First report of reduced sensitivity towards hydrogen peroxide found in the salmon louse Lepeophtheirus salmonis in Norway

•Reduced hydrogen peroxide sensitivity was found in salmon lice in Norway.•Bioassays seperate strains of salmon lice regarding hydrogen peroxide sensitivity.•Up to ten times variance in sensitivity between two strains was recorded.

Reduced sensitivity towards chemotherapeutants in the salmon louse Lepeophtheirus salmonis (Krøyer) is an increasing problem for the fish farming industry. Most fish farmers are dependent on chemical treatments in order to maintain salmon lice numbers below permitted levels. However parasites showing reduced sensitivity contribute to complicating this task. Hydrogen peroxide (H2O2) is used as a delousing agent in bath treatments and until recently treatment failures due to reduced H2O2-sensitivity have not been documented in Norway. The aim of the current study was to develop a bioassay protocol suitable for testing H2O2-sensitivity in L. salmonis. If failed treatments were found to be caused by parasite insensitivity to H2O2 the possibility of this reduced sensitivity being hereditary was looked into. The results show that bioassays permit differentiation between strains of salmon lice with regards to H2O2-sensitivity, coinciding with treatment efficacies. Up to ten times variance in sensitivity between two strains was recorded. The progeny of the least sensitive salmon lice also showed reduced sensitivity to H2O2 in a bioassay, which indicates that reduced sensitivity towards H2O2 is hereditary. The current study presents the first case report of reduced sensitivity towards H2O2 in salmon lice in Norway. This change in sensitivity imposes a threat to the Norwegian fish farming industry and should be monitored closely.