Nocardia seriolae infection in cultured jade perch, Scortum barcoo

Nocardiosis is a chronic infectious disease that afflicts multiple fish species and poses a growing threat to the aquaculture. In this study, Nocardia sp. infection was identified in jade perch, Scortum barcoo, from a pond culture in Guangzhou, China. The infected fish were characterized by dark body color, occasional skin ulcerations, and nodular lesions in the kidney, liver, heart, and other organs with typical granuloma structure under light microscopy. Ziehl-Neelsen staining revealed a large quantity of bacterial aggregates within lesions. Histological examination of the kidney, liver, and spleen demonstrated typical granuloma accompanied by histopathological changes. A bacterial strain with consistent morphology was isolated from multiple tissues. Symptoms of nocardiosis were also observed in healthy fish after artificial infection with the isolated strain. Physiological and biochemical experiments suggested that the isolated strain shared the basic characteristics of Nocardia, and the causative pathogen was subsequently confirmed as Nocardia seriolae by sequencing of 16S ribosomal RNA and the gyrB gene. To our knowledge, this is the first report of nocardiosis in jade perch; it may pose a serious risk to the aquaculture of jade perch due to prolonged incubation prior to symptom emergence and the low efficacy of current treatments in china.