I am sure you must have checked this, but what about an abcessed tooth?
My Deli had the absolute worst breath, to the point, I couldn't even get
him too close without getting a headache from it.  I thought he needed
his teeth cleaned real bad, but he wasn't feeling very well either.  I
took him to the vet after about a week and he wasn't getting any better,
and it was a bad abcessed tooth.  They removed it, put him on antibotics
and now he is smelling sweet as ever (for a ferret!)  Hope you figure out
the problem soon.
 
Best wishes,
Julie
[Posted in FML issue 4185]