Characterization of moxidectin resistant Trichostrongylus colubriformis and Haemonchus contortus.

The development of moxidectin resistance (MOX-R) in sheep parasitic gastrointestinal nematodes already carrying multiple resistances to other anthelmintic groups has made control of these strains very difficult. The anthelmintic resistance patterns of MOX-R strains of Trichostrongylus colubriformis and Haemonchus contortus were characterized to provide an insight into the remaining role of anthelmintics in the control of such strains. Homozygous MOX-R individuals of both genera were unaffected by moxidectin. For MOX-R heterozygotes a dose rate of 200 microg/kg abamectin (ABA) given orally removed 25% of H. contortus while 200 microg/kg MOX given orally achieved a 72% reduction. Doubling the dose rate of ABA improved the mean efficacy to 37%. Consequently, in H. contortus, the degree of dominance differs markedly between the two anthelmintics. A dose rate of 8 mg/kg levamisole and 185 mg/kg napthalophos achieved >95% reduction in worm count of the MOX-R homozygous H. contortus but only 85 and 7%, respectively against the MOX-R homozygous T. colubriformis.