Increased catalase activity — A possible resistance mechanism in hydrogen peroxide resistant salmon lice (Lepeophtheirus salmonis)

Abstract Due to their pathogenic effects and vast reproductive capacity, salmon lice, Lepeophtheirus salmonis , pose a problem for salmonids and salmonid fish farmers in the Northern Hemisphere. Reliance on chemical delousing has resulted in widespread resistance amongst salmon lice throughout most of the salmonid producing world. In an attempt to combat resistant salmon lice, hydrogen peroxide (H 2 O 2 ) has become increasingly used in salmon lice bath treatments. Resistance towards H 2 O 2 has however also been detected in both Scottish and Norwegian salmon lice, while the resistance mechanism(s) still needs to be elucidated. Increased activity of the H 2 O 2 degrading enzyme catalase has been found in several other H 2 O 2 resistant organisms. The aim of the current study was therefore to investigate catalase activity and catalase gene expression in H 2 O 2 resistant and sensitive salmon lice. In the current study, two strains of salmon lice differing in sensitivity towards H 2 O 2 were used. Four different groups of salmon lice were included per strain: pre-adult I females and males, pre-adult II females and adult males. In comparison to the sensitive strain, both the catalase activity and the expression of the catalase gene were increased in the resistant salmon louse strain. This was true for all investigated life stages. The least square mean catalase activity was 2.0 units per mg protein in the sensitive strain and 6.5 units per mg in the resistant strain. The results obtained in this study indicate that increased catalase activity plays a role in H 2 O 2 resistance in salmon lice. To the best of our knowledge, this is the first possible H 2 O 2 resistance mechanism described in arthropods. Relevance The current study strongly indicates that increased catalase expression and subsequent enhanced catalase activity is a mechanism for H 2 O 2 resistance in salmon lice. To the best of our knowledge, this is the first possible H 2 O 2 resistance mechanism described in arthropods. Molecular screening methods can be developed on this basis.