Immune responses and protection in rohu vaccinated against Aeromonas hydrophila infection

In this study, efficacy of three antigenic preparations from the fish pathogen Aeromonas hydrophila was evaluated as vaccine candidate in rohu Labeo rohita. The rabbit anti-rohu globulin conjugate was prepared in rabbit using sera obtained from healthy adult rohu. Three antigens, namely formalin inactivated A. hydrophila (FAH), FAH mixed with Freund's incomplete adjuvant in a 1:1 (v/v) ratio (FAH + A), and extra cellular product were prepared. Protection of rohu against A. hydrophila infection was tested at days 10, 20, and 30 post-vaccination. At the end of the vaccination, fish were challenged with A. hydrophila (dose: 1 x 10(7) cfu ml(-1)) and relative per cent of survival was recorded up to 60th day of post-challenge. Study of cellular immunological parameters including antigen-specific leucocyte proliferation, antigen-specific nitric oxide production, and superoxide anion production exhibited significantly higher (p < 0.05) values on 10th and 20th day of post-vaccination compared with control. Among all the antigenic groups, FAH + A showed most significant responses compared with the other groups and produced maximum antibody on 30th day following vaccination. Among the humoral immune responses, lysozyme activity showed almost similar trend like cellular parameters. The result of the challenge study showed a higher level of survival in all the vaccinated groups, especially in FAH + A group. Our results suggest that vaccine FAH + A can effectively protect rohu against A. hydrophila infection and could offer an appropriate strategy to prevent this infection in rohu farms.