Multilocus Sequence Typing Identifies Epidemic Clones of Flavobacterium psychrophilum in Nordic Countries

Abstract

f Flavobacterium psychrophilum is the causative agent of bacterial cold water disease (BCWD), which affects a variety of freshwater-reared salmonid species. A large-scale study was performed to investigate the genetic diversity of F. psychrophilum in the four Nordic countries: Denmark, Finland, Norway, and Sweden. Multilocus sequence typing of 560 geographically and temporally disparate F. psychrophilum isolates collected from various sources between 1983 and 2012 revealed 81 different sequence types (STs) belonging to 12 clonal complexes (CCs) and 30 singleton STs. The largest CC, CC-ST10, which represented almost exclusively isolates from rainbow trout and included the most predominant genotype, ST2, comprised 65% of all isolates examined. In Norway, with a shorter history (<10 years) of BCWD in rainbow trout, ST2 was the only isolated CC-ST10 genotype, suggesting a recent introduction of an epidemic clone. The study identified five additional CCs shared between countries and five country-specific CCs, some with apparent host specificity. Almost 80% of the singleton STs were isolated from non-rainbow trout species or the environment. The present study reveals a simultaneous presence of genetically distinct CCs in the Nordic countries and points out specific F. psychrophilum STs posing a threat to the salmonid production. The study provides a significant contribution toward mapping the genetic diversity of F. psychrophilum globally and support for the existence of an epidemic population structure where recombination is a significant driver in F. psychrophilum evolution. Evidence indicating dissemination of a putatively virulent clonal complex (CC-ST10) with commercial movement of fish or fish products is strengthened.

Genetic Diversity of Flavobacterium psychrophilum Isolates from Three Oncorhynchus spp. in the United States, as Revealed by Multilocus Sequence Typing

“…For example, the founding and subfounding genotypes of CC-ST10 (i.e., ST2 and ST10) caused severe disease outbreaks in Europe and Chile and were proposed to have given rise to multiple highly pathogenic sublineages (28,(31)(32)(33). The findings of this study support this hypothesis, as all but 2 CC-ST10 isolates recovered in the United States were isolated from rainbow trout undergoing clinical BCWD.…”

“…Van Vliet et al (19,29,32), and the findings from this study suggest that the same may be true for some genotypes found in the United States (Table 2). For example, isolates belonging to CC-ST10 were recovered exclusively from rainbow trout, thus supporting the hypothesis that CC-ST10 strains are particularly adapted to this species, which is highly susceptible to BCWD, or its rearing conditions (32).…”

“…The average gene diversity of the 96 U.S. isolates (i.e., 0.75 Ϯ 0.03) was higher than any previously reported values from other regions of the world, where it varied from 0.43 in France (28) to 0.48 in Chile (33), 0.61 in Nordic countries (32), and 0.68 in Japan (29). However, the observed diversity may depend on the sampling scheme and the epidemiological characteristics of each F. psychrophilum population under investigation.…”

“…For example, isolates belonging to CC-ST10 were recovered exclusively from rainbow trout, thus supporting the hypothesis that CC-ST10 strains are particularly adapted to this species, which is highly susceptible to BCWD, or its rearing conditions (32). A statistical association between ST10 and ST78 (both CC-ST10) and rainbow trout hosts has been demonstrated; however, statistically significant associations between rainbow trout and the other STs in CC-ST10 may become evident as more isolates are analyzed.…”

“…However, the observed diversity may depend on the sampling scheme and the epidemiological characteristics of each F. psychrophilum population under investigation. In particular, the number of fish species in these previous studies varied from 1 to 15 (28,29,32,33), whereas this study focused on 3 Oncorhynchus spp. Interestingly, the average pairwise nucleotide diversity of the 34 STs in the current study (i.e., 5.4 kbp Ϫ1 ) is strikingly similar to that reported in Japan (i.e., 5.4 kbp Ϫ1 , based on 35 STs) (29), which may suggest that the genetic diversity of F. psychrophilum may be roughly comparable between the temperate regions of the Northern Hemisphere.…”