Virulence of Photobacterium damselae subsp piscicida in cultured cobia Rachycentron canadum

An outbreak of serious mortality among the cultured cobia Rachycentron canadum (weighing 3 kg) characterized by the presence of whitish granulomatous deposits on the kidney, liver and spleen occurred in July of 2000 in Taiwan. A non-motile strain CP1 was isolated from kidney and/or liver on tryptic soy agar and/or brain heart infusion agar plates (both supplemented with 1% NaCl, w/v). This strain was characterized and identified as Photobacterium damselae subsp. piscicida using biochemical characteristics and BIONOR mono-Pp tests. The bacterium and its extracellular products (ECP) were lethal to the cobia (weighing 10 g) with LD50 values of 1.03 x 10(4) colony forming units and 1.26 mug protein/g fish body weight, respectively. All the moribund/dead fish exhibited darkness in color with no gross or internal leasions. However, the bacteria could be reisolated from kidney and liver after bacterial challenge. The present results reveal that Ph. damselae subsp. piscicida is the causative agent of fish photobacteriosis in the cobia and the bacterium isolated from sub-adult cobia (chronic form) is virulent to young cobia causing acute form of the disease.