Efficacy of Emamectin Benzoate Table 1. Definition of terms used within emamectin benzoate efficacy analysis. Term Definition Emamectin benzoate treatment: Any site-wide treatment episode where emamectin benzoate was the only sea lice medicine administered and all pens commenced treatment on the same day. Any emamectin benzoate treatment where the mean abundance of mobile L. salmonis fell below 40% of pre-treatment levels in the 12 weeks following treatment. Note: All treatments that were applied before guideline treatment-trigger lice levels had been reached were classified as effective. All stocked farms (owned by the industrial partner) within a loch system were treated for sea lice within two weeks of each other. The sea lice medicine used at the other sites in the loch system was not necessarily emamectin benzoate. Any effective treatment that was administered as part of a loch-wide intervention. The sea lice medicine used at the other sites in the loch system was not necessarily emamectin benzoate. The treatment episodes at the other sites in the loch system were not necessarily effective. PLoS ONE | www.plosone.org 3 February 2008 | Volume 3 | Issue 2 | e1549 Treatment-trigger guidelines: Mean abundance of L. salmonis adult females has reached 0.5 (February to June), or 1.0 (July to January). across whole loch systems, or other pre-agreed geographical areas, to minimise the risk for cross infestation amongst sites. Results Effective treatment: Loch-wide treatment: Effective loch-wide treatment: doi:10.1371/journal.pone.0001549.t001 Efficacy calculations Treatment efficacy was investigated by comparing post- treatment mobile L. salmonis abundance with levels recorded prior to treatment intervention. To establish a meaningful pre-treatment figure with which to compare post-treatment abundance, at least one lice count had to be available for a site in the 16 day period prior to treatment. If lice abundance was monitored more than once during this time, then levels recorded on the date closest to the point of treatment intervention were used as the baseline. To permit analysis at various time points after treatment, only emamectin benzoate episodes where lice levels were monitored in at least three of the 12 weeks following treatment, or before another treatment was applied, were included. Where treatment efficacy is summarised by year, post-treatment lice abundance was examined in two ways: as mean lice per fish; and as a percentage of pre-treatment abundance (mean post- treatment abundance/mean pre-treatment abundance * 100). When using the latter approach it was important to ensure that the percentage change was based on matched pre and post-treatment lice counts. This allowed the percentage change to be estimated for each seven day period in the 83 days following treatment, even though lice levels were not always monitored at all treated sites every week or for the full 12 weeks following treatment. It should be noted that not all treatment episodes included in this analysis were deemed to have been ‘‘effective’’ (Table 1). Data management and statistical analysis Data were stored and managed using a set of structured tables in Microsoft Access 2003. This application was also used to calculate the mean sea lice abundance values and post-treatment lice abundance as a percentage of pre-treatment levels. All figures were constructed using Microsoft Excel 2003 while statistical analyses were performed within Minitab 14.1. A statistical model to determine the factors affecting post- treatment lice abundance was investigated using a General Linear Model (GLM) procedure. Interactions between all available factors were also examined. As treatment lasted for 7 days (day 0 to 6), post-treatment abundance was only analysed for the 7 to 83 day period following treatment initiation. To improve normality and equalise variances, data were logarithmically transformed (ln(x+1)) prior to GLM analysis. To aid interpretation of the final models, least squares means and their 95% confidence intervals were subsequently untransformed. The sea lice treatment screening process resulted in a data set containing 185 emamectin benzoate treatment episodes, 77 of which did not have the necessary pre/post treatment sea lice data required for further efficacy analysis. Of the remaining 108 treatments, 22 were followed-up with another sea lice treatment (either cypermethrin or emamectin benzoate) within 12 weeks. A summary of the treatment episodes in each of the data sets is presented in Table 2. The number of treatment episodes, and sites on which they were administered, varied over the five-year period studied. However, the majority (84%) of sites studied were represented in the final data set used for efficacy analysis. As shown in Figure 1, the profiles of the full and the reduced data sets were broadly similar. In the first year of production the majority of emamectin benzoate treatments were administered in the autumn (Aug–Oct) whereas in the second year most occurred in the spring (Feb–Apr). Few episodes occurred in spring of the first year or winter (Nov–Jan) of the second. The main distinction between the full and the reduced data sets was the proportion of episodes administered in the autumn of the second production year. Several of the emamectin benzoate treatments applied in this period had to be discounted from the efficacy analysis because Table 2. Numbers of sites and episodes of emamectin benzoate treatment in years 2002 to 2006. All treatments No. of sites No. of episodes Treatments with necessary pre/post treatment sea lice data No. of sites No. of episodes 26 31 56 47 25 185 10 18 24 25 13 47 10 21 31 30 16 108 Year 2002 2003 2004 2005 2006 All 22 24 35 31 17 54 Figures are presented for all site-wide emamectin benzoate treatment episodes and for episodes with the necessary pre/post treatment sea lice data required for further analysis. doi:10.1371/journal.pone.0001549.t002