Lack of reversion of a benzimidazole resistant strain of Haemonchus contortusafter six years of levamisole usage

The resistance level of Haemonchus contortuswas studied on a farm where benzimidazole resistance had been detected in 1980 and where, since 1982, only levamisole had been used for worm control. Egg development tests gave ld50values of 0·18, 0·19 and 0·21 μg ml−1thiabendazole for samples taken from ewes in 1988 compared with 0·21 μg ml−1in 1981. Studies on sheep experimentally infected with larvae cultured from the faeces of these ewes confirmed the continued resistance of H contortusto benzimidazoles, and demonstrated the absence of benzimidazole resistance in other nematode species together with the susceptibility of all species to levamisole.