Time-to-response toxicity analysis as a method for drug susceptibility assessment in salmon lice

• Published in: Aquaculture
• Main Authors: Carmona-Antoñanzas, Greta, Humble, Joseph L, Carmichael, Stephen N, Heumann, Jan, Christie, Hayden R L, Green, Darren M, Bassett, David I, Bron, James E, Sturm, Armin
• Other Authors: Biotechnology and Biological Sciences Research Council, Scottish Salmon Producers Organisation, Institute of Aquaculture, University of Stirling, Machrihanish, orcid:0000-0001-9026-5675, orcid:0000-0002-7529-0829, orcid:0000-0003-3544-0519, orcid:0000-0003-2632-1999
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2016
• Subjects: Parasite; Drug susceptibility; Emamectin benzoate; Sea lice; Salmon delousing agent; Atlantic salmon; Salmo salar
• Online Access: http://hdl.handle.net/1893/24074; https://doi.org/10.1016/j.aquaculture.2016.08.007; http://dspace.stir.ac.uk/bitstream/1893/24074/1/Carmona_Antonanzas2016.pdf

The salmon louse Lepeophtheirus salmonis (Krøyer, 1837) is an ectoparasite causing infections ofwild and farmed Atlantic salmon (Salmo salar L.) in the Northern hemisphere.While L. salmonis control at commercial mariculture sites increasingly employs non-medicinal approaches, such as cage designs reducing infection rates and biological control through cleaner fish, anti-parasitic drugs are still a requirement for effective fish health care. With only a limited range of salmon delousing agents available, all of which have been in use for more than a decade, drug resistance formation has been reported for different products. Successful resistance management requires reliable susceptibility assessment, which is usually achieved through L. salmonis bioassays. These tests involve the exposure of parasites to different drug concentrations and require significant numbers of suitable L. salmonis stages. The present study reports an alternative bioassay that is based on time-to-response toxicity analyses and can be carried outwith limited parasite numbers. The assay determines the median effective time (ET50), i.e., the time required until impaired swimming and/or attachment behaviour becomes apparent in 50% of parasites, by conducting repeated examinations of test animals starting at the timepointwhere exposure to a set drug concentration commences. This experimental approach further allows the estimation of the apparent drug susceptibility of individual L. salmonis by determining their time to response, which may prove useful in experiments designed to elucidate associations between genetic factors and the drug susceptibility phenotype of parasites. Three laboratory strains of L. salmonis differing in susceptibility to emamectin benzoate were characterised using standard 24 h bioassays and time-to-response toxicity assays. While both the median effective concentration (EC50) and the ET50 showed variability between experimental repeats, both types of bioassay consistently discriminated susceptible and drug-resistant L. ...