Nocardiosis in large yellow croaker, Larimichthys crocea (Richardson)

Abstract An epizootic in seawater‐cage reared large yellow croaker, Larimichthys crocea , in China was caused by a Nocardia sp. from August to October 2003. The cumulative mortality rate was 15% and the diseased fish were 16 months old with individual length varying from 25 to 30 cm. Multiple, white nodules, 0.1–0.2 cm in diameter, were scattered on the heart, spleen and kidney. The morphology of isolated bacteria from Lowenstein–Jensen medium and tryptic soy agar was bead‐like or long, slender, filamentous rods. Experimental infection indicated that the isolated bacterium was the pathogen responsible for the mortalities. A partial sequence of the 16S rRNA gene of the organism and the type strain of Nocardia seriolae JCM 3360 T (Z36925) formed a monophyletic clade with a high sequence similarity of 99.9%. Based on the morphological, physiological, biological properties and the phylogenetic analysis, the pathogenic organism was identified as N. seriolae . This is the first report on N. seriolae ‐infected large yellow croaker in aquaculture.