Optimization of an inactivated vaccine against a scuticociliate parasite of turbot: Effect of antigen, formalin and adjuvant concentration on antibody response and protection against the pathogen

Philasterides dicentrarchi is a scuticociliate parasite that causes important mortalities in cultured turbot and therefore considerable economic losses. In the present study, we applied for the first time a factorial experimental design to optimize the formulation of an inactivated anti-P. dicentrarchi vaccine on the basis of three of its main components: antigenic dose, concentration of inactivating agent and proportion of adjuvant in the emulsion, and determined the effect of each factor on protection against the parasite and on antibody production. We found that concentration of antigen was the only one of the three factors considered that had a significant effect on the level of specific serum antibodies. In addition, we showed a statistically significant relationship between the level of antibodies and the mean time of survival of the vaccinated fish. The vaccine formulation containing 106 trophozoites/ml, 0.2% formalin and 50% adjuvant generated the longest mean time of survival, 100% protection and the highest levels of antibodies in the vaccinated fish.