Sequential development of resistance to thiabendazole and levamisole in nematodes of sheep

Abstract

Anderson N., Martin P. J., Jarrett R. G., Brown T. H. and Miller D. W. 1988. Sequential development of resistance to thiabendazole and levamisole in nematodes of sheep. International Journal for Parasitology 18: 243–249. On separate set-stocked paddocks. Merino weaners acquired trichostrongylid infections, the Ostertagia spp. component of which differed in susceptibility to benzimidazoles following the use of thiabendazole (TBZ) at treatment frequencies of nil, five to six per year and 17 per year. Levamisole (LEV) used for the first summer treatment reduced egg counts of grazing sheep by an average of 76.6% in year 1 but reductions in each of three subsequent years were not as great as this. An anthelmintic efficiency assay showed that LEV removed 81 and 95% respectively of Ostertagia and Trichostrongylus spp. in sheep infected with worms whose progenitors had not been exposed to LEV in the grazing experiment. Worms exposed to two or four treatments of LEV per year were resistant to LEV with an oral dose of 7.5 mg/kg removing only 32–52% of both Ostertagia and Trichostrongylus populations. Previous use of TBZ was not a predisposing factor in selection for resistance to LEV. The rapid development of resistance to LEV demonstrated that annual monitoring of efficacy was needed when the presence of resistance to TBZ necessitated a change to LEV for the control of trichostrongylid infections.