I haven't a clue about what would cause a hearing ferret to become deaf,
but I would definitely have a good veterinarian check for things like
sinus infection, ear mites, wax buildup, etc.  If nothing is found to be
physically wrong, I would also consider the possibility that the ornery
little guy has just decided to "not hear you".  Try making some awful
noise that would normally cause YOU to "jump" when you can watch his
reaction but he is not paying attention to you.  The "acid test" for our
deaf ferret was the coffee grinder!  Our hearing ferret "jumped" every
time we first turned it on, even though he was used to it & was OK after
being initially startled.  But it got no reaction whatsoever out of our
deaf guy.  Try intentionally crashing all the metal pots & pans in you
kitchen cabinet, or grinding beans in your blender, or doing anything
"unusual" that makes a lot of SUDDEN unpleasant noise.  If he can hear
it he should get a "startled" reaction just like us humans.
 
Good luck - & please be assured that a deaf ferret is no biggie.  The
only thing you'll be missing is he won't come when you call him. :>)
 
Lisa
[Posted in FML issue 4686]