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Haemonchus contortus (Barber's pole worm or "BPW") is the nematode "nemesis" of small ruminant production systems in tropical and subtropical regions of the world. Its reputation derives from a combination of high fecundity and a short generational interval that provides an enviable developmental plasticity for adaptation or resistance to control measures. This review critically examines the historical and current literature on the host-parasite-environment interaction for H. contortus, particularly in sheep, to highlight changes in parasite distribution and ecology on pasture, changes to the seasonal inhibition of fourth stage larvae and the most appropriate models to identify protective responses and assess vaccines. The review also proposes pathways to bring host genetics to fruition and avenues where advances in the parasite genome may complement control measures.

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The primary recommendation is to use a mixture of effective drugs when treating sheep. However, unless the combination treatment is highly effective it is unlikely to delay selection for ML resistance if sheep are being treated and moved to a clean or safe pasture. Where possible, reliance on the ML anthelmintics should be reduced by not using them every year, not using them in low risk stock or by using narrow spectrum and low efficacy drugs such as naphthalophos when appropriate. Anthelmintic treatment should be given as part of a strategic worm control program. It is suggested that IVM-oral and IVM-capsules should not be used when ML resistance is present. In this situation MOX or ABA should be used in combination with other drugs, provided that the chosen ML is effective against the resistant parasite. It is essential to monitor the efficacy of ML and drug combinations by post-treatment worm egg counts, particularly when ML resistance has been detected.

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