Kelleen,
 
I am not a vet, nor a vet tech.  Like you, I run a shelter.  The information
that I have on earmites and ivermectin is that the ivermectin will kill all
mites present but not touch the eggs.  You have to wait until the eggs hatch
and redo the ivermectin to kill those before they breed and lay new eggs.
I'm not sure that the one week span would be sufficient to do much good
unless you were getting adult mites from the other ferrets, infecting the
treated ones.
 
Sounds like you are treating them all at once.  The second dose at two weeks
should eliminate the infestation.  As you know, you do have to clean the
ears of wax before treatment with the ivermectin.
 
As to whether or not a weekly treatment for three weeks would do any harm,
I'll leave that up to the vets.  I do know that ivermectin is very toxic and
should be used sparingly and with care.
 
Dick B.
[Posted in FML issue 1553]