Enhanced Efficacy and Immunogenicity of a Nanoencapsulated ECP-Enriched Aeromonas hydrophila Vaccine in Common Carp (Cyprinus carpio)

Recently, there has been increasing attention regarding the potential application of biodegradable nanoparticles (NPs) as adjuvants in injectable vaccines. In this study, encapsulated antigens of Aeromonas hydrophila in alginate/chitosan polymers were used as injectable vaccines in common carp. Formalin-killed bacteria and extracellular products (ECPs) of A. hydrophila were encapsulated via emulsification method. In total, 480 juvenile common carp (32.1 ± 3.64 g, mean ± SD) were randomly divided into eight triplicate groups (20 fish for each replicate). Groups 1–3 were injected with bacterin, ECP, and bacterin + ECP, respectively. Groups 4–6 were injected with encapsulated bacterin, encapsulated ECP, and encapsulated bacterin + ECP, respectively. Groups 7 and 8 as controls were injected with alginate/chitosan and phosphate-buffered saline (PBS), respectively. Blood samples were collected on days 0, 30, and 60 of the experiment. Immunological parameters, including antibody titer against A. hydrophila, serum lysozyme, bactericidal and complement activities, and nitro blue tetrazolium (NBT) reduction, as well as serum globulin and protein levels, were compared among the groups. Hematological parameters (red blood cell [RBC], white blood cell [WBC], hemoglobin [Hb], and hematocrit (Hct) were evaluated too. Remained fish were challenged with severe A. hydrophila, and mortality was recorded for 10 days. Results showed that anti-A. hydrophila antibody titer, survival rate following bacterial challenge, NBT reduction, serum lysozyme activity, globulin, and protein levels were significantly higher in encapsulated bacterin and encapsulated bacterin + ECP groups compared with the control group (p  < 0.05). Serum bactericidal and complement activity showed no significant change in different groups of sampling points. Then, it can be concluded that the encapsulation of bacterin and bacterin + ECP of A. hydrophila using chitosan/alginate NPs enhanced the protection and immune responses in common carp and this method is a promising candidate for a vaccine adjuvant in fish.