Macrocyclic lactone resistance on new grass leys — the putative role of accidental ‘dose-and-move’ strategies due to use of persistent macrocyclic lactone products

Nematodirosis and teladorsagiosis/trichostrongylosis were detected in 8-week old lambs on a Scottish lowland farm and treated with oral ivermectin. A drench check identified treatment failure in some pastures but not others. Ivermectin resistance was confirmed by faecal egg count reduction test. Grazing history suggested treatment of ewes with injectable moxidectin in the autumn and oral moxidectin post-lambing and movement onto clean grazing played a role in the selection for macrocyclic lactone resistance on this farm.