Isolation and pathogenicity of Streptococcus iniae in offshore cage-cultured Trachinotus ovatus in China

An epizootic outbroke in offshore cage-cultured Trachinotus ovatus, in Hainan provinces of China, causing massive mortality. Necropsy, histopathology and bacteriology experiments were carried out to identify the cause of this disease. The most prominent lesions happened in the brain, spleen and kidney, and a gram-positive bacterium was isolated from diseased fish. Through analysis of 16s rRNA and biochemical profiles, we identified this isolate as a strain of Streptococcus iniae, and named it HN strain. Artificial infection experiments were carried out, the HN strain can cause similar symptom with an LD50 value of 8.9 × 103 colony-forming units (CFU), which confirmed the isolated HN strain was virulent for T. ovatus. The HN strain is sensitive to all of the antimicrobial drugs tested, some of them, such as florfenicol and norfloxacin may be used to control the disease.