Two Genotypes of Streptococcus iniae Are the Causative Agents of Diseased Ornamental Fish, Green Terror Cichlid (Aequidens rivulatus)

Green terror cichlid (Aequidens rivulatus) is a popular tropical freshwater ornamental fish. In 2021, an unknown disease was observed in cultured A. rivulatus in Tianjin, China, with a cumulative mortality rate of 25% within 7 days of onset. The main clinical signs were scale loss, skin ulceration, and slight bleeding. Histopathological observation revealed obvious damage to the liver, spleen, and kidney of diseased fish. In addition, abundant granulomas were observed in the spleen and head kidney of the diseased fish. To define the potential pathogens from A. rivulatus, bacteria were isolated from the visceral tissue of diseased fish with conventional methods. An artificial infection experiment was carried out to prove the pathogenicity of the isolated bacteria. The strains HG-2021-1 and HG-2021-3 were isolated from diseased fish and identified as being responsible for the disease. They were identified as Streptococcus iniae based on physiological and biochemical tests, lctO gene detection, and 16S rRNA gene sequence analysis. According to the result of multilocus sequence typing (MLST), HG-2021-1 and HG-2021-3 belong to different genotypes of S. iniae. Furthermore, they were found to contain the virulence genes pgmA, scpI, cpsD, and pdi, and the median lethal dose (LD50) for A. rivulatus was 1.8 x 10(6 )Colony-Forming Units (CFU)/mL and 6.6 x 10(6) CFU/mL, respectively. To our knowledge, this is the first report of fish coinfected by two genotypes of S. iniae.