Phylogeography and host specificity of pathogenic to sea-farmed fish in the north-east Atlantic.

The present study was undertaken to address the recent spate of pasteurellosis outbreaks among sea-farmed Atlantic salmon () in Norway and Scotland, coinciding with sporadic disease episodes in lumpfish () used for delousing purposes in salmon farms. Genome assemblies from 86 bacterial isolates cultured from diseased salmon or lumpfish confirmed them all as members of the family, with phylogenetic reconstruction dividing them into two distinct branches sharing <88% average nucleotide identity. These branches therefore constitute two separate species, namely and the as-yet invalidly named "". Both species further stratify into multiple discrete genomovars (gv.) and/or lineages, each being nearly or fully exclusive to a particular host, geographic region, and/or time period. Pasteurellosis in lumpfish is, irrespective of spatiotemporal origin, linked almost exclusively to the highly conserved " gv. " (Pac). In contrast, pasteurellosis in Norwegian sea-farmed salmon, dominated since the late-1980s by " gv. " (Pas), first saw three specific lineages (Pas-1, -2, and -3) causing separate, geographically restricted, and short-lived outbreaks, before a fourth (Pas-4) emerged recently and became more widely disseminated. A similar situation involving (Ps) has apparently been unfolding in Scottish salmon farming since the mid-1990s, where two historic (Ps-1 and -2) and one contemporary (Ps-3) lineages have been recorded. While the epidemiology underlying all these outbreaks/epizootics remains unclear, repeated detection of 16S rRNA gene amplicons very closely related to and "" from at least five cetacean species worldwide raises the question as to whether marine mammals may play a part, possibly as reservoirs. In fact, the close relationship between the studied isolates and associated with harbor porpoise (), and their relatively distant relationship with other members of the genus , suggests that both and "" should be moved to the genus .