Co-dominance of Lactococcus garvieae ST97 and Streptococcus agalactiae ST7: a new etiological profile for Tilapia Streptococcosis in China

Streptococcosis represents a major constraint to tilapia aquaculture, causing significant economic losses globally. To date, the Streptococcus spp. have been regarded as comprising the dominant pathogens of streptococcosis. However, real-time epidemiological data on tilapia streptococcosis remain limited. We conducted a systematic epidemiological investigation during 2024 streptococcosis outbreaks in China’s primary tilapia-producing region, isolating 97 bacterial strains from diseased fish with typical streptococcosis symptoms: exophthalmia, swirling behaviour, lethargy, body curvature, and visceral lesions. Molecular identification confirmed S. agalactiae (38.14%, 37/97) and Lactococcus garvieae (49.48%, 48/97) as the dominant pathogens, with no co-infection observed. All S. agalactiae isolates were classified as serotype Ia/ST7, while L. garvieae strains were identified as ST97. Histopathological analysis revealed severe hepatic hydropic degeneration, splenic necrosis, and ocular/brain inflammation in infected fish; L. garvieae induced significantly higher lesion scores than S. agalactiae. Antimicrobial susceptibility testing demonstrated widespread resistance to sulfamethoxazole in both species, but retained susceptibility to florfenicol. Pathogenicity trials showed that L. garvieae strains induced > 60% mortality even at the lowest challenge dose (1.0 × 10⁵ CFU/fish), surpassing S. agalactiae virulence. This study documents a critical diversification of the etiological agents in tilapia streptococcosis, confirming the continued dominance of S. agalactiae Ia/ST7 while revealing the significant emergence of L. garvieae ST97. It provides essential scientific evidence on current pathogen profiles, antimicrobial resistance, and differential virulence, establishing a vital foundation for refining surveillance and developing targeted control strategies against streptococcosis in tilapia aquaculture.