Abstract
The avermectin derivative emamectin benzoate (EMB) has been widely used by salmon industries around the world to control sea lice infestations. Resistance to this anti-parasitic drug is also commonly reported in these industries. The objective of this study was to quantify the number of sea lice potentially exposed to sub-lethal concentrations of EMB while fish clear the drug after treatments. We assessed juvenile sea lice abundance after 38 EMB treatments on six Atlantic salmon farms, in a small archipelago in British Colombia, Canada, between 2007 and 2018. We fitted a standard EMB pharmacokinetic curve to determine the time when fish treated with this product would have EMB tissue concentrations below the recommended target therapeutic level. During the study, we estimated that for each sea lice treatment there was, on average, an abundance of 0.12 juvenile sea lice per fish during the time period when the concentrations of EMB would have been lower than 60ppb, the recommended therapeutic treatment level for sea lice. The findings from this study on metaphylactic anti-parasitic treatments identify a potential driver for drug resistance in sea lice that should be further explored.
Highlights
The avermectin derivative emamectin benzoate (EMB) has been widely used by salmon industries around the world to control sea lice infestations
In light of the urgent need to better understand the impact of different treatment strategies on emerging anti-parasitic drug resistance, and the need to increase the life span of new, in-feed anti-parasitic treatments, we explored sea lice infection after metaphylactic treatments with EMB
There was a total of 40 EMB treatments during our study period, but only 38 treatments had sea lice data for the time period when fish would have had EMB levels below 60 ppb
Summary
The avermectin derivative emamectin benzoate (EMB) has been widely used by salmon industries around the world to control sea lice infestations. Lepeophtheirus salmonis and Caligus spp., is a group of grossly visible host-dependent ectoparasites[1,2] commonly found in all large salmonid aquaculture industries around the world Treatment of these parasites use chemical baths and/or in-feed pharmaceutical products such as the avermectin derivative emamectin benzoate (EMB)[3,4,5]. Given the dynamics of sea lice infestations within salmon farming areas[10], and the size of farm populations treated, the selection pressure for drug resistant sea lice may be significant Despite this there are no studies, which have assessed the level of re-infection during the period when fish would have sub-lethal levels of EMB in their tissues. We determined the re-infection pressure after oral (in-feed) EMB treatments on salmon farms, during the period when animals were clearing the anti-parasitic medication and were likely below the EMB manufacturer’s recommended therapeutic threshold
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