is most likely CRD. So, if I cull the one bird with the secondary infection, having treated the IB with Tylan....I will be OK, and I can still sell eggs? Am I correct on this?
http://umaine.edu/livestock/poultry/mycoplasma-gallisepticum-faq/
http://edis.ifas.ufl.edu/ps034