Depending on multiple factors I think that there are other things that
can make target shaped bite marks, but are you in an area with Lyme
Disease?  I do NOT -- repeat, NOT -- know if ferrets get this but can
recall hearing that dogs do.  I'm going to ask someone who may know if
they get it, but do not know if he will be quickly available so in case
they can get Lyme I'd call the vet and ask the vet if your treating vet
knows and what should be done on a precautionary basis if there is a
possible risk.  I might be that one of the vets at the Ferret Health
List if you have not also posted there yet
http://www.smartgroups.com/groups/ferrethealth
will know but also contact your own vet.
 
Oh, and link to the FHL Complete Archives from the home page of the
website above, and check the FML Archives (addy is in the header of every
issue of the FML) to see if there is info on if ferrets get Lyme Disease.
[Posted in FML issue 4095]