Clade A and Clade B Isolates Are Associated with Pacific Oyster () Disease Outbreaks Across Ireland

Bacteria from the family have been implicated in mass mortalities of farmed Pacific oysters () in multiple countries, leading to substantial impairment of growth in the sector. In Ireland there has been concern that have been involved in serious summer outbreaks. There is evidence that is increasingly becoming the main pathogen of concern for the Pacific oyster industry in Ireland. While bacteria belonging to the clade are also detected frequently in mortality episodes, their role in the outbreaks of summer mortality is not well understood. To identify and characterize strains involved in these outbreaks, 43 isolates were recovered from Pacific oyster summer mass mortality episodes in Ireland from 2008 to 2015 and these were whole-genome sequenced. Among these, 25 were found to be (implicated in disease) and 18 were members of the species complex (role in disease undetermined). Two distinct clades of - clade A and clade B - were found that had previously been described as circulating within French oyster culture. The high degree of similarity between the Irish and French isolates points to translocation of the pathogen between Europe's two major oyster-producing countries, probably via trade in spat and other age classes. isolates were more diverse, but the data reveal a single clone of this species that has spread across oyster farms in Ireland. This underscores that could be transmitted readily across oyster farms. The presence of clades A and B in not only France but also Ireland adds weight to growing concern that this pathogen is spreading and impacting Pacific oyster production within Europe.