The prevalence of anthelmintic resistance in sheep nematodes on the central tablelands of New South Wales.

A survey was undertaken to assess the prevalence of anthelmintic resistance in sheep nematode populations on 40 commercial farms distributed throughout the central tablelands of New South Wales. Representatives of the 2 major groups of broad spectrum anthelmintics with different modes of action (thiabendazole and levamisole) were used at the manufacturer's recommended dose rates. Efficacy was assessed on the basis of the reduction in faecal strongyle egg counts 7 days after treatment. An efficacy of less than 90% using both anthelmintics was obtained on 4 farms. Thiabendazole had an efficacy of less than 90% on a further 21 farms and levamisole had an efficacy of less than 90% on an additional 4 farms. There was no evidence of anthelmintic resistance on 8 farms, while the remaining 3 had insignificant parasite burdens. Based on larval cultures from faeces, Ostertagia and Trichostrongylus were the most significant species in resistant populations. Haemonchus burdens were sporadic and levels of resistance relatively low. Nematodirus burdens were widespread but no evidence of resistance was detected.