ANTHELMINTIC RESISTANCE IN TRICHOSTRONGYLUS COLUBRIFORMIS

Resistance to thiabendazole has been found in two strains of Trichostrongylus colubriformis from sheep. These strains occurred in flocks on research properties where the anthelmintic had been used at frequent intervals for experimental purposes for several years. The LD₅₀ values for the resistant strains were approximately 70 mg/kg and 125 mg/kg respectively, compared with an expected value from published reports of less than 10 mg/kg for susceptible strains. Apparent cross-resistance to the related benzimidazole compound parbendazole was observed at the normal therapeutic dose rate of 20 mg/kg in sheep. Both anthelmintics depressed egg-production in resistant female worms at normal therapeutic dose-rates. Both dl-tetramisole and pyrantel tartrate were effective against one of the benzimidazole-resistant strains of T. colubriformis, but the efficiency of the latter anthelmintic was less than that normally expected against benzimidazole-susceptible strains.