Investigation of deltamethrin resistance in salmon lice (Lepeophtheirus salmonis) provides no evidence for roles of mutations in voltage-gated sodium channels

BACKGROUND The pyrethroid deltamethrin is used to treat infestations of farmed salmon by parasitic salmon lice,Lepeophtheirus salmonis(Kroyer). However, the efficacy of deltamethrin for salmon delousing is threatened by resistance development. In terrestrial arthropods, knockdown resistance (kdr) mutations of the voltage-gated sodium channel (Na-v), the molecular target for pyrethroids, can cause deltamethrin resistance. A putativekdrmutation of anL. salmonissodium channel homologue (LsNa(v)1.3 I936V) has been identified previously. At the same time, deltamethrin resistance ofL. salmonishas been shown to be inherited maternally and to be associated with mitochondrial DNA (mtDNA) mutations. This study assessed potential roles of the above putativekdrmutation as a determinant of deltamethrin resistance in laboratory strains and field populations ofL. salmonis. RESULTS The deltamethrin-resistantL. salmonisstrain IoA-02 expresses the LsNa(v)1.3 I936V mutation but was susceptible to the non-ester pyrethroid etofenprox, a compound against which pyrethroid-resistant arthropods are usually cross-resistant if resistance is caused by Na(v)mutations. In a family derived from a cross between an IoA-02 male and a drug-susceptible female lacking thekdrmutation, deltamethrin resistance was not associated with the genotype at the LsNa(v)1.3 locus (P > 0.05). Similarly, in Scottish field populations ofL. salmonis, LsNa(v)1.3 I936V showed no association with deltamethrin resistance. By contrast, genotypes at the mtDNA loci A14013G and A9030G were significantly associated with deltamethrin resistance (P < 0.001). CONCLUSION In the studiedL. salmonisisolates, deltamethrin resistance was unrelated to the LsNa(v)1.3 I936V mutation, but showed close association with mtDNA mutations.

Automated summary

The study found that deltamethrin resistance in Lepeophtheirus salmonis is unrelated to the LsNa(v)1.3 I936V mutation, but is closely associated with mtDNA mutations.