A New Vibriosis of Cultured Ayu Plecoglossus altivelis

A mass mortality of cultured ayu Plecoglossus altivelis occurred from May to June in 2006 in Hiroshima Prefecture. Most of the diseased fish showed exophthalmos, reddening of the head and curvature of the body. A Gram-negative bacterium was isolated from the brain and kidney of affected fish by using a blood agar medium, but not by non-blood agars. The bacterium was well-cultivable on non-blood agars supplemented with hemin at 25 μg/mL or higher concentrations. The bacterium was classified into the genus Vibrio but differentiated from any known vibrios, based on the results of conventional characterization tests and sequence analyses of 16S rDNA and the non-coding region of the intergenic transcribed spacers between 16S and 23S rDNA. Intraperitoneal injection confirmed the pathogenicity of the bacterium to ayu (mean body weight: 14.8 g) with the LD50 at 1.4 × 103 CFU/fish. Bacteriological and histopathological examinations on the experimentally infected fish revealed that the causative bacterium has high affinity to the brain of ayu.