THE INCIDENCE OF THIABENDAZOLE RESISTANCE IN FIELD POPULATIONS OF HAEMONCHUS CONTORTUS ON THE NORTHERN TABLELANDS OF NEW SOUTH WALES

A survey was undertaken to assess the incidence of thiabendazole resistance in field populations of H. contortus in sheep flocks on 40 farms distributed throughout the sheep grazing area of the northern tablelands of New South Wales. Treatment with thiabendazole at 66 mg/kg reduced H. contortus ova output by 96-100% in 18 flocks while in the remaining 22 there was a range of responses down to levels where the anthelmintic was without apparent effect. Factors which may contribute to the high incidence of anthelmintic resistance are discussed and the possibility of widespread resistance occurring in other sheep raising areas is considered.