Cataracts can be from metabolic, hereditary, congenital, infectious, and
traumatic problems, but juvenile cataracts are likely congenital or
hereditary.  Diet can also play a role, and hypoglycemia from insulinoma
is another cause in older ferrets.
 
I would recommend not to breed this ferret with the unilateral cataract
and to keep a close eye on the good eye (no pun intended).
 
Hope that helps,
Jerry Murray, DVM
[Posted in FML issue 3520]