Please get a diagnosis before using the anticoccidials. It's unlikley to be cocci with one bird and no signs of enteritis. Speak to your vet and get some droppings in and sent to your local VLA (if your vet doesn't do in house). Trouble is, there are a lot of cocci that cause no problems and they are common in young animals with a develooping immunity. Marek's - the right age, but you should be seeing leg weakness/paralysis before death.
If it's fresh a PM would help. If you don't fancy paying for it, and you are use to gralloch, do it yourself. On its back, cut through the rib cage eiher side of the sternum and peel that back. Scissors are better than a knife. Take pictures and maybe mail them to a poultry vet. POst here if you want but I'm not a poultry vet. (Some would say I'm a paltry vet)