Capsulated Lactococcus garvieae caused devastating mortality in Atlantic bluefin tuna, Thunnus thynnus: genomic and histopathologic characterization.

Atlantic bluefin tuna (ABFT; Thunnus thynnus) is among the most valuable commodities in Mediterranean mariculture, and recent increases in seawater temperatures have coincided with the re-emergence of bacterial diseases causing catastrophic losses. During the summer 2025 mortality event, we investigated stranded and moribund ABFT using bacteriological isolation and identification, high-throughput 16S amplicon profiling of tissue-associated bacterial communities, whole-genome sequencing to resolve a complete genome of the etiologic agent, transmission electron microscopy, and gross and histopathological examinations. Across multiple organs, the metagenomic profiles were overwhelmingly dominated by Lactococcus garvieae, supporting a primary systemic bacterial etiology. The isolate displayed a capsulated phenotype, and genome analysis identified a capsule-associated gene cluster consistent with a capsulated lineage. Capsule expression was further confirmed ultrastructurally by transmission electron microscopy. Pathology indicated fulminant septicemia with prominent hemorrhagic lesions and severe cardioperitoneal involvement, including fibrinous epicarditis with abundant Gram-positive cocci, alongside marked hepatic and splenic pathology. Collectively, these data document, for the first time in two decades, the detection of a capsulated L. garvieae serotype or lineage associated with ABFT mass mortality. Rapid etiologic confirmation, mitigation of temperature-related and husbandry-associated stress, and targeted prevention strategies (including vaccination and biosecurity) are recommended to reduce recurrence in warming coastal waters.