This happened to my ferret back in 1987 and I was told by my vet that it
is a dietary problem where there aren't enough lipids in the diet.  The
skin dries out and hair loss begins.  I was given a supplement to put into
the food and within a few weeks the hair had grown back.
 
I assume this is correct because it worked.  Using foods designed for
ferrets this has never happened to me again.
 
-Clint
[Posted in FML issue 3819]