Immune Responses and Protective Efficacy of Immersion and Intraperitoneal Vaccination Against Tenacibaculum maritimum in European Seabass (Dicentrarchus labrax)

Tenacibaculosis, caused by Tenacibaculum maritimum, remains a major bacterial disease affecting marine aquaculture, particularly European seabass farming. This study evaluated the immune response and protective efficacy of a formalin-inactivated T. maritimum vaccine administered by immersion or intraperitoneal (i.p.) injection in European seabass (Dicentrarchus labrax). Each vaccination strategy was evaluated using its corresponding experimental challenge model 36 days post-vaccination. Vaccine performance was assessed by relative percentage of survival (RPS), expression of selected immune-related genes (IL-1β, IL-10, IgT), and specific serum IgM production. Intraperitoneal vaccination followed by i.p challenge provided significant protection, with 87% survival and an RPS of 67, accompanied by elevated serum IgM levels and pronounced systemic immune activation. Immersion-vaccinated fish also showed complete survival; however, the unexpectedly low mortality in the corresponding mock-vaccinated controls limited the discriminatory capacity of the immersion challenge model and prevented a robust assessment of vaccine efficacy according to the defined criteria. Gene expression analysis revealed distinct immune profiles, with immersion vaccination inducing an early but transient response, whereas i.p. vaccination elicited stronger and more sustained systemic activation, particularly within innate and humoral immune pathways. Principal component analysis further demonstrated clear clustering of immune responses according to vaccination route. These findings demonstrate the potential of an autogenous vaccine to protect European seabass against tenacibaculosis and highlight the importance of considering both vaccination route and the challenge model when interpreting vaccine efficacy. Further studies should optimise immersion vaccination protocols and investigate local mucosal immune responses to support the development of effective vaccination strategies for juvenile European seabass.