Expanding Horizons: The First Reported Outbreak of Piscine Lactococcosis in Farmed Gilthead Seabream Sparus aurata in the Northern Tyrrhenian Sea.

Piscine lactococcosis, caused by Lactococcus garvieae, has traditionally been reported in rainbow trout and marine fish in specific regions. However, its first outbreak in farmed gilthead seabream Sparus aurata in the northern Tyrrhenian Sea marks a significant expansion in the distribution of the disease. In 2024, a total of 212 gilthead seabream from three different aquaculture facilities, including one offshore farm with floating cages and two land-based tank farms, were subjected to diagnostic exams during mortality outbreaks. Bacterial isolation and molecular identification confirmed L. garvieae in market-size gilthead seabream collected during mortality outbreaks in the warm season or at seawater temperatures > 18°C. Our results highlight the importance of environmental monitoring and pathogen management in preventing piscine lactococcosis. The outbreaks align with previous studies on L. garvieae infections in marine fish, particularly regarding water temperature. The expanding geographic range of the pathogen necessitates further investigation into its ecology, particularly in Mediterranean aquaculture. This study highlights the need for improved biosecurity measures, early detection methods, and tailored vaccination strategies to mitigate the impact of piscine lactococcosis in gilthead seabream farming. Future research should focus on understanding the environmental triggers and host-pathogen interactions to develop more effective control strategies.