DEVELOPMENT OF AN AUTOGENOUS BACTERIAL VACCINE AGAINST PATHOGENIC Vibrio alginolyticus AND AN ASSESSMENT OF ITS PROTECTIVE EFFICIENCY IN RED TILAPIA HYBRIDS Oreochromis sp.
Several vaccines against bacterial diseases in fish have been developed and some of these have been routinely administered on a commercial scale. However, a single vaccine for a known bacterial pathogen may not confer high protective efficacy as there may exist various serotypes of a particular pathogen. Using red tilapia hybrids, Oreochromis sp as a model fish species, the study aimed to develop an autogenous bacterial vaccine against Vibrio alginolyticus, to implement a vaccination strategy for the delivery of this vaccine in fish and to assess the effects on the immune responses and protective efficiency of this vaccine in fish following experimental challenge with the pathogen.
Vibrio alginolyticus, isolated from local fish farm, was cultured in broth to a density of at least 108 colony forming units (CFU) ml-1, harvested, heat-inactivated at 70oC for 24 h and diluted with phosphate buffered saline (PBS) to a final density of 109 CFU ml-1. Intraperitoneal (IP) and intramuscular (IM) vaccine trials were conducted using 15-20 g red tilapia hybrid juveniles, each receiving 100 μl of the heat-killed bacterial vaccine. Control tilapia were injected with the same volume of 1x PBS. Plasma samples were obtained from both the vaccinated and control fish at 0, 2, 15 and 30 days post-vaccination to assess the effects of vaccination on the immune responses and anti-oxidant activity. Protective efficiency of the vaccine in fish was determined following experimental challenge with live pathogen (100 μl of 106 CFU ml-1 bacterial cells) by intraperitoneal injection and survival rate was monitored for a period of 10 days.
Immune responses of the fish were differentially modulated following vaccination and was affected by the route of vaccination. Anti-protease activity, total serum globulin levels, bacterial inhibition activity and scavenging activity were up-regulated during the early stages post-vaccination using the IM route, while the lysozyme, myeloperoxidase and alkaline phosphatase activities were up-regulated at a later time period post-vaccination. For fish vaccinated via the IP route, various components of the immune responses including amount of total serum globulin levels, myeloperoxidase, lysozyme, alkaline phosphatase, protease, bacterial inhibition and scavenging activities were up-regulated at different time periods post-vaccination. The autogenous vaccine resulted in high protective ability in fish following experimental challenge with the bacterial pathogen having a 75% and 87% relative percent survival (RPS) for the IP and IM vaccination, respectively. Taken together, the results showed the efficacy of developing an autogenous bacterial vaccine for fish. Further studies are required to better understand the effects of the vaccine on the adaptive immune responses of the fish and to assess cross-protective ability of this type of vaccine to various serotypes of the pathogen.