Ringworm in ferrets, although not common, is not unheard of.  Griseofulvin
at 25 mg/kg orally might take care of it if given for about 3 - 4 weeks.
It's very important to really disinfect the entire area though.  The spore
can remain active for up to 2 years, so just getting rid of the ferrets
won't solve the problem.  Chances are they are just passing around the
spore from animal to animal to person to person.
 
Dick B.  - not a vet
[Posted in FML issue 2640]