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Abstract
Fifty four randomly selected farms, located throughout the North Island and the Nelson region of South Island, were surveyed for anthelmintic usage and for sheep nematodes resistant to anthelmintics.
Information on anthelmintic usage was recorded on a standardized questionnaire.
Most fanners had used both benzimidazole and non- benzimidazole broad spectrum drenches on their properties in previous years.
Sheep were being drenched, on average, 7 times within their first year of life but much less frequently thereafter. Commercial interests played the dominant role in helping farmers formulate their drenching policies.
On each farm 24 numbered ewe replacement lambs were sampled for faeces at the beginning of the trial to provide material for egg counts and larval cultures. The lambs were weighed and divided into three groups of eight. One group received thiabendazole (TBZ) at 66 mg/kg, the second levamisole (LEV) at 8mg/kg while the third remained untreated as controls. All were resampled 4 to 10 days later.
On 37 farms (74%) the faecal egg count depression (FECD) following treatment with either drench was 100%. On 7 farms TBZ was less than 100% effective as gauged by FECD. Of these 6 had an FECD above 90% and one below. LEV proved to be less than 100% effective on 8 farms. Of these 5 had FECDs greater than 90% and 3 FECDs less than 90%. Only two farms had lambs with an FECD below 100% for both LEV and TBZ.
Preliminary evidence based on larval cultures suggests that those surviving the TBZ treatment were chiefly Haemonchus and those surviving LEV Trichostrongylus spp. One population of Haemonchus resistant to TBZ has been identified by the egg-hatch technique.