Vaccination against Aeromonas salmonicida in turbot (Scophthalmus maximus L.): Study of the efficacy, morphological changes and antigen distribution

• Oil-adjuvant vaccine conferred higher protection than aqueous based vaccine.
• Oil-adjuvant vaccine caused severe intra-coelomic lesions in the injected turbot.
• Pathological studies contribute to evaluate the efficacy and safety of vaccines.

Aeromonas salmonicida subspecies salmonicida (A. salmonicida), is one of the major pathogens of fish farm, being particularly harmful for salmonid and turbot culture. Vaccination against A. salmonicida generally provides a protective response, especially in Atlantic salmon and rainbow trout. However, there are few data there about vaccine against this pathogen in turbot. In order to evaluate the efficacy of the experimental vaccines against A. salmonicida on this species, we have assessed the degree of protection and the side-effects of different vaccinal formulations. Two groups of fish were intracoelomically injected with aqueous and oil based vaccines containing formalin inactivated A. salmonicida as immunogen. The analysis of lesions was performed by gross and microscopic studies, as well as immunohistochemical methods using anti-cytokeratin, -TNF-α and -iNOS antibodies. Immunohistochemistry was also employed to describe the distribution of vaccinal antigen in turbot tissues.Results revealed significant differences between both groups regarding the protection, immune response, and the distribution of the bacterins. Oil-adjuvanted vaccine showed acceptable levels of protection but caused severe granulomatous coelomitis, so their use may be questionable in terms of animal health and welfare. On the other hand, fish injected with aqueous vaccine showed mild inflammatory reaction and systemic distribution of the bacterins, but the level of protection was insignificant.The present study highlights the importance of the histopathology and immunohistochemistry not only to assess the side-effects of different vaccine adjuvants in turbot, but also to the evaluation of the efficacy and the safety of vaccines against A. salmonicida in this species.