Abstract
Background:
Vaccination plays a vital role in preventing bacterial infections in aquaculture.

Aim:
This study aimed to develop and evaluate a feed-based monovalent vaccine to protect rainbow trout against Aeromonas salmonicida.

Method:
The vaccine was created using a naturally occurring strain of A. salmonicida isolated from rainbow trout. Formalin-killed bacteria were either sprayed onto or incorporated into commercial feed with palm oil and aloe vera oil added as adjuvants at 10%. The experimental diets were tested for stability, palatability, and safety before the trial. Rainbow trout (average weight: 100 ± 5.0 g) were divided into seven groups, including controls with three replicates of each treatment. Groups received either sprayed or incorporated vaccinated feed, with or without adjuvants for two months at 5% of their body weight. Immune responses were assessed through serum lysozyme activity, agglutination antibody titers, and immunoglobulin M (IgM) levels.

Results:
The group receiving incorporated feed with palm oil (IVFP) showed the highest immune response and growth performance. After 14 days, fish were challenged with a virulent A. salmonicida strain (6.3×10⁹ CFU/fish). Survival rates were 69% for fish immunized with bacterin alone and 85–90% for those with adjuvants. Histological analysis revealed minimal kidney and liver damage in vaccinated groups compared to controls.

Conclusion:
These findings highlight the potential of oral vaccines to reduce A. salmonicida pathogenicity in rainbow trout offering a promising solution for sustainable aquaculture.

Key words: Aeromonas salmonicida; Agglutination antibody titer test; Feed based monovalent vaccine; Lysozyme activity; Rainbow trout.