Efficacy evaluation of live attenuated vaccines against Streptococcus agalactiae Ia, administered to Nile tilapia (Oreochromis niloticus) by immersion or intraperitoneal injection.

Streptococcus agalactiae serotype Ia is a major bacterial pathogen in Nile tilapia (Oreochromis niloticus), and vaccines for juvenile fish, especially immersion vaccines, remain unavailable. We evaluated live attenuated vaccine candidates based on deletion of capsule alone (Δcps) or capsule plus hemolysin (Δcps/Δcyl), and compared them with an oil-adjuvanted inactivated vaccine. Nile tilapia were vaccinated by intraperitoneal injection with an inactivated bacterin, live Δcps, or live Δcps/Δcyl, or by immersion with live Δcps/Δcyl. Mock-vaccinated fish served as controls. After approximately 1000° days, fish were challenged intraperitoneally with a virulent S. agalactiae Ia at high, medium, and low challenge doses. ELISA measured antibody responses, and group means ELISA values were plotted against mortality for each challenge dose. Bacterial DNA in immune organs, post-challenge vaccination, and in brain tissue post-challenge. Intraperitoneal vaccination induced significant systemic antibody responses, whereas immersion vaccination induced only a modest, non-significant increase. Bacterial DNA persisted transiently in the spleen and kidney after intraperitoneal vaccination but was not significantly elevated after immersion at 1000°-days. Protection varied with vaccine type and challenge pressure. The oil-adjuvanted inactivated vaccine showed the highest efficacy, with relative percent survival of 95-100%. The live Δcps mutant given intraperitoneally yielded 71 to 100% relative percent survival, the live Δcps/Δcyl mutant given intraperitoneally yielded 38 to 89%, and immersion with Δcps/Δcyl yielded 33 to 61%. Only the inactivated oil-adjuvanted vaccine markedly reduced bacterial DNA in the brain after challenge. Across groups, higher mean ELISA antibody levels were generally associated with lower mortality at the high-, medium-, and low-challenge doses. Intraperitoneal delivery of both inactivated and live attenuated vaccines induced the strongest systemic responses and conferred the greatest protection. However, immersion with the live attenuated Δcps/Δcyl strain still conferred moderate protection and remains a promising approach for juvenile tilapia. The findings support the further development of live attenuated immersion vaccines, including possible prime-boost strategies, and suggest that capsule expression is not required to induce protective immunity against S. agalactiae Ia.