I do not recommend flea collars for ferrets.  They are usually not the source
of the fleas (the dog or cats that go outside are more likely to be the
constant source).  It is this reason they usually don't need constant
protection against fleas.  If they do have fleas I recommend cleaning and
defleaing (with a mild pyrethrin spray) their enviornment then let it air dry
before putting them back into it. And for the ferrets you can usually remove
most of the fleas with a flea comb: if you can't get them all its ok to use a
dilute pyrethin spray (be careful of their eyes) or mild flea shampoo.
I'm not sure what chemical is in the zodiac collar they used, but in
susceptible ferrets (as well as cats and dogs--but not as bad) any chemical
can cause them to get sick, especially if it was a brand new collar and the
ferret licked or ingested some.  This ferret could have had an underlying
problem or it could hve caused the entire problem.  If the ferret is still
sick after the collar is off for 2-3 days then I would recommend a blood test
to check his liver and kidneys.
It is also a good idea to give him a bth to remove any residual chemicals.
Dr Weiss
301-299-4142
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[Posted in FML issue 1125]