Effectiveness of homologous inactivated Streptococcus agalactiae vaccine by immersion bath in Nile tilapia (Oreochromis niloticus)

Abstract

Tilapia is the most important group between cultivated fish. Intensive handling systems are characterized by high rate of stocking of fish making them susceptible to infectious diseases. The streptococcosis is one of the diseases that cause great economic losses. The objective of the study was to test a vaccine administered by immersion bath (b.i.) against Streptococcus agalactiae and challenge with homologous strain. For this study, 421 Nile tilapia (Oreochromis niloticus) with an average weight of 38.38 grams were used. They were divided into two treatments (T1 and T2) and a control group. At T1, the fish were vaccinated with one dose of vaccine by b.i., with a concentration of 5.4 x 108 CFU mL-1. The T2 received two doses of vaccine by b.i., with an interval of 25 days. The control group received a bath of ultrapure water. The fish were challenged by intraperitoneal (i.p.) with a concentration of 3.0 x 108 CFU mL-1 43 days after the first or second dose of vaccine, according to the treatment. The fish were monitored for 16 days after challenge. Compared with the control group results were obtained for T1: p = 0.0805, RR = 0.79 (95% CI: 0.61 – 1.01) and RPS = 21%. For T2 were: p = 0.0296, RR = 0.74 (95% CI 0.56 to 0.96) and RPS = 26%. Mortality after challenge was 57 fish (40.71%) in T1, 51 (38.06%) at T2, and 76 (51.7%) in the control group. There was no significant difference between T1 and T2, p = 0.7445. This result shows that the vaccine tested by b.i. with one and two doses had low efficacy in vaccination of tilapia.