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Abstract
The increasing prevalence of anthelmintic (drench) resistance in gastrointestinal parasite
populations is decreasing the profitability of the Australian sheep industry. Refugia
management can delay its development by not exposing a proportion of the worm
population to chemical control. A dynamic-optimisation model is used to assess the
economic value of refugia for management of the worm species Teladorsagia circumcincta
and macrocyclic lactone drenches in an application to 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 proportion
of the flock not exposed to chemical control. Frequent drench application should
remain the primary method of control. However, its efficacy should be preserved
through refugia management, rather than greatly reducing treatment frequency.