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Diseases of Aquatic Organisms

    DAO prepress abstract   -  DOI: https://doi.org/10.3354/dao03517

    Nocardia seriolae: a serious threat to the largemouth bass Micropterus salmoides industry in Southwest China

    Xueping Lei, Ruoxuan Zhao, Yi Geng*, Kaiyu Wang, Ping Ou Yang, Defang Chen, Xiaoli Huang, Zhicai Zuo, Changliang He, Zhengli Chen, Chao Huang, Hongrui Guo, Weimin Lai

    *Corresponding author:

    ABSTRACT: Nocardia seriolae is the causative agent of nocardiosis in both saltwater and freshwater fish. Here, we report on multiple outbreaks of nocardiosis associated with elevated mortality (23–35%) in farmed largemouth bass in Sichuan, China, from 2017 to 2018. A total of 9 strains isolated from diseased largemouth bass were identified as N. seriolae by phenotypic characterization, 16S rRNA and hsp65 gene sequence analysis. The clinical signs of infected largemouth bass included hemorrhage, skin ulcers and prominent tubercles varying in size in the gill, liver, spleen and kidney. Experimental infection indicated that these isolates were the pathogens responsible for the mortalities. In vitro antibacterial activities of 12 antibiotics against N. seriolae isolates were determined as minimum inhibitory concentrations (MICs). Histopathological observation of diseased fish infected with N. seriolae showed necrotizing granulomatous hepatitis, nephritis, splenitis, epithelial hypertrophy and hyperplasia with degenerative changes of the epithelium in gill. Large quantities of bacterial aggregates were found in the necrotic area of granuloma by Lillie-Twort Gram stain and immunocytochemistry. Our findings indicated that N. seriolae is a serious threat to the largemouth bass Micropterus salmoides industry in Southwest China.