INACTIVATION OF Vibrio parahaemolyticus ISOLATED FROM SEAWATER TO DEVELOP FISH VACCINE

Vibrio parahaemolyticus is a gram-negative bacterium which has been widely reported as the cause for hepatic and kidney necrosis in fishes, especially species of high economic value, in many regions across the world. Five pathogenic strains of V. parahaemolyticus were isolated from seawater samples collected in Thanh Hoa and Nghe An regions, Vietnam. All those strains were characterized by well-known morphological and biochemical characteristics of V. parahaemolyticus. Furthermore, we identified the presence of two common found virulent genes in V. parahaemolyticus (i.e. toxR và tlh) from all isolated strains while the other two genes (i.e. tdh and trh) were missing. Experimental results indicated LD50 values of isolated strains diverged from 105.73 to 107.28 on tilapia (Oreochromis niloticus) and 104.15 to 105.15 on zebrafish (Danio rerio). Then, the strain with the lowest LD50 value, named DH64.1, was selected for producing an inactivated vaccine by using formaldehyde. Consequently, the inactivated vaccine was injected on tilapia with survival rate 100% and the lack of any pathogenic symptom. Finally, vaccinated fish that were challenged with DH64.1 strain at three different challenge doses (i.e. 106, 107, 108 CFU\mL). After a 15 days post-vaccination, the relative percentage survival (RPS) of the vaccine was around 88.66-100%.