Effectiveness of a new cationic lipid-based nanovaccine for enhancing immersion vaccination against Flavobacterium oreochromis in red tilapia (Oreochromis sp.).

Flavobacterium oreochromis (Fo) poses a significant threat to tilapia, leading to economic losses due to mortality. Immersion vaccines, while practical for small fish, often result in limited antigen absorption. This study aimed to develop a cationic lipid-based nanoparticle vaccine using cetyltrimethylammonium bromide (CTAB) and evaluate its efficacy against F. oreochromis in red tilapia. In the first trial, healthy red tilapia were immersion vaccinated for 30 min, with three groups included in the trial: control (non-vaccinated), formalin-killed sonicated cells (FK-SC), or cationic lipid-based nanoparticles (Fo-NV). The second trial followed the same design, with booster vaccinations (FK-SC-B, Fo-NV-B) administered 14 days after the first vaccination. Fish were challenged with virulent F. oreochromis at multiple time points up to 120 days post-vaccination (dpv) in the first trial and up to 180 dpv in the second, with survival recorded for 10 days post-challenge during each challenge. Specific IgM antibody levels were measured at various dpv intervals. The Fo-NV group, characterized by nanoscale size (179 nm) and positive charge (13 mV), showed enhanced stability and mucoadhesion compared to FK-SC. In the first trial, the Fo-NV group had significantly higher relative percentage survival (RPS) (83.3-63.3 %) compared to FK-SC (33.3-10.0 %) during the first three months. In the second trial, the Fo-NV-B group exhibited elevated IgM levels and higher RPS (81.8-58.5 %) compared to control groups over five months. In conclusion, a booster dose of Fo-NV improved vaccine efficacy, enhancing antigen delivery to mucosal surfaces and providing prolonged protection against F. oreochromis infection.