Identification and characterization of two salmon louse (Lepeophtheirus salmonis, Krøyer, 1838) heme peroxidases and their potential as vaccine antigens

Abstract Salmon louse, Lepeophtheirus salmonis, represents a major challenge for salmon farming as current treatments impose welfare issues and are costly, and no prophylactic measures are available. Two salmon louse heme peroxidases (LsPxtl-1 and LsPxtl-2) were tested for their importance in parasite development in vitro and as potential vaccine candidates. LsPxtl-1 possesses two heme peroxidase domains and is expressed in ovaries and gut. LsPxtl-1 knockdown in nauplius II stage resulted in poor swimming performance and parasite death when reaching the copepodid stage, indicating LsPxtl-1 being indispensable for parasite development. LsPxtl-2 encodes one peroxidase domain and was predicted to contain an N-terminal signal peptide and an Eph receptor ligand binding domain. LsPxtl-2 knockdown did not impact survival or phenotype. Immunization with DNA plasmids encoding the peroxidases by single or combined injections, gave non-significant reduction in lice numbers post challenge which correlated with low specific antibody levels post vaccination. Combined injections of both plasmids followed by an LsPxtl-2 protein boost induced higher antibody response with reduced lice numbers post challenge but not significantly different from controls. The findings suggest LsPxtl-1 to play a role for parasite development, current formulations and vaccination modalities did not give significant reduction in lice infestation.