dealing with a recent outbreak of Marek's, I Feel your pain.It is the toughest thing watching your bird suffocating from fowl pox.
Back story: birds got fowl pox while on vacation, turned into wet pox, no b local stores had the vaccine. Finally drove an hour to another feed store to get vaccine. Vaccinate entire flock. Probably lost about 1/2 flock from heat and fowl pox. Ones dying now show no outer symptoms. Buy one day see them trouble breathing look in their mouth entire trachea covered in lesions. No other lesions on body and mouth. Add to cull many at this point so they don't suffer. I am thinking were exposed and maybe the vaccine helped with the outer lesions and eyes but not enough for internal.