[Anthelmintic resistance in sheep nematodes in The Netherlands].

The prevalence of anthelmintic resistant nematodes in sheep was investigated in 1994 with a faecal egg count reduction test (FECRT) and an egg hatch assay (EHA) on 71 farms in the Netherlands. A benzimidazole (oxfendazole) was tested on 70 farms, ivermectin on 51 farms, and levamisole on 35 farms. Based on the FECRT benzimidazole resistance was present on 56 farms, 2 were suspected, on 9 no resistance was observed, while 3 farms could not be tested due to a too low egg output before treatment. No clear indications for resistance to ivermectin and levamisole were found. Benzimidazole resistance was demonstrated in Haemonchus contortus, Cooperia curticei, Ostertagia spp. and/or Trichostrongylus spp. No resistance was seen in species from the genus Nematodirus, Chabertia ovina and/or Oesophagostomum spp. De results of the EHA confirmed the results of the FECRT. On 59 farms the ED50 value was > 0.12 microgram ml-1 thiabendazole.