Economic management of anthelmintic resistance:model and application*

The increasing prevalence of anthelmintic (drench) resistance in gastrointestinal parasitepopulations is decreasing the profitability of the Australian sheep industry. Refugiamanagement can delay its development by not exposing a proportion of the wormpopulation to chemical control. A dynamic-optimisation model is used to assess theeconomic value of refugia for management of the worm species Teladorsagia circumcinctaand macrocyclic lactone drenches in an application to Western Australian sheepflocks. A low rate of refugia (2 per cent) is most profitable under standard circumstancesbecause it slows the development of resistance, but also reduces the proportionof the flock not exposed to chemical control. Frequent drench application shouldremain the primary method of control. However, its efficacy should be preservedthrough refugia management, rather than greatly reducing treatment frequency.