I lost one last week mate, exactly the same symptoms.... She was fine but seemed a little thin, I put extra food in, chicken wings + rabbit, but the next day she was shaky and really weak just not right... I bought her in and syringe fed her milk and egg mixed and she drank well then she slept for a day nearly, but the following night she seemed to get worse and died.... I was gutted mate, my little lad cried his eyes out and cried again at bed time ( the whole family get attached to them) she was only around 18 months old and was a good little ferret, I planned to breed off her in the future.