The 2001 Compendium of Animal Rabies Prevention and Control at Part
III.B.1.(a) states:
"All dogs, cats, and ferrets should be vaccinated against rabies and
revaccinated in accordance with Part II of this Compendium.  If a
previously vaccinated animal is overdue for a booster, it should be
revaccinated with a single dose of vaccine.  Immediately following the
booster, the animal is considered currently vaccinated and should be place
on an annual or triennial schedule depending on the type of vaccine used."
 
Part II states that 1 ml of IMRAB 3 should be administered subcutaneously
annually.
 
The import of this change is that once a ferret has been properly
vaccinated against rabies, and even though their annual booster shot is
late (by any amount of time), the ferret will be considered currently
vaccinated immediately after the booster shot is received.  There is no
waiting period.
[Posted in FML issue 3265]