Development and immunoprotective evaluation of a chitosan-based nanovaccine against Aeromonas hydrophila in Yangtze sturgeon (Acipenser dabryanus).

The Chinese sturgeon (Acipenser sinensis), a critically endangered species, faces heightened susceptibility to infectious disease during intensive juvenile rearing. Given the close phylogenetic relationship between the Chinese sturgeon and the Yangtze sturgeon (Acipenser dabryanus), the latter was employed as a surrogate model to evaluate a chitosan-encapsulated, inactivated Aeromonas hydrophila nanovaccine prepared using an emulsion-based method. Safety assessments over a 28-day post-vaccination period revealed no mortality in either immersion- or injection-vaccinated groups, and histological examination of the liver, intestine, and spleen detected no vaccine-associated adverse effects. Immune response analyses revealed distinct tissue-specific and time-dependent expression patterns of IL-1, TNF-α, MHC I, MHC II and IgM. Injection immunization induced stronger systemic immune responses in the liver and spleen, whereas immersion immunization elicited more pronounced mucosal responses in the intestine, demonstrating route-dependent modulation of systemic versus mucosal immunity. Serum ELISA analyses showed a significant increase in A. hydrophila-specific IgM levels by day 21 post-immunization, with the immersion group exhibiting a more sustained antibody response. Evaluations of serum immune enzyme activities revealed only moderate fluctuations, with no evidence of physiological stress. Challenge experiments demonstrated that both vaccination routes conferred comparable protective efficacy, yielding relative percent survival (RPS) values of 77.78% for immersion and 75% for injection. Overall, these results demonstrate that chitosan-based antigen encapsulation enables effective and safe vaccine delivery. Importantly, immersion immunization provides protection comparable to injection while offering clear advantages in operational simplicity and scalability, highlighting its potential for bacterial disease control in sturgeon aquaculture.