Sheep blowfly strike: the cost of control in relation to risk.

Sheep blowfly strike (ovine cutaneous myiasis) is a widespread economic and welfare problem in sheep husbandry in many parts of the world. Strike incidence is determined by a complex interaction of fly abundance, host susceptibility and climate, combined with farmer husbandry and intervention strategies. Sheep farmers adopt a range of approaches to the type and timing of the management used for the control of blowfly strike, the rational basis for which is often not robust. Here a deterministic model, based on existing data relating to fly abundance, seasonal risk and strike incidence, is used to compare the variable costs associated with different strike management strategies. The model shows that not employing prophylactic treatment is the lowest cost strategy only where strike risk is low. In all other circumstances, prophylactic treatment incurs lower costs than not doing so, because the deaths associated with strike outweigh the costs of prophylactic treatment. Lamb treatment, in particular, has a substantial effect on strike and cost reduction, since lambs are the most abundant age-class of animals and are at the highest risk over the period when fly abundance is the greatest. Early-season treatment of ewes before shearing is also an important component of the lowest cost strategies, particularly when the blowfly season is extended. While the rational choice of the most appropriate strike management strategy is essential in the context of farm economics, welfare considerations lend added importance to treatment decisions that reduce strike incidence.