Effect of anthelmintic treatment of ewes at parturition and lambs at weaning on the build up of Haemonchus contortus populations in lambs in the Netherlands

Abstract

In two groups of ewes and lambs, grazing pastures which were used respectively by calves and sheep in the previous year, infection with Haemonchus contortus was virtually prevented by a single treatment of the ewes with fenbendazole before turning out. In another group, grazing a pasture which was not grazed in the previous year, a delay of approximately eight weeks in the build up of high H contortus burdens in lambs was obtained by this treatment, compared with a group of untreated ewes grazing similar pasture. In two groups of lambs treated with fenbendazole at weaning and subsequently grazed on pastures which could be considered free or almost free of H contortus respectively, the build up of high pasture infectivity levels for H contortus also took at least six to eight weeks. It is suggested that treatment of ewes at parturition and lambs at weaning might delay the build up of high H contortus burdens in the lambs by less than the six to eight weeks observed in these experiments when such treatment is less than 100 per cent effective.