I had a roo that I had to put down 2 weeks ago. I thought it was Marek's, classic symptoms. My vet did a necropsy, and didn't find any tumors. So his feeling was that my roo had contracted Eastern Equine Encephalitis. On reading, turns out that 204 sentinel chickens in Florida have gotten EEE. And I hear it's a big problem in the midwest. The symptoms are the same. It's spread by mosquitos.
It might be easier to take a chicken for a blood sample to rule out EEE, and check for deficiencies. I can't say that my roo didn't have Marek's, but there were no tumors. He was not sick and didn't act sick. He did waste away from not eating enough. One day I thought he was staggering, the next day he's like hopping crawling, and that night he can't move. Legs and one wing paralyzed. Looked like one foot forward one foot back, and his left eye appeared different, first he kept it closed, then when he did open it, the pupil looked small, and he was blind in that eye. He couldn't eat normally, kept missing the food. Then he started losing strength in his neck. I was going to try tube feeding, but just trying exhausted him. Very sad, still makes me tear up. Everyone else is good so far.