Search

ABSTRACTFlavobacterium psychrophilum causes bacterial coldwater disease (BCWD) and is responsible for substantial losses in farm and hatchery-reared salmonids (Family Salmonidae). Although F. psychrophilum infects multiple economically important salmonids and is transmitted horizontally, the extent of knowledge regarding F. psychrophilum shedding rates and duration is limited to rainbow trout (Oncorhynchus mykiss). Concurrently, hundreds of F. psychrophilum sequence types (STs) have been described using multilocus sequence typing (MLST), and evidence suggests that some variants have distinct phenotypes, including differences in host associations. Whether shedding dynamics differ among F. psychrophilum variants and/or salmonids remains unknown. Thus, three F. psychrophilum isolates (e.g., US19, US62, and US87) in three MLST STs (e.g., ST13, ST277, and ST275) with apparent host associations for coho salmon (O. kisutch), Atlantic salmon (Salmo salar), or rainbow trout were intramuscularly injected into each respective fish species. Shedding rates of live and dead fish were determined by quantifying F. psychrophilum loads in water via quantitative PCR. Both live and dead Atlantic and coho salmon shed F. psychrophilum, as did live and dead rainbow trout. Regardless of salmonid species, dead fish shed F. psychrophilum at higher rates (e.g., up to ~108–1010 cells/fish/hour) compared to live fish (up to ~107–109 cells/fish/hour) and for a longer duration (5–35 days vs 98 days); however, shedding dynamics varied by F. psychrophilum variant and/or host species, a matter that may complicate BCWD management. Findings herein expand knowledge on F. psychrophilum shedding dynamics across multiple salmonid species and can be used to inform future BCWD management strategies.IMPORTANCEFlavobacterium psychrophilum causes bacterial coldwater disease (BCWD) and rainbow trout fry syndrome, both of which cause substantial losses in farmed and hatchery-reared salmon and trout populations worldwide. This study provides insight into F. psychrophilum shedding dynamics in rainbow trout (Oncorhynchus mykiss) and, for the first time, coho salmon (O. kisutch) and Atlantic salmon (Salmo salar). Findings revealed that live and dead fish of all fish species shed the bacterium. However, dead fish shed F. psychrophilum at higher rates than living fish, emphasizing the importance of removing dead fish in farms and hatcheries. Furthermore, shedding dynamics may differ according to F. psychrophilum genetic variant and/or fish species, a matter that may complicate BCWD management. Overall, study results provide deeper insight into F. psychrophilum shedding dynamics and will guide future BCWD management strategies. more...

Bacterial coldwater disease (BCWD), caused by Flavobacterium psychrophilum, results in significant losses among multiple salmonid (family Salmonidae) species. Molecular epidemiology and serotyping studies have suggested that some variants are host specific; however, these associations have not been evaluated by cross‐challenging fish species with putatively host‐associated F. psychrophilum isolates via more natural (i.e. immersion) exposure routes. To this end, F. psychrophilum isolates US19‐COS, US62‐ATS and US87‐RBT, each originally recovered from diseased coho salmon (Oncorhynchus kisutch), Atlantic salmon (Salmo salar) or rainbow trout (O. mykiss), and belonging to a host‐associated multilocus sequence typing clonal complex (e.g. CC‐ST9, CC‐ST232 or CC‐ST10), were PCR‐serotyped, evaluated for proteolytic activity, and used to challenge adipose fin‐clipped 4‐month old Atlantic salmon, coho salmon and rainbow trout via immersion. Findings showed US87‐RBT caused disease and mortality only in rainbow trout (e.g. 56.7% survival probability). US19‐COS and US62‐ATS caused more mortality in coho salmon and Atlantic salmon but also caused disease in both other host species, albeit to a lesser extent. Observed survival differences may be due to variant antigenic/virulence determinants as differences in serotype and proteolytic activity were discovered. Collectively, results highlight the intricacies of F. psychrophilum‐host interactions and provide further in vivo evidence that some F. psychrophilum MLST variants are host specific, which may have implications for the development of BCWD prevention and control strategies. [ABSTRACT FROM AUTHOR] more...

Objective: Globally, flavobacteria (family Flavobacteriaceae and Weeksellaceae) are leading causes of disease‐related losses in fish‐farms and hatcheries. One route flavobacteria gain access to aquaculture facilities is via source water. Ultraviolet (UV) light treatment of source water has been effective in reducing the risk of disease outbreaks caused by nonflavobacteria; however, the UV dose required to inactivate flavobacteria has been understudied. The primary objective of this study was to examine the efficacy of UV light treatments for reducing the viability of fish‐pathogenic and fish‐associated Flavobacterium and Chryseobacterium species in a planktonic form. Methods: Sixty‐five flavobacterial isolates belonging to ten Flavobacterium spp. and Chryseobacterium spp. were exposed to a low (25 mJ/cm2) and high (126 mJ/cm2) dose of UV light via a collimating beam apparatus under in vitro conditions, after which treatment efficacy was determined via culture. Result: All assayed flavobacteria were reduced by an average of ~1000‐fold or ~100,000‐fold at the low and high UV doses, respectively; however, substantial differences in reduction at the same UV dose were noted among isolates of the same flavobacterial species, including F. psychrophilum, F. columnare, and F. oreochromis. In addition, F. psychrophilum multilocus sequence typing variants ST10 and ST78, which are two of the most widespread variants in the United States of America, were among the least susceptible to ultraviolet light. Conclusion: Overall, results demonstrate that viable flavobacteria can be reduced substantially by ultraviolet doses of 25–126 mJ/cm2, suggesting such treatments represent a promising tool for minimizing flavobacterial loads in hatcheries and aquaculture facilities, thereby enhancing biosecurity and reducing the risk of epizootics. Impact statementIn this study, ultraviolet light effectively reduced multiple fish disease‐causing flavobacteria under laboratory conditions. Thus, ultraviolet light treatment of water is a promising tool for reducing harmful flavobacteria in fish farms and hatcheries, thereby potentially improving fish health and aquaculture sustainability. [ABSTRACT FROM AUTHOR] more...

Flavobacterium psychrophilum, the etiological agent of bacterial coldwater disease and rainbow trout fry syndrome, causes considerable losses in salmonid aquaculture globally. Systemic F. psychrophilum infections in rainbow trout (Oncorhynchus mykiss) lead to a range of clinical signs, including ulcerative lesions in the skin and muscle and splenitis. Previous studies offered an integrative analysis of the skeletal muscle response to F. psychrophilum infection in rainbow trout. However, little is known about the molecular mechanism of immune response in the spleen, which is an important immune organ of rainbow trout. Here, we investigated the timecourse splenic transcriptome profiles in uninfected rainbow trout (CK) and F. psychrophilum-infected rainbow trout at day 3 and day 7 (D3, D7) by RNA-seq analyses. Among the 7,170 differentially expressed genes (DEGs) in the three comparisons (D3 vs. CK, D7 vs. CK, D3 vs. D7), 1,286 DEGs showed consistent upregulation or downregulation at D3 and D7 and were associated with pattern recognition, acute-phase response, complement cascade, chemokine and cytokine signaling, and apoptosis. The Real time quantitative PCR (RT-qPCR) analysis of eight DEGs confirmed the accuracy of the RNA-Sequencing (RNAseq) data. Our results reflected a general process from pathogen recognition to inflammatory cytokine generation and delineated a putative Toll-like receptor signaling pathway in rainbow trout spleen, following F. psychrophilum infection. Taken together, these results provide new insights into the molecular mechanism of the immune response to F. psychrophilum infection and are a valuable resource for future research on the prevention and control of bacterial coldwater disease during salmon culture. [ABSTRACT FROM AUTHOR] more...

Lake whitefish (Coregonus clupeaformis; LWF) is an economically and ecologically valuable native species to the Great Lakes, but recent declines in their recruitment have generated significant concern about their future viability. Although studies have sought to identify factors contributing to declining recruitment, the potential role(s) of infectious diseases has not been thoroughly investigated. In 2018 and 2019, adult LWF were collected from Lakes Superior, Michigan, and Huron for clinical examination and bacteriological analyses. Herein, we describe the first isolation of Flavobacterium psychrophilum, aetiological agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), from systemically infected adult LWF. Bacterial isolates were yellow‐orange, Gram‐negative, filamentous bacilli that were oxidase and catalase positive, and produced a flexirubin‐type pigment in 3% potassium hydroxide. Isolate identity was confirmed via F. psychrophilum‐specific PCR, and multilocus sequence typing revealed three new singleton sequence types (STs) that were distinct from all previously described F. psychrophilum STs. The prevalence of F. psychrophilum infections was 3.3, 1.7, and 0.0% in Lakes Superior, Michigan and Huron respectively. Findings illustrate the potential for F. psychrophilum to cause systemic infections in adult LWF and highlight the need for future studies to investigate the bacterium's potential role in declining LWF recruitment. [ABSTRACT FROM AUTHOR] more...

Aquatic pathogens are a major concern for fish hatchery production, fisheries management, and conservation, and disease control needs to be addressed. Two important salmonid pathogens are Myxobolus cerebralis and Flavobacterium psychrophilum that cause whirling disease and bacterial coldwater disease (BCWD), respectively. Innate disease resistance is a potential option for reducing disease‐related mortality in hatchery‐reared rainbow trout (Oncorhynchus mykiss, Walbaum). Two experiments were conducted to assess pathogen resistance of first‐generation (F1) rainbow trout created by crossing M. cerebralis‐ and F. psychrophilum‐resistant strains. In the first experiment, we exposed two rainbow trout strains and one F1 cross to six treatments: control (no exposure), mock injection, F. psychrophilum only, M. cerebralis only, F. psychrophilum then M. cerebralis, and M. cerebralis then F. psychrophilum. Results indicated that the F1 cross was not resistant to either pathogen. In the second experiment, we exposed five rainbow trout strains and four rainbow trout crosses to F. psychrophilum. The second experiment indicated that at least one rainbow trout cross was F. psychrophilum‐resistant. Achieving dual resistance may be possible using selective breeding but only some multigenerational strains are suitable candidates for further evaluation. [ABSTRACT FROM AUTHOR] more...

ABSTRACT Flavobacterium psychrophilum, the etiological agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome, causes great economic losses in salmonid aquaculture worldwide. Recent molecular studies have uncovered important epidemiological and ecological aspects of this pathogen; however, such data are lacking for F. psychrophilum populations affecting aquaculture in China. Herein, F. psychrophilum phenotype, genotype, and virulence were characterized for isolates recovered from epizootics in multiple salmonid aquaculture facilities across China. Thirty-one F. psychrophilum isolates, originating from four provinces and three host fish species, were predominantly homogeneous biochemically but represented 5 sequence types (STs) according to multilocus sequence typing (MLST) that belonged to clonal complex CC-ST10 or 3 newly recognized singleton STs. PCR-based serotyping classified 19 and 12 F. psychrophilum isolates into molecular serotypes 1 and 0, respectively, showing an obvious relationship with host species. Antimicrobial susceptibility analysis via broth microdilution revealed reduced susceptibility to enrofloxacin, flumequine, and oxolinic acid, moderate susceptibility to gentamicin, erythromycin, and florfenicol, and variable susceptibility to ampicillin and oxytetracycline. In vivo challenge experiments confirmed the ability of two representative Chinese F. psychrophilum isolates to induce typical signs of BCWD and mortality in 1-year-old rainbow trout (Oncorhynchus mykiss). Findings collectively demonstrate (i) that BCWD outbreaks in China studied thus far are caused by F. psychrophilum lineages that are common on other continents (e.g., CC-ST10) and others that have not been reported elsewhere (e.g., ST355, ST356, ST357), (ii) that F. psychrophilum molecular serotypes distinguish isolates from different host fish species, even within STs, and (iii) reduced F. psychrophilum antimicrobial susceptibility against compounds used for BCWD control in China. IMPORTANCE Flavobacterium psychrophilum causes substantial economic losses in salmonid aquaculture worldwide. Although this bacterium is also believed to be a disease source in China, published reports of its presence do not yet exist. Herein, F. psychrophilum was linked to multiple disease outbreaks in several salmonid aquaculture facilities within four Chinese provinces, and polyphasic characterization revealed that most isolates were genetically distinct from strains recovered on other continents. Analyses further revealed the predominating molecular serotypes, antimicrobial susceptibility profiles, and pathogenic potential of two representative recovered isolates. Collectively, the results presented here provide important data on the epidemiology and disease ecology of F. psychrophilum in China and pave the way for targeted prevention and control methods to be pursued in the future. more...

Flavobacteriumpsychrophilum causes bacterial coldwater disease (BCWD) in salmonids, resulting in significant losses worldwide. Several serotyping and genetic studies of F. psychrophilum have suggested some geno‐/serotypes may be either host‐specific or generalistic in nature; however, this association has not been adequately explored in vivo using more natural exposure routes. Herein, F. psychrophilum isolate US19‐COS, originally recovered from coho salmon (Oncorhynchuskisutch) and belonging to multilocus sequence typing clonal complex (CC) CC‐ST9, and isolate US53‐RBT, recovered from rainbow trout (Oncorhynchusmykiss) and belonging to CC‐ST10, were serotyped via PCR, evaluated for proteolytic activity and utilized to determine their median lethal dose in immersion‐challenged coho salmon fingerlings. US19‐COS belonged to serotype 0, hydrolysed casein and gelatin but not elastin, led to fulminant multiorgan infections and elicited severe gross and microscopic pathology. In contrast, US53‐RBT, belonging to serotype 2, hydrolysed all three substrates, but did not lead to detectable infections, disease signs or mortality in any exposed coho salmon despite proving virulent to rainbow trout in previous experiments. This study provides in vivo evidence for potential host specificity of some F. psychrophilum genotypes that can also be serologically distinct, a matter of importance towards better understanding F. psychrophilum disease ecology and epidemiology. [ABSTRACT FROM AUTHOR] more...

The Gram‐negative bacterium, Flavobacterium psychrophilum, is endemic to California, USA, where it is an important pathogen in salmonid aquaculture, especially in rainbow trout (Oncorhynchus mykiss). Disease outbreaks caused by F. psychrophilum in rainbow trout fingerlings can approach 90% mortality, resulting in millions of dollars of economic losses annually. The focus of this study was to investigate the genetic diversity of 49 F. psychrophilum isolates collected from disease outbreaks in 17 salmonid hatcheries in California, USA, from 2015 to 2018 using multilocus sequence typing. Results suggest California F. psychrophilum isolates are diverse, representing 11 distinct sequence types (STs), three of which were previously undescribed. Still, the majority of genotyped isolates (n = 41) belonged to a single clonal complex (CC), CC‐ST10, which is the largest CC worldwide and has been linked to disease outbreaks on several continents. Results of this study provide evidence of marked intraspecific genetic diversity of F. psychrophilum from California. The biological significance of this genetic variability is unclear but could have implications for future vaccine development and treatments. Further studies investigating the virulence, antigenic, and antimicrobial susceptibility profiles of F. psychrophilum are warranted to better understand the epizootiology of this pathogen in the Western United States. [ABSTRACT FROM AUTHOR] more...

Flavobacterium psychrophilum can be a significant mortality source in hatcheries where trout and salmon (Family Salmonidae) are propagated and raised. Preventing F. psychrophilum outbreaks and controlling its spread in hatcheries is challenged by limited effectiveness of current therapeutics and lack of efficacious commercial vaccines. Consequently, one of the few available options to limit outbreaks and spread within a facility may be modifications to hatchery-rearing practices. We developed an individual-based model to evaluate how rearing practices could limit F. psychrophilum -induced losses of Oncorhynchus mykiss in flow-through, partial-reuse raceway systems. Evaluated practices included frequency of removal of dead individuals, the probability that hatchery personnel found fish that died from infection, and water flow rate through raceways. Practices were assessed under different assumptions about initial number of infected individuals and virulence levels associated with infection. Increasing the frequency of removal of dead individuals led to the largest reductions in cumulative mortality due to disease as well as reductions in peak daily infection rates and time until peak daily infections occurred when compared to changes in flow rates. Under high initial infection, high virulence, and low flow conditions, increasing the frequency of removal from once to two or three times per day decreased cumulative mortality by approximately 150 thousand fish. When initial infection prevalence was high, disease outbreaks were acute with peak daily infection rates occurring 39 to 53 days post-placement in outdoor raceways. Conversely, when initial infection prevalence was low, infection course was more chronic. Increasing the frequency of dead fish removal to hourly and the probability of hatchery personnel finding dead fish in raceways when checks were performed resulted in modest gains in survival, although these results were likely conditional on model assumptions. If the probability of hatchery personnel finding dead fish was lower than what was assumed in our evaluations, larger gains from more frequently checking for dead fish may have resulted. We believe there is considerable potential in using simulation models to explore the benefits of different rearing practices in cultured fish. To that end, we have made the simulation model used in this study available for those interested in adapting or expanding it to address other research questions. • We used simulations to evaluate how hatchery-rearing practices may reduce Flavobacterium outbreaks in raceway systems. • Evaluated practices included flow rate, frequency of removing dead fish, and how likely hatchery workers found dead fish. • Increasing removal frequency of dead fish from once to two or three times per day had the largest effect on mortality. • Course of infection was related to initial infection prevalence, with high initial prevalence leading to acute outbreaks. • Simulation modeling may be valuable for evaluating hatchery practices that improve fish production in hatcheries. • The computer code used herein is available for download to address other questions concerning hatchery-rearing practices. [ABSTRACT FROM AUTHOR] more...

Flavobacterium psychrophilum is the causative agent of bacterial coldwater disease and can cause significant mortality in salmonid aquaculture. To better evaluate disease prevention or treatment methods for F. psychrophilum in the laboratory, a waterborne challenge model that mimics a natural outbreak is needed. Here we report on the development of a waterborne challenge model for F. psychrophilum in which we incorporated variables that may influence challenge success: specifically, scarification prior to bacterial exposure and culture of F. psychrophilum under iron-limited culture conditions to potentially increase the probability of establishing disease. Additionally, two F. psychrophilum strains, CSF 259-93 and THC 02-90, were used in this model to test whether there were virulence differences between strains. Mortality was significantly higher in scarred fish than unscarred fish (81.5 vs. 19.4%), supporting the hypothesis that disruptions in the dermal layer enhance mortality in F. psychrophilum waterborne challenges. Although mortality differences were not significant between iron-replete and iron-limited treatments, mortality was high overall (> 30%). There was a significant difference in mortality between CSF 259-93 and THC 02-90 treatments, although both strains caused high mortality in injection challenges. In conclusion, this waterborne challenge model can be used to evaluate potential disease prevention and treatment methods. [ABSTRACT FROM AUTHOR] more...

Background: Understanding the distribution and abundance of pathogens can provide insight into the evolution and ecology of their host species. Previous research in kokanee, the freshwater form of sockeye salmon (Oncorhynchus nerka), found evidence that populations spawning in streams may experience a greater pathogen load compared with populations that spawn on beaches. In this study we tested for differences in the abundance and diversity of the gram-negative bacteria, Flavobacterium spp., infecting tissues of kokanee in both of these spawning habitats (streams and beaches). Molecular assays were carried out using primers designed to amplify a ~200 nucleotide region of the gene encoding the ATP synthase alpha subunit (AtpA) within the genus Flavobacterium. Using a combination of DNA sequencing and quantitative PCR (qPCR) we compared the diversity and relative abundance of Flavobacterium AtpA amplicons present in DNA extracted from tissue samples of kokanee collected from each spawning habitat. Results: We identified 10 Flavobacterium AtpA haplotypes among the tissues of stream-spawning kokanee and seven haplotypes among the tissues of beach-spawning kokanee, with only two haplotypes shared between spawning habitats. Haplotypes occurring in the same clade as F. psychrophilum were the most prevalent (92% of all reads, 60% of all haplotypes), and occurred in kokanee from both spawning habitats (streams and beaches). Subsequent qPCR assays did not find any significant difference in the relative abundance of Flavobacterium AtpA amplicons between samples from the different spawning habitats. Conclusions: We confirmed the presence of Flavobacterium spp. in both spawning habitats and found weak evidence for increased Flavobacterium diversity in kokanee sampled from stream-spawning sites. However, the quantity of Flavobacterium DNA did not differ between spawning habitats. We recommend further study aimed at quantifying pathogen diversity and abundance in population-level samples of kokanee combined with environmental sampling to better understand the ecology of pathogen infection in this species. [ABSTRACT FROM AUTHOR] more...

The article discusses a study on a multiplex polymerase chain reaction (mPCR) method that can simultaneously detect Flavobacterium psychrophilum, Lactococcus garvieae, Pseudomonas aeruginosa and P. putida from pure cultures, spiked samples and diseased/naturally infected fish. A survey of the rainbow trout Oncorhynchus mykiss from 32 farms in northeast Turkey was done for the four major bacterial pathogens between 2006 and 2009. It indicates the importance in aquaculture of a rapid, sensitive and specific diagnostic methods to detect bacterial pathogens causing the disease. It points to the mPCR assay that allows fast and reliable diagnosis of disease in a single PCR assay. more...

Flavobacterium psychrophilum is the causative agent of bacterial cold-water disease (CWD) and rainbow trout fry syndrome (RTFS) in salmonids. These diseases are a major problem in the aquaculture industry in Spain, and a better understanding of the epidemiology of F. psychrophilum isolates is necessary to improve management strategies. In this study, to investigate genetic variability of this bacterium, pulsed-field gel electrophoresis after DNA digestion with endonuclease StuI, plasmid profiling analysis and antimicrobial susceptibility testing were undertaken with 25 isolates of F. psychrophilum from Spain. These isolates were classified into 17 patterns by PFGE analysis, which were grouped into four clusters and seven independent branches. Twenty isolates (80%) possessed plasmids of 3.5 kb ( n = 13) or 5.5 kb ( n = 7). No plasmids were associated with antibiotic resistance to oxytetracycline (OTC) or florfenicol (FLO). Twenty isolates (80%) had minimum inhibitory concentrations (MICs) to OTC of between 2.4 and 9.7 μg mL−1, and all isolates were susceptible to FLO. A relationship between the origin of the isolates and PFGE genotypes was found. Plasmid profile typing correlated with PFGE profile typing, whereas no correlation was found between antimicrobial susceptibility testing and PFGE profiles. These results suggest that the population of F. psychrophilum with pathogenic potential in northern Spain is quite heterogeneous. [ABSTRACT FROM AUTHOR] more...

The article presents the findings of research concerning the role of upregulated proteins in the pathogenesis of Flavobacterium psychrophilum as potential vaccine candidate antigens. The study cites that Flavobacterium psychrophilum has been known to cause bacterial coldwater disease (CWD) and rainbow trout fry syndrome. The pathogenicity of the Gram-negative bacteria can be traced to factors that include host tissue adherence, production of proteolytic enzymes, and iron acquisition mechanisms. For research purposes, 15 rainbow trout were obtained from the Aquaculture Research Institute. The study cites that future research in Flavobacterium psychrophilum should also examine outer membrane proteins that are involved in host-pathogen interactions. more...

Flavobacterium psychrophilum is the aetiological agent of rainbow trout fry syndrome and bacterial cold water disease. This study examined the genetic diversity of F. psychrophilum isolates retrieved from multiple epizootics at rainbow trout, Oncorhynchus mykiss, rearing facilities and from spawning coho salmon, O. kisutch. A total of 139 isolates were confirmed as F. psychrophilum by PCR assay and were further typed using pulsed-field gel electrophoresis (PFGE). Multiple epizootics at three proximally located rainbow trout rearing facilities were numerically dominated by three PFGE profiles, which accounted for 76% of all trout isolates. In coho salmon, 19 PFGE profiles were differentiated by PFGE and four numerically dominant PFGE profiles represented 56% of all coho salmon isolates. PFGE analysis also indicated that the average similarity of macrorestriction patterns of F. psychrophilum isolates was greater in rainbow trout than in coho salmon (88% vs. 70%). Furthermore, it was not unusual to isolate multiple PFGE profiles from a single coho salmon sample whereas only two PFGE profiles were shared between two sample dates separated by 1 month. It is clear that the domestic rainbow trout aquaculture facilities studied here were primarily affected by a complex of genetically related strains whereas spawning coho salmon supported a much more genetically diverse collection of F. psychrophilum. [ABSTRACT FROM AUTHOR] more...

The article cites a study that uses a simple polymerase chain reaction (PCR) assay to differentiate 16S rRNA alleles of Flavobacterium psychrophilum. The microassay experiments and the 16S rRNA microassay tests conducted at different times were scored independently by different investigators. The distribution of 16S rRNA alleles relative to host species is also examined. more...

Flavobacterium psychrophilum , the etiological agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), causes significant economic losses in salmonid aquaculture, particularly in rainbow trout ( Oncorhynchus mykiss ). Prior studies have used multilocus sequence typing (MLST) to examine genetic heterogeneity within F. psychrophilum At present, however, its population structure in North America is incompletely understood, as only 107 isolates have been genotyped. Herein, MLST was used to investigate the genetic diversity of an additional 314 North American F. psychrophilum isolates that were recovered from ten fish host species from 20 U.S. states and 1 Canadian province over nearly four decades. These isolates were placed into 66 sequence types (STs), 47 of which were novel, increasing the number of clonal complexes (CCs) in North America from 7 to 12. Newly identified CCs were diverse in terms of host association, distribution, and association with disease. The largest F. psychrophilum CC identified was CC-ST10, within which 10 novel genotypes were discovered, most of which came from O. mykiss experiencing BCWD. This discovery, among others, provides evidence for the hypothesis that ST10 (i.e., the founding ST of CC-ST10) originated in North America. Furthermore, ST275 (in CC-ST10) was recovered from wild/feral adult steelhead and marks the first recovery of CC-ST10 from wild/feral fish in North America. Analyses also revealed that at the allele level, the diversification of F. psychrophilum in North America is driven three times more frequently by recombination than random nucleic acid mutation, possibly indicating how new phenotypes emerge within this species. IMPORTANCE is the causative agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), both of which cause substantial losses in farmed fish populations worldwide. To better prevent and control BCWD and RTFS outbreaks, we sought to characterize the genetic diversity of several hundred Flavobacterium psychrophilum is the causative agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), both of which cause substantial losses in farmed fish populations worldwide. To better prevent and control BCWD and RTFS outbreaks, we sought to characterize the genetic diversity of several hundred F. psychrophilum isolates that were recovered from diseased fish across North America. Results highlighted multiple F. psychrophilum genetic strains that appear to play an important role in disease events in North American aquaculture facilities and suggest that the practice of trading fish eggs has led to the continental and transcontinental spread of this bacterium. The knowledge generated herein will be invaluable toward guiding the development of future disease prevention techniques., (Copyright © 2019 American Society for Microbiology.) more...