The
development of resistance was almost inevitable as more infested sheep were being treated
and some of these treatments were likely to be inappropriate, e.g. pour-on pyrethroids
being used when undiagnosed scab mites were present. Resistancewas first confirmed to the
synthetic pyrethroids in 1995 (Synge et al., 1995) and in the winter of 1996–1997 there were
at least 20 cases of resistance (Coles, 1998).