The Efficacy of Emamectin Benzoate against Infestations of Lepeophtheirus salmonis on Farmed Atlantic Salmon (Salmo salar L) in Scotland, 2002–2006 Fiona Lees1,2, Mark Baillie2, George Gettinby1, Crawford W. Revie2* 1 Department of Statistics and Modelling Science, University of Strathclyde, Glasgow, United Kingdom, 2 Department of Computer and Information Sciences, University of Strathclyde, Glasgow, United Kingdom Abstract Background: Infestations of the parasitic copepod Lepeophtheirus salmonis, commonly referred to as sea lice, represent a major challenge to commercial salmon aquaculture. Dependence on a limited number of theraputants to control such infestations has led to concerns of reduced sensitivity in some sea lice populations. This study investigates trends in the efficacy of the in-feed treatment emamectin benzoate in Scotland, the active ingredient most widely used across all salmon producing regions. Methodology/Principal Findings: Study data were drawn from over 50 commercial Atlantic salmon farms on the west coast of Scotland between 2002 and 2006. An epi-informatics approach was adopted whereby available farm records, descriptive epidemiological summaries and statistical linear modelling methods were used to identify factors that significantly affect sea lice abundance following treatment with emamectin benzoate (SLICEH, Schering Plough Animal Health). The results show that although sea lice infestations are reduced following the application of emamectin benzoate, not all treatments are effective. Specifically there is evidence of variation across geographical regions and a reduction in efficacy over time. Conclusions/Significance: Reduced sensitivity and potential resistance to currently available medicines are constant threats to maintaining control of sea lice populations on Atlantic salmon farms. There is a need for on-going monitoring of emamectin benzoate treatment efficacy together with reasons for any apparent reduction in performance. In addition, strategic rotation of medicines should be encouraged and empirical evidence for the benefit of such strategies more fully evaluated. Citation: Lees F, Baillie M, Gettinby G, Revie CW (2008) The Efficacy of Emamectin Benzoate against Infestations of Lepeophtheirus salmonis on Farmed Atlantic Salmon (Salmo salar L) in Scotland, 2002–2006. PLoS ONE 3(2): e1549. doi:10.1371/journal.pone.0001549 Editor: Ross Thompson, Monash University, Australia Received October 5, 2007; Accepted January 14, 2008; Published February 6, 2008 Copyright: 2008 Lees et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The database underpinning this work was created as part of two UK government funded research projects (MAFF LINK-ENV12; DEFRA-VM0213). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. *E-mail: crawford.revie@cis.strath.ac.uk Introduction Commercial farming of Atlantic salmon (Salmo salar L) has developed rapidly since the 1970’s, with global production exceeding one million tonnes per annum since 2002 [1]. Atlantic salmon farming is currently dominated by the aquaculture industries of Norway and Chile, however Scotland and Canada are also major producers. As intensive marine aquaculture developed, the threat posed to fish health and production by infestations of parasitic copepods emerged as one of the greatest challenges facing the industry [2]. Not only can these aquatic parasites inhibit growth and cause extensive damage, extreme infestation can lead to host mortality [3]. It has also been suggested that caligid copepods, commonly referred to as sea lice, originating from salmon farms may pose a risk to wild salmonid populations [4–8]. In Scotland two species of sea lice parasitise farmed salmonids: Lepeophtheirus salmonis (Krøyer 1837) and Caligus elongatus (Nord- mann 1832). Of the two species, L. salmonis is the larger and more abundant [9]. Whereas C. elongatus is known to parasitise more than 80 species of fish, the major species of interest L. salmonis is principally confined to salmonids [10]. In response to the challenges presented by sea lice infestation, salmon producers on the west coast of Scotland have developed integrated health management programmes based on previous research into the epidemiology of sea lice [11–14] and farm management practices [15–18]. Some of these management strategies have proven to be successful and, together with the availability of more effective ectoparasitic medicines, have helped to reduce the abundance of L. salmonis and C. elongatus on Scottish farms over the past decade [19]. Nevertheless, sea lice remain a persistent problem and the cost of controlling these parasites is substantial [18]. The availability and use of medicines to control sea lice burdens in Scotland has changed considerably in the last decade and since 2005 only two therapeutants have been in common use; the topical treatment cypermethrin (ExcisH, Novartis Animal Health) and the in-feed treatment emamectin benzoate (SLICEH, Schering Plough Animal Health). Both ectoparasiticides are widely used and, since obtaining UK Market Authorisation in 2000, the use of PLoS ONE | www.plosone.org 1 February 2008 | Volume 3 | Issue 2 | e1549