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

Abstract

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.

Co‐occurrence of heterogeneous Flavobacterium psychrophilum isolates within the same Chilean farm and during the same infectious outbreak

“…Under field conditions, differences in the antigenic and genetic features of each F. psychrophilum isolate could determine infection and the percentage of fish mortality, as has been reported in a number of previous in vivo studies (Bruce et al, 2021;Knupp et al, 2021). In fact, epidemiological typing studies will have implications for management strategies (e.g.…”

Persistence of heterologous Flavobacterium psychrophilum genetic variants in microcosms simulating fish farm and hatchery environments

“…Multilocus sequence typing (MLST) has been applied to >1500 F. psychrophilum isolates worldwide, revealing over 260 distinct sequence types (STs; https://pubmlst.org/organisms/flavobacterium-psychrophilum). Importantly, the wealth of diversity within this species has been linked to phenotypic variation with respect to host associations, virulence, ultraviolet light susceptibility, and antimicrobial susceptibility (Knupp et al, 2019; Knupp et al, 2023; Knupp, Faisal, et al, 2021; Knupp, Kiupel, et al, 2021; Sundell et al, 2019; Van Vliet et al, 2017). It remains unclear, however, if such intraspecific diversity contributes to F. psychrophilum persistence in different environments.…”

Varying Flavobacterium psychrophilum shedding dynamics in three bacterial coldwater disease-susceptible salmonid (Family Salmonidae ) species

“…Multilocus sequencing typing of >1,500 F . psychrophilum isolates has revealed >260 different sequence types (STs) worldwide ( https://pubmlst.org/organisms/flavobacterium-psychrophilum ), some of which differ in host species association ( 15 – 17 ), virulence ( 18 , 19 ), and prevalence in either hatchery-reared or wild/feral fish populations ( 15 , 20 ). Notably, most F. psychrophilum STs in North America differ from those that have been reported from other continents (e.g., Asia, Europe, South America) ( 15 , 17 , 20 – 26 ), suggesting that the F. psychrophilum variant evaluated by Madetoja et al ( 14 ) is likely distinct from many causing losses in the USA.…”