The value of refugia in managing anthelmintic resistance: a modelling approach

The increasing prevalence of anthelmintic (drench) resistance in gastrointestinal parasite populations is decreasing the profitability of the sheep industry in Western Australia (WA). A strategy, referred to as refugia management, has the potential to delay the development of resistant parasites by not exposing a proportion of the worm population to chemical control. A dynamic bioeconomic model is used to assess the economic value of refugia for management of the worm species Teladorsagia circumcincta and Macrocyclic Lactone drenches in Western Australian sheep flocks. A low rate of refugia (2 per cent) is most profitable under standard circumstances because it slows the development of resistance, but also reduces the cost accruing to production losses that arise from maintaining a proportion of the worm population in refugia. The optimal drenching strategy is to drench heavily early in the time horizon to minimise worm populations in subsequent years. This study highlights the optimality of minimising production losses by maintaining high levels of consistent control.