Derivation of the economic value of R-0 for macroparasitic diseases and application to sea lice in salmon

Background: Macroparasites, such as ticks, lice, and helminths, are a concern in livestock and aquaculture production, and can be controlled by genetic improvement of the host population. Genetic improvement should aim at reducing the rate at which parasites spread across the farmed population. This rate is determined by the basic reproduction ratio, i.e. R-0, which is the appropriate breeding goal trait. This study aims at providing a method to derive the economic value of R-0. Methods: Costs of a disease are the sum of production losses and expenditures on disease control. Genetic improvement of R-0 lowers the loss-expenditure frontier. Its economic effect depends on whether the management strategy is optimized or not. The economic value may be derived either from the reduction in losses with constant expenditures or from the reduction in expenditures with constant losses. Results: When R-0 <= 1, the economic value of a further reduction is zero because there is no risk of a major epidemic. When R-0 > 1 and management is optimized, the economic value increases with decreasing values of R0, because both the mean number of parasites per host and frequency of treatments decrease at an increasing rate when R-0 decreases. When R-0 > 1 and management is not optimized, the economic value depends on whether genetic improvement is used for reducing expenditures or losses. For sea lice in salmon, the economic value depends on a reduction in expenditures with constant losses, and is estimated to be 0.065(sic)/unit R-0/kg production. Discussion: Response to selection for measures of disease prevalence cannot be predicted from quantitative genetic theory alone. Moreover, many studies fail to address the issue of whether genetic improvement results in reduced losses or expenditures. Using R-0 as the breeding goal trait, weighed by its appropriate economic value, avoids these issues. Conclusion: When management is optimized, the economic value increases with decreasing values of R0 (until the threshold of R-0 = 1, where it drops to zero). When management is not optimized, the economic value depends on whether genetic improvement is used for reduced expenditures or production losses. For sea lice in salmon, the economic value is estimated to be 0.065 (sic)/unit R-0/kg production.