I have been vaccinating my ferrets for the past 13 years (distemper)and
never experienced any reactions.  Vaccine, licensed for ferrets, are
impossibly to obtain here in New Zealand but there was a vaccine Canvac DM
which stated "One dose for dog or 4 ferrets".  And I have used this dosage
ever sins.
 
Bill.
[Posted in FML issue 1819]