Haemonchus contortus: the then and now, and where to from here?

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.