Mycobacterium marinum is the causative agent of splenic and renal granulomas in half-smooth tongue sole (Cynoglossus semilaevis Günther) in China

The half-smooth tongue sole (Cynoglossus semilaevis Günther) is a major cultivated marine fish species of high economic value. However, splenic and renal granulomas disease is a major disease of C. semilaevis and results in high mortality; its first outbreak was reported in 2009, and the pathogen responsible was believed to be associated with an unidentified virus. In this study, we investigated the prevalence of this disease in three fish farms located in different cities; we isolated homogeneous colonies from affected fish from each farm. Healthy half-smooth tongue sole were subjected to bacterial challenge tests by intraperitoneal injection of 0.5 × 107 cfu/fish of the isolates from each farm. The accumulative mortality rate was >80% within 30 days post infection. Diseased sole displayed symptoms similar to those seen in naturally infected fish. These strains were all identified as Mycobacterium marinum based on bacterial morphology, analytical profile index identification and 16S rDNA sequencing. The results of this study suggest that M. marinum is the causal agent of splenic and renal granulomas disease in half-smooth tongue sole; therefore, more attention should be paid to the occurrence of this bacterium in the flat fish aquaculture industry in China.