Molecular identification and characterization of Streptococcus agalactiae serotype Ia (ST7) associated with streptococcosis outbreaks in farmed tilapia in San Martín, Peru.

Tilapia (genus Oreochromis) contribute significantly to food security and aquaculture development in the Department of San Martín, an administrative region in the Peruvian Amazon. During the final quarter of 2024, an investigation was carried out following reports of infectious disease outbreaks associated with unusually high mortality in farmed tilapia located in the provinces of Mariscal Cáceres and Bellavista. A total of 153 samples were analysed, including 120 broodstock, 24 fry, and 9 juvenile specimens. PCR testing detected Streptococcus agalactiae in at least 1 juvenile sample from each of Farms A and B, representing the first confirmed cases of S. agalactiae associated with the outbreaks. In the hatchery, microbiological analysis of broodstock and fry samples yielded 6 bacterial isolates compatible with Streptococcus spp., which were subsequently confirmed as S. agalactiae by PCR. At Farm C, S. agalactiae was confirmed through PCR, and 3 bacterial isolates (P01, P02, and P03) were recovered and comprehensively characterized. Molecular analyses confirmed all isolates as S. agalactiae. Further characterization identified serotype Ia using conventional PCR and sequence type ST7 through multilocus sequence typing (MLST). Three representative isolates obtained from diseased juvenile tilapia were further characterized by whole-genome sequencing and MLST analysis, confirming their classification as S. agalactiae serotype Ia sequence type ST7. Clinical signs observed during the outbreaks included erratic swimming, lethargy, exophthalmos, fin erosion, and hepatic alterations, consistent with streptococcosis. This study represents the first molecular characterization of S. agalactiae serotype Ia ST7 associated with streptococcosis outbreaks in farmed tilapia in Peru.