MUTUAL RESISTANCE TO AVERMECTINS AND MILBEMYCINS - ORAL ACTIVITY OF IVERMECTIN AND MOXIDECTIN AGAINST IVERMECTIN-RESISTANT AND SUSCEPTIBLE NEMATODES

To determine whether there is mutual resistance to avermectin and milb emycin anthelmintics, ivermectin and moxidectin sheep drenches were te sted against ivermectin-resistant and susceptible isolates of Ostertag ia circumcincta and Trichostrongylus colubriformis in sheep. None of t he isolates had been exposed to moxidectin previously. The dosages of ivermectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubriformis were 23 times and six times larger, respectively, than the dosages required to remove the same percentage of susceptible isolates. The dosages of moxidectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubrif ormis were 31 times and nine times larger, respectively, than the dosa ges required to remove the same percentage of susceptible isolates. It is concluded that the worms resistant to ivermectin were also resista nt to moxidectin.