i had a similar problem to this over the last 2 years with 3 different ferrets,all 3 died in the end,most likey cause is something in the genes,best way to deal with it is to breed with another ferret that is completley unrelated which should dilute the problem.if you want pm me and i can give you a more detailed breakdown,my other half is a vetrinary nurse and one of the vets has helped me a lot!!!