Karen,

Duffel is a collie so the dermatologist opted not to risk a negative reaction
to ivermectin.  When I suggested testing to see if he could take ivermectin
her reply was that it took a while for test results to come back, and that
milbemycin (interceptor) was just as effective as ivermectin and had fewer side
effects - except for the cost.  The liquid ivermectin that they give to cattle
is apparently very cheap.  So we soldier on.

Thanks for everyone's input.

Ellen