By the latter part of the grazing season, most lambs will have received wormer treatments involving one or more of the group one, two or three products. Any worms resistant to these groups will have survived these treatments and, because they can live inside a sheep for months, they accumulate over the season. This population of worms in the lambs is therefore made up of a higher proportion of anthelmintic resistant worms than the population in refugia when the levels of resistance are low.
Treatment with an effective product, such as Zolvix™, will remove these worms and stop them from reproducing and adding their genes to the overall population. This can help slow down the development of resistance on farm. However Zolvix must be incorporated into a worm control programme when the level of resistant worms is low.1