Willawong Hill
Songster
Congenital defect is just a birth defect- it's possible your chicken was just born with unusual eyes if she was always like this. It does look a lot like ocular mareks but that does seem to take a little while to show up so if she hatched with pupils like this it's unlikely. If it is mareks and she's been like this for months then your flock has already been exposed so putting her down wouldn't make a difference. If it is ocular Mareks only then it should only affect the eyes, though it seems like some chickens can get multiple types of mareks.