Vaccination modality and injection site influence the immune response and protection against sea lice Lepeophtheirus salmonis infestation in Atlantic salmon (Salmo salar).

Sea lice (Lepeophtheirus salmonis) infestations represent a challenge to Atlantic salmon (Salmo salar L.) aquaculture. We evaluated recombinant peroxinectin-like proteins (LsPxtl-1 and LsPxtl-2) and whole sea lice (WSL) crude proteins as vaccine antigens, administered intraperitoneally (i.p.) or via a combination of i.p./subcutaneous fin-base injection (IP-F). Fish were immunized, boosted, smoltified, and challenged with copepodites, and systemic (plasma) and mucosal (mucus) antibody responses were assessed alongside lice burden. LsPxtl-1 (p < 0.05) reduced lice counts compared to PBS control but not against the empty plasmid PBS group. Elevated systemic IgM/IgT and IgT responses were observed, but levels did not correlate with lice reduction. Mucus antibody responses were generally weak, but suggest IgT may contribute to local protection, particularly in the IP-F group. Results demonstrate that antigen source and vaccine modality influence protection, and protective immunity likely involves additional mechanisms beyond measurable antibodies. Further optimization of antigens, formulations, and delivery strategies is required to achieve reliable vaccine-based control of salmon lice in aquaculture.