Although I've never seen a case of ringworm in a ferret, I can't say its not
possible, but not that common.  There are several types of ringworm and only
one of these types (M. canis) fluoresces with a UV light 60% of the  time.
 So there are many cases of ringworm that  don't fluoresce (light up) with a
UV light.  Scaly, dead skin (flakes of skin) can also fluoresce and give a
false positive reading.  The best way to diagnose ringworm is to pluck a few
hairs from the area and place them in a culture which will turn red if its
positive (takes about 7-10 days).
I caught the tail end of this conversation, but I hope it helps.
Dr Weiss
301-299-4142
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[Posted in FML issue 1117]