Abstract: Nocardiosis is a long-term and problematic bacterial disease that can infect many kinds of culture fishes and cause great economic losses to the aquaculture. The nodular disease was identificated in pond farming African catfish Clarias gariepinus. The fish were dark in color, some presented with skin ulceration, and the off-white nodules dispersed throughout the internal organs, such as liver and kidney. Many typical granuloma were characterized under the light microscope via wet mount preparations, and the bacteria were observed clustedred together in bright red after acid fast stain onto a slides. Histopathology revealed nodular lesion in internal organs, and associated with different degrees of pathologic lesion. A characteristic bacterial strain were isolated, which could experimently infect African catfish and induce histopathological changes similar to the natural cases. Based on the biological characteristics and phylogenetic analysis, the isolated bacteria were identified as Nocardia seriolae. To our knowledge, this was the first report of nocardiosis in African catfish, it might be closely relate to the breeding pattern of the fish in south China.

Key words: Nocardia seriolae, Clarias gariepinus, granuloma, infection