Emergence of clonal Lactococcus garvieae ST95 associated with high mortality in cage-cultured Tilapia in northern Vietnam

Résumé fourni par la source

Lactococcus garvieae is an emerging pathogen in aquaculture, with increasing reports of outbreaks in tilapia farming across Asia. In this study, we investigated two disease outbreaks in tilapia (Oreochromis spp.) cage farms in Northern Vietnam in 2024. Affected fish exhibited clinical signs including exophthalmia, scale hemorrhages, and internal lesions such as hepatobiliary swelling and ascites, empty gut and intestinal hemorrhage. Bacterial isolates obtained from diseased fish were initially misidentified as Streptococcus agalactiae based on biochemical profiling, but were subsequently confirmed as L. garvieae using MALDI-TOF Mass Spectrometry and 16S rRNA sequencing. Whole-genome sequencing of eight isolates revealed remarkable genomic homogeneity, with all strains assigned to multilocus sequence type ST95 and showing ≤ 9 SNPs across the core genome. Comparative phylogenomic analysis placed the Vietnamese isolates within a clonal subclade together with ST95 genomes from fish and human sources in East Asia, indicating recent expansion and possible regional transmission. Antimicrobial resistance gene screening detected only the mdt(A) and lsaD, consistent with high resistance to erythromycin, tetracycline, and doxycycline observed phenotypically. No plasmid-associated resistance genes were identified. Our findings highlight the emergence of a clonal L. garvieae ST95 lineage in Vietnamese aquaculture, characterized by high phenotypic antimicrobial resistance, and high genomic clonality. All L. garvieae strains from tilapia carried the full complement of 53 known virulence genes, including capsule biosynthesis genes, hemolysins (hly1-3), and LPxTG-anchored adhesins, underscoring their high pathogenic potential and explaining the severity of the clinical manifestations. These results underscore the importance of routine genomic surveillance, and strengthened biosecurity to mitigate the risk of further spread of L. garvieae in farmed fish populations. Although L. garviae is susceptible to only some antimicrobials traditionally used in aquaculture, the development of vaccines is needed.