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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.

Overwintering of horse cyathostomes as inhibited third stage larvae (L3) and the effect of repeated oxfendazole (ofz)treatment on strongyle infections were studied in an experiment with two groups of three Shetland ponies. Both groups were grazed together from May 28 to November 11, 1986 and subsequently housed. Treatments with 10 mg ofzkg−1were given on May 26, July 1 and July 28 and again one week before each group was necropsied in December and April, respectively. Worm populations of both groups were dominated by inhibited early L3. The proportion of fourth stage larvae (L4) was significantly higher in April than in December. The faecal egg counts showed that the ofztreatment in May was more effective than those in July. This resulted in high pasture larval counts from the end of August onwards. The final treatments revealed a low efficacy against L3 and L4 but not against adult worms.