http://www.smartgroups.com/message/readmessage.cfm?gid=1423922&messageid=2044
 
There is no peer-reviewed or even scientific studies on prolonged
duration of vaccines in ferrets.  Most of the general consensus is
extrapolated from dogs, in which studies have shown that in adult dogs
with a history of a properly administered series of puppy distemper
shots and several annual boosters, the duration of subsequent distemper
boosters (and parvo as well) may be up to 5 years.  But no studies have
ever been done in ferrets to show whether this is true.
 
So I can't say that there is any scientific proof to this claim.  For
this reason, under normal circumstances, annual vaccinations are still
recommended.
 
With kindest regards,
Bruce Williams, DVM
[Posted in FML issue 3954]