emamectin benzoate (SLICEH) has grown dramatically [19]. While both medicines are effective against all parasitic stages of sea lice, the major advantage of emamectin benzoate is that it can offer sustained periods of louse clearance [20–22]. Furthermore, as an in-feed therapeutant, it can be safely and effectively administered during adverse weather conditions and whole sites/ loch systems can be medicated in a coordinated manner. Cypermethrin bath treatments are more labour intensive and can be stressful to the fish; these interventions are applied more frequently toward the end of the production cycle when salmon are larger and in-feed treatments are consequently more costly. In Canada, emamectin benzoate (SLICEH) is available under Emergency Drug Release and another in-feed medicine, teflu- benzuron (CalicideH, Skretting), has an INAD (Investigational New Animal Drug) approval. However, salmon producers in British Columbia only have access to emamectin benzoate [23]. Until recently, various brands of emamectin benzoate were the only sea lice medicines available in Chile, but the topical treatment deltamethrin (Alpha MaxH, Pharmaq) was licensed for use in late 2007. It should be noted that Atlantic farmed salmon in this region are parasitised by Caligus rogercresseyi (Boxshall and Bravo 2000) rather than L. salmonis and C. elongatus. Dependence on such a limited range of ectoparasiticides has raised concerns that resistance in lice will become an increasing problem for salmon farmers unless new medicines become available and/or existing therapeutants are appropriately managed [24,25]. While there have been anecdotal reports of reduced sensitivity and potential resistance of sea lice to emamectin benzoate, particularly in Chile, few published studies have examined the efficacy of emamectin benzoate on infestations of L. salmonis in farmed Atlantic salmon since regular treatment began in Scotland in 2002 [26,27]. As described in a recent report by Gustafson et al [28], efficacy studies usually rely on the availability of an untreated control group with which to compare medicated subjects. However, in a commercial farm setting where untreated salmon could suffer extensive damage from sea lice infestation this option can rarely be justified. In this situation an alternative strategy is to monitor on- going efficacy by comparing post-treatment lice abundance with levels recorded prior to treatment intervention [23,26,28]. Using an epi-informatics approach, this study aims to examine the efficacy of emamectin benzoate against infestations of the mobile stages of L. salmonis in a commercial setting over a five-year period. Through examining sea lice abundance and treatment records drawn from over 50 salmon farms located along the west coast of Scotland between 2002 and 2006, this study identifies factors associated with the efficacy of emamectin benzoate treatment interventions. Materials and Methods Data set Sea lice abundance and treatment data, in addition to site stocking records, were drawn from 56 commercial Atlantic salmon (Salmo salar L) farms located along the west coast of Scotland between 2002 and 2006. As in earlier analyses, [19,29] sites were divided above and below the 57uN line of latitude and referred to as ‘‘North’’ and ‘‘South’’ regions respectively. Those sites noted as ‘‘Western Isles’’ included farms on the east coast of South Uist and on both the east and west coasts of Harris and Lewis. All farms had implemented an integrated health management programme that included the routine monitoring of sea lice abundance throughout the production cycle. All sites were owned and managed by Marine Harvest (Scotland). Sites were stocked with a single year class of fish and for the most part operated an 18 to 24 month production cycle. Typically Efficacy of Emamectin Benzoate farms were stocked between January and June of the first production year and harvested between August and December of the second. At some sites, fish were introduced in October and others were not harvested until spring of the third year. Occasionally stocks were split at the end of the first production year. The number of stocked sites varied from year to year, but all farms were fallowed for a minimum of six weeks between production cycles. Treatment records provided the start and end date of each sea lice treatment episode in addition to the type of medicine(s) applied and the quantity administered. The number of pens treated was recorded which, when compared with stocking data, allowed each episode to be classified as either a full or partial site treatment. Stocking records also made it possible to categorise treatments as having taken place in the first or second year of production. Episodes were further classified by the season in which they occurred with months grouped together according to mean sea water temperatures. In this analysis, spring is defined as February to April; summer as May to July; autumn as August to October; and winter as November to January. Water temperatures were lowest in the spring months and highest in the autumn. Where possible the industrial partner (Marine Harvest) performed routine lice counts weekly, however because monitor- ing can at times be logistically difficult, e.g. due to adverse weather conditions, farms sometimes deviated from this protocol and observations at some locations became more sporadic. Depending on the size of the farm, 10 to 30 fish from between two and six pens were randomly monitored for lice at each sample point [30,31]. Fish were removed by dip net and anaesthetised before being visually inspected for lice. Lice counts were classified according to species with L. salmonis further differentiated by one of five gender/life-stages: chalimus, pre-adult, adult male, non-gravid female and gravid female. In the analyses presented here the pre- adult, adult male, non-gravid female and gravid female stages of L. salmonis have been aggregated and are reported as L. salmonis mobiles. The mobile stages, so called due to their ability to move around and between fish, are the stages against which treatments are typically targeted as they tend to cause the greatest damage to the host. Treatment episode selection In the period 2002 to 2006, 561 sea lice treatment episodes were available for analysis, 258 of which included the use of the in-feed medicine emamectin benzoate (SLICEH). All but one of the sites studied had at least one treatment episode involving this medicine. To ensure consistency only site-wide treatment episodes, where fish in all pens began treatment on the same day, were selected for inclusion within this analysis. Mixed treatment episodes, where some pens were medicated with emamectin benzoate and other pens with a different theraputant, were not included. Treatments that occurred on sites that did not follow the typical two-year production cycle, i.e. those that were stocked between July and August, were also excluded from this analysis because such treatments could not readily be classified as first or second year interventions. All emamectin benzoate treatments were adminis- tered as medicated feed at 50 mg kg21 fish for 7 consecutive days. In contrast to some other regions [28], strategic sea lice treatments are encouraged in Scotland; often in the early spring of the second year of production [32]. On some occasions, such interventions are made even when lice levels are low and mean louse abundance is below treatment-trigger guidelines (see Table 1). The aim of strategic treatment is to disrupt the life- cycle of L. salmonis and prevent levels rising in the latter part of the production cycle. Such treatments are typically co-ordinated PLoS ONE | www.plosone.org 2 February 2008 | Volume 3 | Issue 2 | e1549