Prevalence of benzimidazole resistance of nematodes in sheep in The Netherlands

A survey was carried out in The Netherlands to determine the prevalence of benzimidazole resistance in sheep nematodes. It was shown that 47 per cent of the 59 investigated flocks harboured trichostrongylid populations with resistance to benzimidazoles, as demonstrated by an LD50 value of 0·12 μg ml–1thiabendazole or higher in an in vitro egg hatch assay. On these farms Haemonchus contortuswas found to be the dominant species. In two flocks resistant Ostertagiaand, or, Trichostrongylusspecies populations were found. Farms with resistance were revisited to determine the anthelmintic efficacy in vivo by means of treatment and egg count. In only four of the 19 flocks which showed resistance in vitro was an efficacy of less than 95 per cent in vivo achieved. This means that there is a discrepancy between the in vivo and in vitro results. Resistance was found more frequently on pure sheep farms than on mixed farms. It is suggested that mixed farms offer more refuge for susceptible H contortuspopulations.