Flavobacterium psychrophilum associated with septicaemia and necrotic myositis in Atlantic salmon Salmo salar: a case report

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

Flavobacterium 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.

Immersion challenge of three salmonid species (family Salmonidae) with three multilocus sequence typing variants of Flavobacterium psychrophilum provides evidence of differential host specificity

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.

First detection of Flavobacterium psychrophilum in juvenile Siberian sturgeons (Acipenser baerii) and description of the pathological findings

Flavobacterium psychrophilum affects many cultured fish species and is considered one of the most important bacterial pathogens causing substantial economic losses in salmonid aquaculture worldwide. Here, F. psychrophilum was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and nested PCR as the aetiological agent causing mortality in diseased juvenile Siberian sturgeons (Acipenser baerii) reared on a freshwater fish farm. Diseased sturgeons were lethargic and displayed dark skin pigmentation, increased mucus production and the presence of skin ulcerations and haemorrhages specially on the ventral side and the base of fins. The histological examination of fish revealed proliferative branchitis, ulcerative and necrotizing dermatitis and myositis, lymphoid tissue atrophy, liver and kidney degeneration and thrombosis. To the best of our knowledge, this is the first report describing the infection of Siberian sturgeons by F. psychrophilum. The detection of F. psychrophilum in diseased Siberian sturgeons and the description of the pathological findings observed during the outbreak may contribute to a better understanding of the bacterium pathogenicity and the range of fish species susceptible to infection.

Immersion challenge model for Flavobacterium psychrophilum infection of Atlantic salmon (Salmo salar L.) fry

Flavobacterium psychrophilum is the causative agent of bacterial cold-water disease (CWBD) and rainbow trout fry syndrome (RTFS), which affect salmonids. To better understand this pathogen and its interaction with the host during infection, including to support the development of resistant breeds and new vaccines and treatments, there is a pressing need for reliable and reproducible immersion challenge models that more closely mimic natural routes of infection. The aim of this present study was to evaluate a challenge model developed previously for rainbow trout for use in Atlantic salmon. First, preliminary challenges were conducted in Atlantic salmon (n = 120) and rainbow trout (n = 80) fry using two F. psychrophilum isolates collected from each fish species, respectively; fish had been pretreated with 200 mg/L hydrogen peroxide for 1 h. Thereafter, the main challenge was performed for just one F. psychrophilum isolate for each species (at 2 × 10⁷  CFU/mL) but using larger cohorts (Atlantic salmon: n = 1187; rainbow trout: n = 2701). Survival in the main challenge was 81.2% in Atlantic salmon (21 days post-challenge) and 45.3% in rainbow trout (31 days post-challenge). Mortalities progressed similarly during the preliminary and main challenges for both species, demonstrating the reproducibility of this model. This is the first immersion challenge model of F. psychrophilum to be developed successfully for Atlantic salmon.

Assessment of Flavobacterium psychrophilum-associated mortality in Atlantic salmon (Salmo salar) and brook trout (Salvelinus fontinalis)

Salmonid diseases caused by infections of Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease, remain difficult to manage as novel, pathogenic strains continue to emerge in aquaculture settings globally. To date, much of the research regarding treatment options and vaccine development has focused on rainbow trout (Oncorhynchus mykiss), but other inland-reared salmonids are also impacted by this Gram-negative bacterium. As such, Atlantic salmon (Salmo salar) and brook trout (Salvelinus fontinalis) were injection-challenged with a variety of previously reported F. psychrophilum strains isolated from disease diagnostic cases in salmonids, as well as a standard and well-studied F. psychrophilum strain (CSF 259-93) known to be virulent in rainbow trout. In three separate virulence assessments (Trials A, B and C), strains US063 (isolated from lake trout; Salvelinus namaycush) and US149 (isolated from Atlantic salmon) caused a significantly higher cumulative per cent mortality (CPM) relative to other strains in Atlantic salmon (p <.001 for all trials), with US149 causing significantly greater mortality than US063 in Trials A (CPM 97% vs. 65%, p =.008) and B (CPM 96% ± 2.3% vs. 81.33% ± 4.8%, p =.014). Trial C used a lower dose (1.86 × 10⁸  CFU/mL) for US149, resulting in a lower mortality (78.67% ± 9.33%) relative to Trials A and B. CSF259-93 did not cause significant mortality in any trials. In brook trout, the strain 03-179 (originally isolated from steelhead trout; Oncorhynchus mykiss) was significantly more virulent than any other (CPM 100% ± 0%, p <.001), followed by US063 (73% ± 3.8%) and US149 (40% ± 6.1%,) respectively. Again, CSF259-93 did not cause significant mortality relative to a mock challenge treatment. Results provide information about the applicability of strain selection in F. psychrophilum virulence testing in Atlantic salmon and brook trout, demonstrating the high virulence of US063 and US149 for these salmonid species. This information is applicable for the development of therapeutics and vaccines against F. psychrophilum infections and demonstrates the reproducibility of the experimental challenge model.

Histological and hematological alterations of silver catfish Rhamdia quelen highly parasitized by Lernaea cyprinacea

The main objective of this study was to evaluate the histological and hematological alterations of silver catfish Rhamdia quelen (Quoy & Gaimard, 1824) against intense parasitism by anchor worm Lernaea cyprinacea Linnaeus, 1758. Thirteen fish specimens were collected at the Fish Genetic Improvement Unit (EPAGRI/CEDAP), Brazil. After acclimatization, the fish were anesthetized for blood collection, and the samples were used for counting thrombocytes, total leukocytes, differential leukocytes and for quantification of total erythrocytes. Afterwards, the animals were euthanized, the parasites removed and quantified, and the portions of the affected tegument fixed in 10% buffered formalin. All the analyzed animals were parasitized by L. cyprinacea with mean intensity of 192 specimens per fish. The hematological profile of the animals was different from that reported for healthy fish. Immature leukocytes were the most abundant cells, followed by monocytes, lymphocytes, neutrophils, and periodic acid-Schiff-positive granular leukocytes. A small number of neutrophils were present in the bloodstream of parasitized fish, while histological examination evidenced the accumulation of this cell type near the place of infection by the parasite. Total leukocytes correlated significantly and positively (r = 0.6, p = 0.030) with the intensity of L. cyprinacea infection and the fish weight. Histological changes included hemorrhage, liquefactive necrosis and connective tissue disarrangement, necrosis, granulomatous inflammation, mononuclear and polynuclear inflammation, as well as hyperplasia and hypertrophy of the alarm and mucous cells. The L. cyprinacea infection intensity caused deleterious changes both in the hematological profile and the integument of R. quelen, confirming the pathogenic potential of this parasite to the affected fish species.

Antimicrobial susceptibility of Flavobacterium psychrophilum isolates from the United Kingdom

Routine application of antimicrobials is the current treatment of choice for rainbow trout fry syndrome (RTFS) or bacterial coldwater disease (BCWD) caused by Flavobacterium psychrophilum. In this study, the antimicrobial susceptibilities of 133 F. psychrophilum isolates, 118 of which were from the UK, were evaluated by broth microdilution and disc diffusion methods following VET04-A2 and VET03-A guidelines of Clinical and Laboratory Standards Institute (CLSI), respectively. Isolates were categorized as wild type (fully susceptible, WT) or non-wild type (NWT) using normalized resistance interpretation (NRI)-determined cut-off values (CO_WT ). Broth microdilution testing showed that only 12% of UK isolates were WT to oxolinic acid (MIC CO_WT  ≤ 0.25 mg/L) and 42% were WT for oxytetracycline (MIC CO_WT  ≤ 0.25 mg/L). In contrast, all the isolates tested were WT (MIC CO_WT  ≤ 2 mg/L) for florfenicol, the main antimicrobial for RTFS control in the UK. Disc diffusion-based CO_WT values were ≥51 mm for 10 μg amoxicillin, ≥44 mm for 30 μg florfenicol, ≥30 mm for 2 μg oxolinic acid and ≥51 mm for 30 μg oxytetracycline. There was a high categorical agreement between the classifications of the isolates by two testing methods for florfenicol (100%), oxytetracycline (93%) and oxolinic acid (99%).

Optimization of efficacy of a live attenuated Flavobacterium psychrophilum immersion vaccine

This study was aimed at optimizing the efficacy of a recently developed live attenuated immersion vaccine (B.17-ILM) as a promising vaccine against bacterial coldwater disease (BCWD) caused by Flavobacterium psychrophilum in salmonids. Rainbow trout (RBT) fry were vaccinated by immersion, and different parameters affecting vaccination such as fish size, vaccine delivery time, dose, duration of protection, booster regimes and vaccine growth incubation time were optimized. Specific anti-F. psychrophilum immune response was determined by ELISA. Protective efficacy was determined by challenging with a virulent strain of F. psychrophilum (CSF-259-93) and calculating cumulative percent mortality (CPM) and relative percent survival (RPS). All vaccinated fish developed significantly higher levels of serum antibody titers by week 8 when compared to their respective controls. Immersion vaccination for 3, 6 and 30 min produced significant protection with comparable RPS values of 47%, 53% and 52%, respectively. This vaccine provided significant protection for fish as small as 0.5 g with an RPS of 55%; larger fish of 1 g and 2 g yielded slightly higher RPS values of 59% and 60%, respectively. Fish vaccinated with higher vaccine doses of ∼10(10) and 10(8) colony forming units mL(-1) (cfu ml(-1)) were strongly protected out to at least 24 weeks with RPS values up to 70%. Fish vaccinated with lower doses (∼10(6) and 10(5) cfu mL(-1)) had good protection out to 12 weeks, but RPS values dropped to 36% and 34%, respectively by 24 weeks. Vaccine efficacy was optimum when the primary vaccination was followed by a single booster (week 12 challenge RPS = 61%) rather than two boosters (week 12 challenge RPS = 48%). Vaccination without a booster resulted in a lower RPS (13%) indicating the necessity of a single booster vaccination to maximize efficacy. This study presents key findings that demonstrate the efficacy and commercial potential for this live attenuated BCWD vaccine.

Bacteriophage remediation of bacterial pathogens in aquaculture: a review of the technology

Bacteriophages have been proposed as an alternative to antibiotic usage and several studies on their application in aquaculture have been reported. This review highlights progress to date on phage therapies for the following fish and shellfish diseases and associated pathogens: hemorrhagic septicemia (Aeromonas hydrophila) in loaches, furunculosis (Aeromonas salmonicida) in trout and salmon, edwardsiellosis (Edwardsiella tarda) in eel, columnaris disease (Flavobacterium columnare) in catfish, rainbow trout fry syndrome or cold water disease (Flavobacterium psychrophilum) in trout and salmon, lactococcosis (Lactococcus spp.) in yellowtail, ulcerative skin lesions (Pseudomonas aeruginosa) in freshwater catfish, bacterial hemorrhagic ascites disease (Pseudomonas plecoglossicida) in ayu fish, streptococcosis (Streptococcus iniae) in flounder, and luminescent vibriosis (Vibrio harveyi) in shrimp. Information is reviewed on phage specificity, host resistance, routes of administration, and dosing of fish and shellfish. Limitations in phage research are described and recommended guidelines are provided for conducting future phage studies involving fish and shellfish.

Immunohistochemical diagnosis of tenacibaculosis in paraffin-embedded tissues of Senegalese sole Solea senegalensis Kaup, 1858

A sensitive and specific immunohistochemical technique was developed to improve the diagnosis of tenacibaculosis and to better understand its pathogenesis. Senegalese sole Solea senegalensis Kaup, 1858 were inoculated subcutaneously with a bacterial suspension of Tenacibaculum maritimum, and samples were taken at different hours post-inoculation. Sections from different organs were used as positive controls. In addition, a total of 128 field samples from different organs collected from tenacibaculosis outbreaks were used. Tenacibaculum maritimum antigens were detected in several organs of experimentally infected Senegalese sole and in at least one of the tissues from fish suffering from natural tenacibaculosis previously confirmed by culture and PCR-based methods. In fish collected during outbreaks, a strong positive reaction was detected in ulcerative skin areas. Moreover, bacterial antigen was identified inside scale pockets and in sites of the skin with mild lesion. In kidney and spleen, evident immunostaining of bacterial antigen was detected in both naturally and experimentally infected fish. Besides, the presence of T. maritimum in the intestinal tract without associated histological changes suggests that this organ may act as a reservoir for T. maritimum. The results of this study confirm the usefulness of IHC for the diagnosis of tenacibaculosis in paraffin-embedded tissues.

Multilocus sequence typing identifies epidemic clones of Flavobacterium psychrophilum in Nordic countries

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.

Intramuscular challenge of rainbow trout (Oncorhynchus mykiss) with two Norwegian field strains of Flavobacterium psychrophilum

In recent years there has been an increasing occurrence of Flavobacterium psychrophilum infections in farmed salmonids in Norway. The current study describes two field isolates of F. psychrophilum collected from farmed rainbow trout (Oncorhynchus mykiss) fingerlings and post smolts in Norway. Virulence of the two isolates was tested in vivo by intramuscular (IM) and/or intraperitoneal (IP) challenge of disease free, un-vaccinated rainbow trout. The isolates were concluded to be highly virulent compared to a reference isolate as they yielded high mortality after IM challenge even at low challenge doses. The more virulent of the two isolates was further used to establish a challenge model to evaluate the efficacy of vaccines against infections with F. psychrophilum. Three groups were included in the vaccination-challenge study; a vaccinated group given a 6 antigen (Ag) component vaccine containing F. psychrophilum antigens (6 Ag/F.psy(+)), a control vaccinated group administered a similar 5 antigen component vaccine without F. psychrophilum antigens (5 Ag/F.psy(-)), and a non-injected negative control group. Results from the IM challenge demonstrated that 1) our challenge model is able to discriminate between protected and unprotected experimental groups and 2) that the vaccine induced protection is specific against F. psychrophilum as mortality in the 5 Ag/F.psy(-) group was equally high as in the negative control, while the 6 Ag/F.psy(+) induced a high level of protection (RPS60 = 86.7%). The present study is one of the first to describe protection against F. psychrophilum infections induced by a multicomponent injection vaccine.

Genetic variation among Flavobacterium psychrophilum isolates from wild and farmed salmonids in Norway and Chile

AIMS

To aim of the study was to describe the genetic relationship between isolates of Flavobacterium psychrophilum with a main emphasis of samples from Chile and Norway. The isolates have been obtained from farmed salmonids in Norway and Chile, and from wild salmonids in Norway, but isolates from North America and European countries are also included in the analysis.

METHODS AND RESULTS

The study is based on phylogenetic analysis of 16S rRNA and seven housekeeping genes (HG), gyrB, atpA, dnaK, trpB, fumC, murG and tuf, and the use of a multilocus sequence typing (MLST) system, based on nucleotide polymorphism in the HG, as an alternative to the phylogenies. The variation within the selected genes was limited, and the phylogenetic analysis gave little resolution between the isolates. The MLST gave a much better resolution resulting in 53 sequence types where the same sequences types could be found in Chile, North America and European countries, and in different host species.

CONCLUSIONS

Multilocus sequence typing give a relatively good separation of different isolates of Fl. psychrophilum and show that there are no distinct geographical or host-specific isolates in the studied material from Chile, North America and Europe. Nor was it possible to separate between isolates from ulcers and systemic infections vs isolates from the surface of healthy salmonids.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows a wide geographical distribution of Fl. psychrophilum, indicating that the bacterium has a large potential for transmission over long distances, and between different salmonid hosts species. This knowledge will be important for future management of salmonids diseases connected to Fl. psychrophilum.