Development of Immersion and Oral Bivalent Nanovaccines for Streptococcosis and Columnaris Disease Prevention in Fry and Fingerling Asian Seabass (Lates calcarifer) Nursery Farms

In the present study, bivalent nanovaccines of S. iniae and F. covae were administered by immersion vaccination at 30 and 40 days after hatching (DAH), and the third vaccination was orally administered by feeding at 50 DAH. Five different groups were designed: group 1 was set as the control group, and groups 2-4 were vaccinated using the same protocols. ELISA revealed that the levels of total IgM and specific IgM to S. iniae, and F. covae were significantly elevated in groups 2-4 at 10, 20, and 30 days after vaccination (DAV). A qRT‒PCR analysis of immune-related genes revealed significantly higher IgT expression in the vaccinated groups compared with the control group, as revealed by 44-100-fold changes in the vaccinated groups compared with the control (p < 0.001) at every tested time point after vaccination. All vaccinated groups expressed IgM, MHCIIα and TCRα at significantly higher levels than the control group at 10 and/or 20 DAV (p < 0.05). In the S. iniae challenge tests, the survival of vaccinated groups 2-4 ranged from 62.15 ± 2.11 to 75.70 ± 3.36%, which significantly differed from that of the control group (44.44 ± 1.92%). Similarly, all vaccinated groups showed higher survival rates of 68.89 ± 3.85 to 77.78 ± 5.09% during F. covae challenge than the control groups (50.00 ± 3.33%) (p < 0.05).