First report on sensitivity of Caligus elongatus towards anti-louse chemicals and identification of mitochondrial cytochrome C oxidase I genotypes

Caligus elongatus is a generalist sea louse infesting a wide range of wild and farmed fish in the North Atlantic. Like its relatives Lepeophtheirus salmonis and Caligus rogercresseyi, C. elongatus is a well known pathogen in farmed salmonids, and it also infests farmed cod. The mitochondrial gene cytochrome C oxidase I (mtCOI) is used as a marker for C. elongatus identification at the species and subspecies level, with two genotypes (possibly sibling species) being identified thus far. Sea lice can cause problems in farmed fish when abundant. Salmon farms use chemical, non-chemical and preventive treatments for controlling lice numbers. It is highly plausible that C. elongatus could develop resistance to chemical delousing treatments, similar to its related species, L. salmonis and C. rogercresseyi, which have developed reduced sensitivity against several anti-louse chemicals. In the current study, we aimed to provide data on C. elongatus sensitivity status towards four major anti-louse chemicals (azamethiphos, deltamethrin, emamectin benzoate and hydrogen peroxide). We collected a number of lice from wild saithe and farmed Atlantic salmon from three localities in the West coast of Norway. Lice were kept in lab culture and six-dose bioassays were carried out on the louse offspring for the four anti-louse chemicals. In addition, we screened the samples for the two mitochondrial genotypes using a TaqMan probe assay. The results showed that most of the C. elongatus specimens were sensitive to all the delousing chemicals tested. Lice from one of the farms had lower immobilization rate at the highest H₂O₂ concentration than lice from wild fish, which might be due to sensitivity loss. The mtCOI genotyping revealed the presence of only one genotype (genotype 1) in our sample set. The current study provides, for the first time, the baseline sensitivity levels of C. elongatus to four major anti-lice chemicals used on salmon farms. This would provide a good reference point for future studies or monitoring programs to keep a regular check on the sensitivity levels of this parasite.