The protective effect of the Phe362Tyr mutation in salmon lice’ (Lepeophtheirus salmonis) AChE when exposed to full-scale azamethiphos bath treatments

Abstract Organophosphates are applied for medicinal bath treatments of salmon lice ( Lepeophtheirus salmonis ) infested farmed salmonids. This chemical class remains important despite the development of resistant parasites, due to few available treatment options. The protective effect of the Phe 362 Tyr mutation in one of the acetylcholinesterase (AChE) genes of L. salmonis has previously been studied in small-scale treatments with the organophosphate azamethiphos. The current study was aimed at investigating the protective effect of this mutation in field treatments of commercial fish farms. In addition the effect of different methods of sampling on the occurrence of salmon lice with zero, one or two copies of Phe 362 Tyr (homozygote sensitive (SS), heterozygote (RS) and homozygote resistance (RR) respectively) were investigated. Salmon lice were collected prior to and after azamethiphos treatments in four fish farms. The resistance genotypes were determined for each parasite. No SS lice were found in any of the farms post-treatment. Both RR and RS genotypes protected the salmon lice from the effect of azamethiphos, but the protective effect of RR genotype was greater than of RS. The study thereby emphasizes the strong selection pressure towards resistant parasites imposed by an azamethiphos treatment event in farms where resistant parasites are present. The distribution of SS, RS and RR genotypes were not found to differ between lice from different net pens, sampled at different dates or picked off fish of different health statuses in a study with repeated sampling from a single farm.