I had a similar incident happen with one of my roos. He was about 3, and started not acting himself, but was still eating & drinking. However, lost a lot of weight, his comb shriveled, eventually face turned pale, & finally after about a month of going down hill, he finally died. Just never did figure out what happened! Then, after investigating, I'm pretty sure was some form of cocci~even though I wormed, & treated with Sulmet, gave electrolytes, etc etc. Hated it so~was my fav! None other have been sick that I can tell. Never seen a chicken last so long~usually get sick & will die rapidly. Not sure what else I could of done. Still wish I knew what happened for sure!