Three worm control strategies, on a commercial farm in south east England, were monitored parasitologically for a grazing season. Three peaks of larval infection in June, late August and late October/early November occurred on paddocks grazed by dosed ewes and lambs on a new ley (preventive strategy). Lambs dosed and moved on to a hay aftermath in July (evasive strategy), had a mean egg count of 2090 by late August, attributable to infection with Haemonchus contortus. Three weekly drenching of lambs (suppressive strategy), suppressed faecal egg counts until September when the treatment interval was extended due to other on-farm commitments. In addition to these observations, autoinfection, with Haemonchus contortus, was seen to occur in lambs grazing summer turnips. An in vitro egg hatch assay, and critical efficacy trial, confirmed the presence of a benzimidazole resistant strain of Haemonchus contortus on this farm. The implications of these findings are discussed.
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