Mine have always been late bloomers. Out of all of them, only 3 layed early or, what most call on time. Except my little serama, she was laying at 13 weeks old!!
Wow! I would have thought that a hatchery would have this under control. Assumptions can be dangerous! I wouldn't have ordered from Mt Healthy anyway. After their outbreak with salmonella, I was staying far, far away. My chickens are salmonella free. I would like to keep it that way...
I can't tell you how she got it. Have you brought in any new chickens in the last 6 months? I'm sorry, I didn't reread your original post so I'm not sure of the age, sorry. You can usually trace it back if you have good records of your flock history. New birds, visiting farms, chicken shows...
Your necropsy indicates the presence of Marek's. There is no treatment. I would almost bet that the Marek's is first and the others are secondary infections brought on by the Marek's. I'm sorry. This isn't a good necropsy report for the longevity of your flock.
Are they also going to Necropsy? If so, please post the final report. I'm glad that you're not dealing with Coryza!!!
You never mentioned the blindness and paralysis before. I do hope that they are doing a full necropsy. I'm concerned that the MG is a secondary infection, not the primary...
I wish I knew more about Coryza and the vaccine but, I don't. I would think that with Coryza in your flock you would never be able to sell birds, only eggs. I'm not sure what the success rate is for the vaccine or, if the birds would also be considered carriers. I do know that the information...
I think you are dealing with Coryza. There is no cure. You can treat, you can find threads on here that will tell you the proper meds to use, but, there isn't a cure. The symptoms will go away but, the chickens will remain carriers for life and will infect any other birds that you bring in...