I do not think Lyme disease has been confirmed in ferrets, but there
are several diseases that ticks can transmit (Ehrlichia, Rocky Mountain
Spotted Fever, Babesia, etc.).  Thus preventing ticks is the way to go.
Frontline and Revolution can be used to prevent ticks.  Frontline is the
better of the two for ticks.  (Note: Advantage does NOT work on ticks)
 
Sounds like it is time to have your ferret's skin "spot" looked at to
make sure it is not something more serious (tumor, etc).
 
Hope that helps,
Jerry Murray, DVM
[Posted in FML issue 3738]