First Detection of Lactococcus formosensis subsp. formosensis in Rainbow Trout (Oncorhynchus mykiss) in Europe.

Lactococcus garvieae, Lactococcus petauri and Lactococcus formosensis are etiological agents of piscine lactococcosis, a disease reported in Italy since the early 1990s and linked to significant aquaculture losses. To the best of our knowledge, this study reports the first detection of L. formosensis subsp. formosensis in farmed rainbow trout (Oncorhynchus mykiss) in Europe. In 2024, a total of 70 trout were sampled in the Piedmont region (Northern Italy), as part of a health monitoring programme. Bacteriological and molecular analyses showed the presence of L. garvieae in four fish (5.7%) and the identification of L. formosensis subsp. formosensis in one single fish exhibiting mild clinical signs via 16S-23S rRNA ITS sequencing. Phylogenetic analysis confirmed the clustering of the isolate within the L. formosensis clade, clearly distinct from L. garvieae and L. petauri. Virulotype comparison showed greater similarity between L. petauri and L. formosensis subsp. formosensis, while the latter exhibited a distinct biochemical profile and was highly susceptible to antibiotics commonly used in aquaculture. Nevertheless, monitoring and advanced diagnostics are essential to clarify the role of this bacterium and to develop any necessary preventive and control measures.