Epidemiological characteristics of Streptococcus agalactiae in tilapia in China from 2006 to 2020

Abstract

Group B Streptococcus (GBS) is the major causative pathogen of streptococcosis in tilapia, leading to a massive economic loss in the aquaculture industry worldwide. To date, the epidemiological characterization of tilapia GBS in China remains limited. Thus, the aim of this study was to determine the phenotype and genotype of GBS strains in tilapia in China from 2006 to 2020. A total of 678 GBS strains were isolated from farmed tilapia in the past decade. The CAMP reaction of GBS strains was investigated, and the cfb gene of the CAMP negative strains was sequenced and analysed. The genotypic distribution of these strains was determined by molecular serotyping, multi-locus sequence typing (MLST), virulence genotyping, and prophage genotyping. All GBS strains were divided into two serotypes (Ia and Ib), two sequence types (STs; ST7 and ST261), five virulence genotypes (V1–V5), and three prophage genotypes (P1-P3). A total of six genotypes (I–VI) were obtained based on serotyping, MLST, virulence genotyping, and prophage genotyping. Among them, strains in genotypes I, II, III, and IV were primarily prevalent from 2006 to 2015, and those in genotypes V and VI were the dominant epidemic strains from 2012 to 2020. Moreover, genotype VI strains showed a substantial population explosion from 2011 to 2020 and were mainly distributed in the geographically proximate provinces of Guangdong, Guangxi, Hainan, and Fujian. This is first report of tilapia Ia-GBS CAMP negative strains, and the strains were probably from the same hatchery. Genetic diversity in tilapia GBS strains has been high in China over the past decade, especially from 2010 to 2015. The genotypes of the tilapia GBS strains underwent obvious variation, and the prevalent strains in recent years have harboured more virulence-related genes. This study provides a better understanding of the epidemiology, pathogenesis, and evolutionary mechanism of tilapia GBS strains.