Effectiveness of Feed-Based Monovalent Aeromonas Vaccine in Farmed Carp

<i>Aeromonas hydrophila</i> (<i>A. hydrophila</i>) is responsible for causing abdominal dropsy, swimming abnormalities, skin ulcerations, and pale gills in fish. Vaccination is an essential strategy for disease prevention in aquaculture. This study evaluated the efficacy of an oral vaccine against <i>A. hydrophila</i> in <i>Ctenopharyngodon idella</i> (<i>C. idella</i>). The vaccine was formulated as feed-based monovalent pellets, incorporating or spraying formalin-killed <i>A. hydrophila</i> on/into commercial feed with 30% crude protein. Mineral and fish oils were used as adjuvants at 10% of the feed. Prior to the trial, the experimental feed groups were subjected to quality and safety tests. Grass carp fingerlings (20 ± 5 g) were divided into seven groups (<i>n</i> = 20 per group): sprayed vaccinated feed with fish oil (SVFF), incorporated vaccinated feed with fish oil (IVFF), sprayed vaccinated feed with mineral oil (SVFM), incorporated vaccinated feed with mineral oil (IVFM), sprayed vaccinated feed (SVF), incorporated vaccinated feed (IVF), and a control group. Feed was provided at 3% of body weight for 60 days. Immunomodulation was investigated through lysozyme activity, antibody titers, and immunoglobulin M (IgM). The IVFF group showed significantly enhanced immunity and growth performance, with an 87% protection rate, 13% mortality, and the highest relative percentage survival (83%) following intraperitoneal <i>A. hydrophila</i> (6.8 × 10⁹ CFU/mL) challenge. Histological analysis indicated minimal pathological changes in the IVFF group compared to controls. Fish oil as an adjuvant enhanced immunity without adverse health effects. Overall, this study demonstrated that feed-based monovalent vaccines effectively improve immune responses and provide protection against <i>A. hydrophila</i> in <i>C. idella.</i>