Streptococcus agalactiae Serotype VII, an Emerging Pathogen Affecting Snakeskin Gourami (Trichogaster pectoralis) in Intensive Farming

Snakeskin gourami (Trichogaster pectoralis) is a freshwater fish species that is being increasingly cultivated in Southeast Asia. The expansion of farms and intensive farming practices has led to the unexplained mortality of snakeskin gourami and tremendous economic losses in many farms. We investigated the unusual mortality of snakeskin gourami at 22 farms in Central Thailand. The moribund fish showed darkened skin, erratic swimming, exophthalmos, and haemorrhaging around the eyeballs, with cumulative mortality between 20% and 45%. Our necropsy findings revealed an enlarged liver and anterior kidney, splenomegaly, haemorrhage in most internal organs, pericarditis, and brain congestion. Histopathology revealed haemorrhaging and congestion of the blood vessels in the liver with infiltration of lymphocytes, enlarged blood vessels with mononuclear and lymphocyte infiltration in the meninges, and cerebral parenchyma were observed. Severe necrotising and suppurative pericarditis with myocardial infarction were found. Epidemiological studies and laboratory diagnosis revealed that Streptococcus agalactiae was predominantly isolated from the moribund fish. Laboratory investigations of the representative 33 isolates of S. agalactiae using mass spectrometry, multiplex polymerase chain reaction assay, pulse-gel electrophoresis, and serotyping suggested that all the isolates were S. agalactiae serotype VII, which is different from the serotype III isolated from diseased tilapia in Thailand. An experimental challenge using three representative isolates of S. agalactiae on snakeskin gourami caused clinical signs, gross lesions, and pathological changes, with high mortality exceeding 60%, which is similar to the mortality in most natural infections. Moreover, S. agalactiae was recovered from the spleen, kidneys, and liver of all the challenged fish. Taken together, this study provides important information that S. agalactiae serotype VII is virulent in snakeskin gourami and can potentially spread among these fish in fish farms. Appropriate preventive measures and the control of animal movements should thus be implemented.