Efficacy of Feed-Based Genome-Free Bacterial Vaccine Against Aeromonas hydrophila Infection in Red Tilapia (Oreochromis sp.)

<i>Aeromonas hydrophila</i> causes motile <i>Aeromonas</i> septicemia (MAS), a disease with a high mortality rate in tilapia culture. Feed-based vaccines with the incorporation of inactivated whole-cell bacteria into the feed offer promising tools to control MAS. Currently, the incorporation of genome-free bacteria as bacterial vaccine through the implementation of SimCells^® technology into the feed has become a particular interest. <b>Background/Objectives</b>: This study investigates the efficacy of a feed-based vaccine incorporating genome-free <i>A. hydrophila</i> (FBV-GFAH) against MAS infection in red tilapia. <b>Methods</b>: The vaccine was prepared and delivered at 5% fish body weight for three consecutive days in weeks 0 (prime vaccination) and 2 (first booster vaccination), orally. Throughout a five-week experimental period, the immune-related genes (IL-1<i>β</i>, MHC-II, CD4, IgT, and IgM) expression in the hindgut and head kidney of the fish was determined using RT-qPCR assay. Lysozyme (serum) and overall IgM (serum, gut lavage, and skin mucus) productions were also detected. <b>Results</b>: Fish vaccinated with FBV-GFAH showed a significant (<i>p</i> ≤ 0.05) improvement in relative percent survival compared with unvaccinated fish following bacterial challenge. FBV-GFAH induced the expression of immune-related genes in the hindgut and head kidney, especially after booster vaccination. Furthermore, serum lysozyme activity and overall IgM production in serum, skin mucus, and gut lavage were also significantly (<i>p</i> ≤ 0.05) improved in the FBV-GFAH vaccinated fish than the unvaccinated fish. <b>Conclusions</b>: This study showed that FBV-GFAH is a promising feed-based vaccine technology to control MAS in cultured tilapia.