Tenacibaculum spp. as the aetiological agent of atypical winter-ulcer in Norwegian farmed salmon. Tenacibaculum

Seventy-six yellow-pigmented putative Tenacibaculum isolates cultured from fifteen outbreak cases in Norwegian fish farms were sequenced and assembled. The intra-outbreak diversity assessed with cgMLST amongst T. finnmarkense isolates involved was significant and in only 2 two outbreaks were the majority of isolates recovered regarded as highly similar or clonal. Thus fish to fish transmission, while present, does not appear to be the main form of transmission of infection by which the infection spreads. Our results suggest that the majority of post-late winter tenacibaculosis outbreaks in post-smolts relatively newly transferred to cold seawater in Norway are caused by T. finnmarkense and dominated by T. finnmarkense subsp. finnmarkense.