First Documented Outbreaks of Lactococcus garvieae in Gilthead sea bream (Sparus aurata) in the Gulf of Follonica, Tuscany, Italy

Lactococcus garvieae is an emerging pathogen responsible for lactococcosis, a haemorrhagic septicemia affecting fish associated with high mortality and widespread distribution. The disease primarily affects rainbow trout ( Oncorhynchus mykiss ), but L. garvieae has also been reportedin various freshwater and marine fish species, as well as in mammals, including humans. This study describes the first documented outbreaks of L. garvieae in gilthead sea bream ( Sparus aurata ), occurring in the Gulf of Follonica (Tuscany, Italy), following a previous episode in European sea bass ( Dicentrarchus labrax ) in the same area in 2023. Affected fish showed erratic and uncoordinated swimming. Macroscopic lesions included ocular and skin lesions, pericarditis, splenomegaly and haemorrhages in internal organs. Histopatological findings revealed severe fibrinous-necrotic pericarditis and epicarditis associated with bacterial aggregates and widespread granulomatous inflammation in the liver and spleen. Identification of L. garvieae was achieved employing multiple techniques: biochemical tests, miniaturized Analytical Profile Index (API rapid ID 32 strep, Biomerieux), MALDI-TOF MS (MicroFlex LT, Bruker), and molecular methods, including two independent end-point PCR assays followed by sequencing of the 16S–23S ITS region and DNA gyrase subunit B (gyrB) gene. Antimicrobial susceptibility testing of selected isolates was carried out using disk diffusion and minimum inhibitory concentration (MIC) methods, following standardised guidelines. This study presents new data on the characteristics of lactococcosis in gilthead sea bream ( Sparus aurata ), confirming Lactococcus garvieae as a primary pathogen and highlighting its potential impact on Mediterranean aquaculture, as well as the importance of implementing preventive strategies, including vaccination.