Streptococcus agalactiae Serotype Ia ST7 CC1 in Farmed Nile Tilapia in Latin America: Age-Dependent Disease and Antimicrobial Susceptibility of an Emerging Clonal Lineage

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Abstract
Recently, a strain of Streptococcus agalactiae serotype Ia sequence type 7 clonal complex 1 (SaIa ST7 CC1) has emerged in Latin American tilapia aquaculture as an international threat. This study evaluated outbreaks of acute streptococcosis occurring between 2021 and 2025 on commercial Nile tilapia (Oreochromis niloticus) farms located in six Latin American countries, with an aim to combine molecular, clinical, pathological and environmental data. In total, 360 moribund or recently dead fish at various production stages (larvae/fry, pre grow-out and grow out) were examined, and 25 S. agalactiae isolates were serotyped, subjected to real time PCR analysis multilocus sequence typing (MLST), virulence and antimicrobial resistance gene profiling and antimicrobial susceptibility testing. All isolates belonged to SaIa and had the same ST7 CC1 MLST profile, which created a highly homogeneous cluster that grouped with reference SaIa ST7 CC1 strains previously isolated from tilapia farms in Asia. These results are consistent with the regional spread of a single clonal line. At larval and fry stage, SaIa ST7 CC1 was associated with hyperacute septicemia, gastrointestinal hemorrhage and frequent intestinal intussusception; while in pre grow out and grow out fish neurological signs were more prominent followed by ocular signs, systemic hemorrhages and coelomic lesions. Histopathological examination showed profuse colonization of brain, spleen, liver, and intestine by Gram positive cocci accompanied by significant acute circulatory and inflammatory lesions and few chronic granulomatous responses consistent with a fast-progressing high aggressive infectious process. All outbreaks occurred during extended episodes of hot water (>32°C) with large day–night thermal gradients and reduced dissolved oxygen, suggesting that thermal stress may exacerbate disease expression in systems affected. All SaIa ST7 CC1 strains exhibited phenotypic susceptibility to florfenicol and amoxicillin, but 84% (21/25) and 100% (25/25) of them exhibited intermediate susceptibility to oxytetracycline and enrofloxacin, respectively. Five of the 21 isolates (23,8%) with intermediate susceptibility to oxytetracycline carried tetracycline resistance genes (tetM, tetO). These findings identify SaIa ST7 CC1 as a clinically relevant threat of emerging thermally facilitated and geographically expanded streptococcosis for tilapia production in Latin America. Immediate priorities include screening of imported broodstock using MLST or whole genome sequencing, harmonized regional molecular surveillance, climate adaptive farm management practices, prudent antimicrobial use and serotype matched vaccination and breeding strategies that improve both disease- as well as heat-resilience.
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