038 | Piscine lactococcosis in Mediterranean fish farms involving multiple lactococcus species affecting key aquaculture species

Giuseppe Esposito¹, Silvia Colussi^1|2, Fabio Bondavalli¹, Pierluigi Acutis¹, Andrea Gustinelli³, Giorgia Bignami³, Lucio Fariano⁴, Marialetizia Fioravanti³, Elena Bozzetta¹, Caterina Faggio^5|6, Paolo Pastorino¹, Marino Prearo¹ | ¹Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d’Aosta, Torino, Italy; ²DIANA - Dipartimento di Scienze animali, della nutrizione e degli alimenti, Università Cattolica del Sacro Cuore, Piacenza, Italy; ³Dipartimento di Scienze Mediche Veterinarie, Università degli Studi di Bologna, Italy; ⁴Azienda Agricola Canali Cavour, Regione Sagnassi, Centallo [CN], Italia; ⁵Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, Università degli Studi di Messina, Italy; ⁶Dipartimento di Biotecnologie Marine Ecosostenibili Stazione Zoologica Anton Dohrn, Napoli, Italy.

Piscine lactococcosis is increasingly identified as a rising bacterial threat in both marine and freshwater aquaculture, with significant economic and health consequences. This study combines three complementary investigations that together enhance our understanding of the distribution, diversity, and pathogenicity of Lactococcus species in Mediterranean fish farms. A retrospective study identified Lactococcus petauri in European seabass, Dicentrarchus labrax(Linnaeus, 1758), from land-based farms along Italy’s southwestern coast. Originally misclassified in 2012, the isolate demonstrates that L. petauri has circulated in Italian marine aquaculture for over a decade. The strain exhibited broad antimicrobial resistance, including β-lactams, fluoroquinolones, and trimethoprim/sulfamethoxazole, highlighting the value of re-evaluating historical bacterial collections with updated molecular tools. The second study documents the first outbreak of piscine lactococcosis caused by Lactococcus garvieae in gilthead seabream, Sparus aurata (Linnaeus, 1758), in the northern Tyrrhenian Sea, Italy. In 2024, 212 fish across three farms (one offshore, two land-based) were affected. Bacteriological and molecular analyses confirmed L. garvieae as the aetiological agent, with disease linked to summer conditions and seawater temperatures exceeding 18°C. The third study presents the first European record of Lactococcus formosensis subsp. formosensis in farmed rainbow trout, Oncorhynchus mykiss (Walbaum, 1792). In a 2024 Piedmont health survey, L. garvieae was found in four fish (5.7%) and L. formosensis in one specimen using 16S-23S rRNA ITS sequencing. Phylogenetic and biochemical analyses confirmed its identity and close relation to L. petauri. The isolate was highly susceptible to common aquaculture antibiotics, and no clinical signs or mortality were observed, highlighting the need for ongoing surveillance. Overall, these findings expand the recognized host range and distribution of Lactococcus species in Mediterranean aquaculture, underscoring the importance of advanced diagnostics, routine monitoring, and effective biosecurity and vaccination measures to manage piscine lactococcosis in Europe.