Scratch is usually around 7% protein and should only be used in very small amounts.. I never buy or use it. It also doesn't have the added vitamins. minerals, and amino acids that formulated rations do.
The older birds are showing SOME resistance to the disease IF it's Marek's.. It won't effect 100% with symptoms even if 100% are infected with the virus.
In older birds.. symptoms of Marek's may look different.. the sciatic nerves may not be effected at all. They may have slow crop, it may come and go. They may have slow wasting. Or they may never show any signs before they pass.. They may be more susceptible to other things like parasites. Here is a fairly informative article.. Marek's doesn't pass through hatching eggs..
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
Older birds also have MORE natural immunity to the coccidia in their environment than a new coming juvenile will.. Coccidiosis doesn't cause imbalance coupled with continued feeding in MY experience.
Layer feed is reduced in protein (usually 16%) and increased in calcium (often 4-5%). Doing so to early can stunt growth, delay the onset of egg laying.. and also excess calcium fed long term to birds not in lay such as juveniles, broody's, roosters, and those in molt.. *can* cause major issues including kidney failure, gout, and even sudden death. There is a genetic component to this statement though.. some birds (roosters) feed the excess calcium will still live long lives of 8+ years. The thing is that we have no way of knowing their genetic predisposition.
Feathers are made from 90% protein and importantly it's amino acids. 12 week old chicks would not have fully molted their adult feathers in yet and would also be growing body wise.. muscle doesn't grow without protein.
If it were me.. according to the description I've read.. I'd skip the coccidiosis treatment and go straight to nutritional support. Still suspecting Marek's.
A link to make contact with your state vet.. at least have a conversation with them about what you're seeing, what the prevalence is that they see in your area, and how they suggest you move forward..
https://dec.alaska.gov/eh/vet/
Please remember I'm only sharing an opinion and possibility as there are MANY factors at play. It's not my intention to fear monger or to be negative Nelly. Nor do I intend to discount other considerations!
Pulling for you all.
