101 Asian Fisheries Science 17 (2004): 101-110 Asian Fisheries Society, Manila, Philippines 101 Virulence and Histopathology of Vibrio anguillarum like (VAL) Bacterium Isolated from Hatchery Produced Juveniles of Lates calcarifer (Bloch) I.S. AZAD, A.R. THIRUNAVUKKARASU, M. KAILASAM, and J.J.S. RAJAN Central Institute of Brackishwater Aquaculture (CIBA) 75, Santhome High Road R.A.Puram, Chennai-600 028 India Abstract Hatchery produced Asian sea bass Lates calcarifer (Bloch) juveniles (120 days post hatch) maintained in a fish hatchery, Central Institute of Brackishwater Aquaculture (CIBA), were used in the present study. Diseased and moribund fish showed haemorrhages at the bases of dorsal, pectoral and anal fins. The affected fish stopped taking food. Bacteria isolated from the aseptic kidney samples drawn from the moribund fish belonged to the genus Vibrio as evi- denced through selective growth on TCBS agar. Colony morphology, growth and the biochemi- cal characteristics suggested that the bacterium was Vibrio anguillarum-like (VAL). This VAL bacterium, injected at varying doses in Liza macrolepis and Oreochromis mossambicus revealed LD 50 end points of 10 4.17 and 10 5.47 CFU, respectively. Exposure of L. calcarifer juveniles to graded levels of virulent bacterium via injection and bath showed that a cell density of 10 4.5 CFU/fish and 10 6.3 CFU per ml, respectively, can kill half of the exposed population. Investiga- tions carried out on the histopathology of the infected fish indicated severe necrotic degenera- tive changes in the gills with mild to extensive lamellar fusion and hyperplasia of naturally infected seabass juveniles, while that of the experimentally infected fish showed no discernible changes. Natural infection showed wide spread chronic histopathological manifestations while, the experimental infection accounted for acute phase pathogenesis – related manifestations. Introduction Disease due to Vibriosis has been a major cause of concern in brackishwater and marine aquaculture systems. Development and expansion of aquacultural activities have resulted in the enhancement of vibriosis asso- ciated fish kills. Annual losses due to this disease in Japan alone, were