It seems to me that everyone who raises chickens should get Gail Daemerow's book The Chicken Health Handbook. Her older verison which is the one I have can be purchased used for $5.50 with no shipping charges https://www.amazon.com/Chicken-Heal...7&sr=8-3&keywords=the+chicken+health+handbook Her updated version is more. https://www.amazon.com/Chicken-Heal...preST=_SX218_BO1,204,203,200_QL40_&dpSrc=srch It is a very good guide on all things chicken related for the backyard flock. It also has advice that helps you avoid problems. If you have invested in chickens invest in a book. I suggest once you have this problem resolved that you read the book cover to cover. I find that when something happens with my flock I often remember something I read in the handbook and it helps me in my research.
Marek's is a good guess. How we defeated Marek's in our flock...well since we breed our own, we just let sick chicks die or culled them. We seem to have a flock that is largely Marek's resistant now. Over time we learned that saving birds that are sick can often create a carrier bird that survives but passes on the disease to other birds. Better to cut your losses. We quickly isolate any sick bird and don't go to extreme measures to save it. Sounds cruel, but to the other birds that don't get sick it is a blessing.
Marek's is a good guess. How we defeated Marek's in our flock...well since we breed our own, we just let sick chicks die or culled them. We seem to have a flock that is largely Marek's resistant now. Over time we learned that saving birds that are sick can often create a carrier bird that survives but passes on the disease to other birds. Better to cut your losses. We quickly isolate any sick bird and don't go to extreme measures to save it. Sounds cruel, but to the other birds that don't get sick it is a blessing.