First Investigation of the Optimal Timing of Vaccination of Nile Tilapia (Oreochromis niloticus) Larvae against Streptococcus agalactiae

Abstract

To investigate early immune responses and explore the optimal vaccination periods, Nile tilapia at 1, 7, 14, 21, 28, 35, and 42 days after yolk sac collapse (DAYC) were immersed in formalin-killed Streptococcus agalactiae vaccine (FKV-SA). A specific IgM was first detected via ELISA in the 21 DAYC larvae (0.108 g) at 336 h after vaccination (hav), whereas in the 28–42 DAYC larvae (0.330–0.580 g), the specific IgM could be initially detected at 24 hav. qRT–PCR analysis of the TCRβ, CD4, MHCIIα, IgHM, IgHT, and IgHD genes in 21–42 DAYC larvae immunized with the FKV-SA immersion route for 24, 168, and 336 hav revealed that the levels of most immune-related genes were significantly higher in the vaccinated larvae at all DAYCs than in the control larvae (p < 0.05) at 336 hav. Immunohistochemistry demonstrated stronger IgM signals in the gills, head kidney, and intestine tissues at 21, 28, and 35 DAYC in all vaccinated larvae compared with the control. Interestingly, at all DAYCs, FKV-SA larvae exhibited significantly higher survival rates and an increased relative percent survival (RPS) than the control after challenge with viable S. agalactiae, particularly in larvae that were immunized with FKV-SA at 168 and 336 hav (p < 0.05).

Adjuvant Effects of a CC Chemokine for Enhancing the Efficacy of an Inactivated Streptococcus agalactiae Vaccine in Nile Tilapia (Oreochromis niloticus)

“…Fifty percent of the water was replaced every three days. Prior to conducting the experiment, the internal organs (liver, spleen, and head kidney) of five randomly selected tilapia were examined for the presence of tilapia lake virus (TiLV) and bacterial pathogens [ 29 ]. PCR and microbiological techniques were used to confirm the health status of the fish.…”

“…The fish in the control group were injected via the intraperitoneal (IP) route with 0.1 mL of phosphate-buffered saline (PBS, pH 7.4), while the fish in the second and third groups were also IP injected with 0.1 mL of the inactivated S. agalactiae vaccine (WC) or 1 × 10 6 CFU of formalin-killed S. agalactiae containing 10 µg of r On -CC1 (WC+CC). During this period, growth parameters were investigated using protocols described previously [ 29 ] at 29 days postvaccination (dpv), one day before the challenge test.…”

“…The concentration was 1 × 10 8 CFU/mL. Following a 30-day vaccination period, 20 fish from each replicate ( n = 80/group) were intraperitoneally injected with 0.1 mL of the bacterial solution, based on preliminary median lethal dose (LD 50 ) data from a previous study [ 29 ]. The challenge dose was 1 × 10 7 CFU/fish.…”

“…The mortality of the injected fish was recorded daily for 14 days, and confirmation of the disease in moribund fish involved isolation on TSA to verify the presence of the S. agalactiae bacterial pathogen. Relative percent survival (RPS) was also investigated with a previously reported method [ 29 ].…”