Identification and Pathogenicity of <i>Lactococcus</i> Species in Nile Tilapia (<scp><i>Oreochromis niloticus</i></scp>) and Asian Sea Bass (<scp><i>Lates calcarifer</i></scp>)

Lactococcosis, primarily caused by Lactococcus garvieae, is prevalent in various freshwater fish species globally. Yet, its significance in Thai fish aquaculture remains poorly understood. This study investigated Lactococcosis-associated diseases across four red tilapia (Oreochromis sp.) and one Asian sea bass (Lates calcarifer) farm in Thailand in 2022. Chronically diseased fish displaying clinical signs such as exophthalmia or corneal opacity, erratic swimming, scale loss and skin haemorrhage, as well as apparently healthy specimens, were studied. Bacterial isolation was performed from the fish internal organs. Gram-positive bacterial isolates were selected and later identified as L. lactis (n = 4), L. taiwanensis (n = 1), L. garvieae (n = 2) and L. petauri (n = 2) based on bacterial 16S rDNA and gyrB partial sequences. Sub-adults of Nile tilapia were intracoelomic (IC) injected with representatives of each of the three Lactococcus species (L. garvieae, L. lactis and L. taiwanensis) at varying doses (10³-10⁷ CFU/fish), while juvenile Asian sea bass were IC injected with L. petauri at doses ranging from 10² to 10⁸ CFU/fish. Control groups received culture medium alone. By the end of the 21-day test, mortality rates from all challenge groups ranged from 0% to 14.82%. Some clinical signs were observed in groups challenged with higher doses of the bacteria. Enzyme-linked immunosorbent assay (ELISA) showed that survivor fish exposed to high bacterial doses elicited specific antibodies IgM. In summary, this study identifies the presence of Lactococcus species in farmed fishes, which exhibit low virulence in Nile tilapia and Asian sea bass under laboratory-controlled challenges. These bacterial isolates appear to be opportunistic rather than pathogenic, potentially indicating underlying health challenges within the affected farms.