Passive transfer of hyperimmune serum anti Streptococcus agalactiae and its prophylactic effect on Nile tilapia experimentally infected

Abstract

The Streptococcus agalactiae bacteria is a potent agent which causes outbreaks of bacterial diseases in fish. The stress caused by management and poor environmental quality makes tilapia susceptible to infections, including by bacterium. The aim of this study was to evaluate the resistance of the Nile tilapia immunized with hyperimmune serum against S. agalactiae subsequently challenged with homologous strain of the same bacteria. After determining the DL 50 of S. agalactiae, 36 tilapias were distributed in 4 aquariums, 2 for the control group and 2 for the group via coelomic, inoculated with the DL 50 for anti-S. agalactiae antibodies production. On the 21st and 28th day blood was collected for the obtainment of hiperimmune serum used in passive transference. Then, 30 tilapias were distributed in 3 aquariums submitted to 3 treatments (GI: control; GII: immunized with inactivated-serum; GIII: immunized with non-inactivated serum). After 48 hours, 7, 14, 21, 28 and 35 days blood collections were done for anti-S. agalactiae antibodies titration using the direct agglutination test. For survival evaluation 30 tilapias were distributed in 3 aquariums and submitted to 3 treatments (GI: control; GII: immunized with inactivated-serum; GIII: immunized with non-inactivated serum). After 48 hours of inoculation tilapias were challenged with 100 mu L of S. agalactiae and monitored twice a day for 35 days. The results showed that titers of antibodies were detected by direct agglutination until the 21st day after passive transference and during the same period the protection between the groups immunized with inactivated serum and non-inactivated serum contending anti-S. agalactiae antibodies was of 80% after challenge with S. agalactiae. In the end, the serum-inactivated serum and active groups had 60 and 80% protection, respectively, while 100% of the fish in the control group developed severe signs of infection and were euthanized. There was no statistical difference in the survival rate between the immunized groups.