In Vivo Experiments Provide Evidence That Flavobacterium psychrophilum Strains Belonging to Multilocus Sequence Typing Clonal Complex ST191 Are Virulent to Rainbow Trout

Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease (BCWD), causes significant economic losses worldwide, particularly in farmed Rainbow Trout Oncorhynchus mykiss. Over the last decade, multilocus sequence typing has revealed >30 clonal complexes (CCs) globally, comprised of >320 F. psychrophilum sequence types (STs). Despite the large number of CCs worldwide, CC-ST10, which is currently the largest CC affecting Rainbow Trout, has been the primary focus of F. psychrophilum virulence studies, leaving the role of other CCs as primary causes of BCWD epizootics unclear. To this end, fingerling Rainbow Trout were experimentally challenged with F. psychrophilum strains belonging to the CC now recognized as the second largest in the world (CC-ST191) alongside CC-ST10 strains. Cumulative percent mortality was 100% in 7-month-old Rainbow Trout and between 27.8% and 61.1% in 8-month-old Rainbow Trout. All examined F. psychrophilum STs were virulent to Rainbow Trout, and no significant differences in virulence between CC-ST10 and CC-ST191 were detected. Due to their wide distribution and high pathogenic potential, both CC-ST191 and CC-ST10 F. psychrophilum strains are excellent candidates for further research aimed at preventing and controlling BCWD.