Bacterial Cold-Water Disease (Flavobacterium psychrophilum)

Overview

What is Bacterial Cold-Water Disease (Flavobacterium psychrophilum)?

Bacterial Cold-Water Disease (BCWD, the term generally used in North America), also known as Rainbow Trout Fry Syndrome (RTFS) in Europe, is caused by Flavobacterium psychrophilum, a Gram-negative, non-flagellated bacterium (family Flavobacteriaceae, phylum Bacteroidota) that is either non-motile or moves by gliding rather than via flagella 1. It is one of the three species recognized as the "big three" fish-pathogenic flavobacteria, alongside F. columnare (columnaris disease) and F. branchiophilum (bacterial gill disease) 1.

F. psychrophilum is a psychrophilic pathogen of cold-water salmonid hatcheries, principally rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar), and coho salmon (O. kisutch). It causes skin/gill lesions and systemic disease with reported mortality of 30–90% in juvenile fish, and has been reported from Europe, North America, Chile, Japan, Korea, and Australia 4 13. Its host range is wider than salmonids: in China it has been isolated in recent years from farmed cyprinids (crucian carp, grass carp, and silver carp) and identified as a causal agent of overwintering mortality syndrome 16 17.

Clinical Signs of Bacterial Cold-Water Disease (Flavobacterium psychrophilum)

Common Signs

• Physical Signs:
  • RTFS episodes tend to occur at 10–14 °C 13, although outbreaks also occur at much lower temperatures. The first Norwegian Atlantic salmon case, for example, occurred below 5 °C 21. Necrotic lesions are often seen on the skin surrounding the dorsal fin and tail. Affected fish show haemorrhage (mouth, dorsal fin, operculum, pelvic fin, trunk), splenomegaly with blurred spleen margins, trunk lesions, and an eroded tail and mouth 13.
  • Affected fish often show dark pigmentation on one or both sides of the body.
  • Skin and muscle lesions are common and usually appear on the caudal peduncle, although they can occur elsewhere on the body. Early in the disease, especially in trout, a whitish discolouration appears at the margin of the adipose fin and gradually spreads until it almost completely surrounds the caudal peduncle.
  • In some cases the adipose fin and the skin on the dorsal side of the peduncle become necrotic, exposing the underlying muscle. In extreme cases this progressive degeneration of skin and muscle can leave the vertebral column exposed.
  • On gross examination, fish may also show unilateral or bilateral exophthalmia, gill lesions, and lesions with exposed muscle on the lateral and dorsal body and on the caudal peduncle.
  • In coho salmon reaching the grow-out phase in estuarine water, clinical signs are similar to those of the freshwater phase. Affected fish show descaling, erosions, and single or multiple ulcers with yellow pigmentation and white or haemorrhagic borders. The yellow colour comes from flexirubin, a pigment characteristic of the genus Flavobacterium 28.
• Behavioural Signs: Fish are lethargic, gather in the corners of the tank, or swim in a spiral pattern.

Progression of Clinical Signs

• Early Stages: Disease onset is closely tied to water temperature. Experimentally raising water temperature accelerated the appearance of clinical disease, while increasing salinity delayed onset and improved survival, suggesting a window in early infection where environmental management can buy time for diagnosis and treatment 9.
• Advanced Stages: Untreated RTFS/BCWD progresses to the lesions described above, with high mortality. In the most advanced stages, fish may turn completely dark or dark only around the caudal peduncle, and some die without showing any external lesions.
• Chronic Cases: Chronic infection can cause spinal deformities (scoliosis and lordosis), and F. psychrophilum can be isolated from the deformed vertebrae.
• Impact on Fish Health: Both live and dead fish shed F. psychrophilum into the water, but dead fish shed the bacterium at substantially higher rates and for longer durations than live fish, meaning that mortality events feed forward into further transmission pressure within a population if not managed promptly 10. In Chile's national surveillance, flavobacteriosis was the leading infectious cause of freshwater mortality in rainbow trout and coho salmon in 2025 25, and the leading reason for antimicrobial use in the freshwater phase 26.

Causes of Bacterial Cold-Water Disease (Flavobacterium psychrophilum)

Etiology

• Causative Agent: F. psychrophilum is a Gram-negative rod (members of the genus measure 0.3–0.5 μm in diameter) that is non-flagellated and either non-motile or displays gliding motility; it is fastidious and generally does not grow on standard media such as trypticase soy agar 1. It shows limited genomic diversity worldwide, with the MLST clonal complex CC-ST10 being the main clonal complex infecting farmed rainbow trout worldwide 5.
• Key Virulence Factors: Colony spreading motility and proteolytic activity are required for pathogenicity. In a comparative study, the only non-virulent isolate tested was deficient in both traits 6. The gldD and gldG genes, part of the gliding-motility/Type IX secretion system (T9SS), are required for adhesion, biofilm formation, proteolytic activity, and full virulence; mutants deficient in either gene showed significantly reduced virulence in rainbow trout challenge models 7.
• Transmission Methods:
  • Horizontal Transmission: Both live and dead fish shed F. psychrophilum into the surrounding water; dead fish shed the bacterium at markedly higher rates and for longer durations than live fish, and shedding dynamics vary by bacterial genetic variant and host species 10.
  • Vertical / Egg-Associated Transmission: F. psychrophilum has been isolated from milt, ovarian fluids, and in close association with eggs, supporting a vertical transmission route that operates alongside horizontal transmission 11. Epidemic genotypes have also been isolated from eggs and sexual fluids, so trade in eggs and gametes, not only live fish, can carry the bacterium to new regions 22.

Risk Factors

• Environmental Factors: In one experimental study, raising water temperature to 18 °C shortened the time to clinical disease, an unexpected result since outbreaks are typically reported at colder temperatures. In the same study, 1% salinity delayed onset and improved cohabitant survival (42.6% vs 17.9%) 9. In the field, cases in coho salmon during the estuarine grow-out phase are explained by freshwater from rivers and rainfall lowering salinity. This helps infections that started in fresh water persist and spread horizontally, and creates conditions that favour new infections 28.
• Farm Management Practices: Prompt removal of dead and moribund fish is a key control point, since dead fish are disproportionate shedders of F. psychrophilum into hatchery and farm water 10.

Diagnosis

Diagnostic Methods

• Clinical Examination: Gross lesions (haemorrhage and trunk lesions) provide a presumptive diagnosis, but clinical signs overlap with other bacterial diseases, so laboratory confirmation is required for species-level diagnosis.
• Laboratory Tests:
  • Bacterial Culture: F. psychrophilum is fastidious and requires specialized low-nutrient media (e.g., TYES or Shieh's medium); it does not grow on trypticase soy agar 1.
  • PCR-Based Methods: A quantitative real-time PCR (qPCR) assay targeting the rpoC gene allows highly sensitive and specific detection and quantification of F. psychrophilum directly from water and fish tissue samples without the need for culturing 12. A multiplex PCR assay enables simultaneous, accurate differentiation of F. psychrophilum from two other major salmonid pathogens, Yersinia ruckeri and Aeromonas salmonicida subsp. salmonicida, in a single reaction 3.
  • Environmental DNA/RNA (Water Sampling): In a 12-week study of two freshwater recirculating aquaculture system (RAS) units on a commercial Atlantic salmon pre-smolt farm, F. psychrophilum was detected in water samples at higher and more stable levels than in gill swabs, which showed only minor fluctuations. This supports non-lethal water sampling as a way to track the bacterium in RAS, although a positive water sample does not by itself indicate active infection, and no clinical F. psychrophilum disease was observed during the study 18.

Differential Diagnosis

• Other Flavobacterial Diseases:
  • Columnaris disease (F. columnare): also produces yellowish skin lesions, eroded fins, and gill necrosis, but outbreaks have historically been associated with summer, whereas F. psychrophilum frequently causes disease below 10 °C 1.
  • Bacterial gill disease (F. branchiophilum): damage is confined to the gills (bacterial growth over the gill surface, excess mucus, and fused lamellae), with fish otherwise appearing normal until death 1. Affected fish show increased opercular movements, gasping at the surface, and respiratory distress 29.
  • Other yellow-pigmented flavobacteria: previously uncharacterized Flavobacterium and Chryseobacterium species have caused lesions very similar to BCWD, including erosion of the adipose fin, multifocal ulcers, and exophthalmia 1. Telling them apart requires laboratory identification, and MALDI-TOF mass spectrometry has been used to differentiate F. psychrophilum from F. psychrophilum-like species 29.
  • Tenacibaculosis (Tenacibaculosis (Tenacibaculum spp)): causes skin erosions, ulcers, and eroded fins, head, and gills 29. It should be considered when ulcers appear after transfer to brackish or sea water, such as in coho salmon in estuaries 28.
• Diseases with Overlapping Signs:
  • Yersiniosis (Yersiniosis (enteric redmouth disease)): lethargy, darkening of the skin, exophthalmia, and haemorrhages in and around the mouth and throat 31.
  • Furunculosis (Furunculosis (Aeromonas spp.)): lethargy, darkened skin, exophthalmia, haemorrhages at the base of the fins, enlarged spleen, and skin lesions and ulcers 29.
  • Viral diseases of fry: infectious pancreatic necrosis (Infectious Pancreatic Necrosis (IPN)) and infectious haematopoietic necrosis (IHN) both cause darkened skin and exophthalmia, with irregular swimming in IPN and lethargy in IHN 29.
  • Whirling disease (Myxobolus cerebralis): causes blacktail, spinal and cranial deformities, and exophthalmia in young rainbow trout. BCWD can also produce blacktail, spiral swimming, and spinal deformities, and both pathogens can be present in the same hatchery 30.
  • Saprolegniasis (Saprolegniasis): a frequent cause of skin lesions in fresh water. In Chile, mycosis was the leading infectious cause of freshwater mortality in Atlantic salmon in 2025 25.
• Confirming the Diagnosis: Because these signs overlap, diagnosis relies on culture on low-nutrient media and PCR (see Laboratory Tests above). A multiplex PCR assay allows F. psychrophilum, Y. ruckeri and A. salmonicida subsp. salmonicida to be identified from the same sample 3.

Treatment and Prevention

Treatment Options

• Current Treatments:

  • Antibiotics: Antimicrobial resistance (AMR) profiling of F. psychrophilum shows a consistent pattern across geographically distant farming regions: florfenicol retains high effectiveness, while resistance to oxytetracycline and quinolones is widespread. In Slovenia, 86.3% of isolates were non-wild-type (reduced susceptibility) for oxytetracycline and enrofloxacin and 90.2% for oxolinic acid, while all isolates were wild-type for florfenicol and erythromycin; reduced oxolinic acid susceptibility was linked to a T83A or T83V substitution in the gyrA gene 4. In Chile, MIC-based testing found non-wild-type frequencies of 70% for oxytetracycline and 38–45% for the quinolones oxolinic acid, flumequine, and enrofloxacin, but only 2% for florfenicol (8% by disk diffusion) 2. Florfenicol susceptibility is not universal, however: an earlier Chilean study using a different interpretive cut-off (normalized resistance interpretation, non-susceptible at >2.35 µg/mL) classified 92.5% of 40 isolates as resistant to florfenicol 27, and reduced florfenicol susceptibility has also been reported in 52.8% of Canadian isolates, alongside field reports of lost treatment efficacy in Chile 4.

• Vaccines:

  • Subunit and Attenuated Candidates: Vaccine development has been hampered by the bacterium's antigenic and genetic diversity. Candidate antigens (OmpH, OmpA, FspA, and lipopolysaccharide) have been identified using sera from convalescent rainbow trout, and an attenuated exbD2 mutant strain achieved 81.8% relative percent survival (RPS) after intramuscular immunization and challenge 8.
  • Polyvalent Immersion Vaccine: A polyvalent immersion vaccine combining three formalin-inactivated F. psychrophilum strains gave 84% RPS in rainbow trout fry against a heterologous strain challenge, with vaccinated fish also showing a delayed mortality onset compared to unvaccinated controls 13.
  • Vaccine Implementation: Commercial vaccines against F. psychrophilum remain available in only a limited number of countries (Chile, Norway), and none are suitable for the juvenile fry that bear the highest disease burden, leaving antibiotics as the primary recourse during an outbreak in most regions 13. In Norway, an injectable vaccine for larger fish works well, but no immersion vaccine is on the market 20.
  • Autogenous Vaccines: Because several serotypes and antigenic groups can co-circulate within a single farm, and each farm tends to have its own dominant serotype, autogenous (site-specific) vaccines built on each farm's dominant isolates have been recommended for Chilean salmonid farms 23.

• Experimental Alternatives:

  • Phage-Derived Endolysins (in vitro only): Ely174, an endolysin from an F. psychrophilum bacteriophage, lysed F. psychrophilum in vitro once the outer membrane had been permeabilised. Protein engineering improved its activity and thermal stability, and fusing it with the C-terminal domain of the Type IX secretion system protein SprA let it lyse untreated F. psychrophilum cells without an outer-membrane permeabiliser, although slowly and only at high concentrations 19.

Preventive Measures

• Biosecurity Protocols:

  • Iodine-based egg disinfection, standard in salmonid hatcheries, reduces surface bacterial load but does not sterilise eggs, and intra-ovum infection has been reported 11.
  • Experimental: bacteriophages showed short-term (24 h) control of F. psychrophilum on eyed eggs without affecting egg survival 11.
  • Prompt removal of dead and moribund fish to reduce the disproportionate shedding load they contribute to hatchery and farm water 10.

• Farm Management Practices:

  • Managing water temperature and salinity where feasible, which experimentally influenced RTFS onset and fry survival 9.

• Genetic Selection:

  • Selective breeding for BCWD resistance is already applied in commercial rainbow trout lines. In one important breeding population, adding genomic information raised prediction accuracy for resistance from 0.27 (pedigree-based) to 0.36. The trait behaved as oligogenic there, with its largest association peaks on chromosomes Omy25 and Omy8 14.
  • Resistance architecture can differ between populations. In two French commercial rainbow trout populations, 29-day survival after bath challenge was 71.3% vs 50.7%, and the 11 QTLs detected showed limited overlap between the two populations. Complement-system genes (e.g., C3, Cfb) sat in two of the QTL regions. Breeding programmes may therefore need population-specific genetic markers 15.

Case Studies

Real-World Examples

• Notable Outbreaks:

  • Arrival of an Epidemic Clone in Rainbow Trout (Norway, 2008 onwards): In Norway, F. psychrophilum was considered a minor problem in brown trout and salmon hatcheries until 2008, when an epidemic of systemic disease caused high mortalities on several rainbow trout farms. Of 117 Norwegian isolates typed by MLST, 46% belonged to clonal complex CC-ST10, and ST2 was the only CC-ST10 genotype found in the country. Given Norway's short (<10-year) history of BCWD in rainbow trout, this points to the recent introduction of an epidemic clone. CC-ST10 genotypes have been isolated from juvenile and adult fish, eggs, and sexual fluids, so trade in live fish, eggs, and gametes can all spread the clone 22. The disease went on to cause high mortality in fry and small fish at several Norwegian rainbow trout hatcheries, and also caused disease in large rainbow trout in low-salinity areas 20.
  • First Atlantic Salmon Case (Norway, Winter 2008–2009): The first Norwegian case of increased mortality in Atlantic salmon occurred in 60–100 g smolts in fresh water on a land-based farm in Western Norway, at water temperatures below 5 °C, with 7.0% accumulated mortality. Affected fish showed septicaemia and necrotic myositis (swollen dark spleen, pale liver, serohaemorrhagic ascites, and haemorrhage in abdominal fat and muscle), and two different genotypes of the bacterium were present in the same outbreak 21.
  • Mixed Strains Within the Same Outbreak (Chile, 2018–2019): F. psychrophilum was isolated from diseased Atlantic salmon and rainbow trout on five geographically separated freshwater farms (68 isolates). Each farm had a predominant serotype (types 0 and 4 in Atlantic salmon, types 1 and 2 in rainbow trout), but several antigenic groups co-existed within some farms during the same outbreak, and 71% of isolates carried the virulent CSF259-93 16S rRNA allele. In experimental challenges, type-2 isolates from rainbow trout were the most pathogenic 23.
  • Infected Broodstock and Hatchery Losses (Great Lakes Basin, United States): A 5-year study of Michigan broodstocks detected F. psychrophilum in all broodstocks except captive lake trout and brook trout. Prevalence was highest in feral Chinook salmon from the Lake Michigan watershed (mean 63.2%), and captive broodstocks had significantly lower prevalence than feral ones. Despite this, the bacterium was rarely detected in the progeny during hatchery rearing, yet heavy losses from clinical BCWD outbreaks still occurred 24.

• Lessons Learned: The Norwegian experience shows how quickly a single epidemic clone can establish itself once introduced, and that the movement of eggs, gametes, and live fish is a key route of spread 22. Good biosecurity, egg disinfection, and avoiding the movement of infected fish to new areas have kept systemic infection in Norwegian rainbow trout to a handful of sites per year 20. The Chilean data show that one outbreak can involve several serotypes at once, which complicates vaccine design and led the authors to recommend autogenous (site-specific) vaccines built on each farm's dominant isolates 23. The Great Lakes study shows that broodstock infection status alone does not predict which hatchery cohorts will suffer outbreaks 24.

Data Insights

Disease Impact by Country

Norway

• Incidence:

  • Systemic infection with F. psychrophilum in rainbow trout is a notifiable disease in Norway (national list, category F). It was confirmed at 1, 4, 1, 1, and 2 sites in 2021, 2022, 2023, 2024, and 2025, respectively. The two 2025 rainbow trout sites involved sequence types ST169 (a highly diverse group thought to include environmental variants) and ST187 (previously found in skin ulcers of Atlantic salmon). In the fjord system where the bacterium had been found in recent years, no disease was detected in large rainbow trout in 2025 20.
  • In Atlantic salmon, F. psychrophilum was detected at 14 sites spread along the coast (production areas PO3–PO13) in 2025, on four in ten sites in fish recorded as healthy and on three in ten in association with disease. At a broodstock facility with skin ulcers, one isolate belonged to ST15, not previously found in Norway. The disease can still be a challenge in the freshwater (smolt production) phase 20.

• Treatment & Management:

  • No immersion vaccine is on the Norwegian market. An available injectable vaccine for larger fish works well, and a combination vaccine containing F. psychrophilum is used actively in the area exposed to the disease. General biosecurity, egg disinfection to reduce possible vertical transmission, and avoiding the movement of infected fish to new areas are the main control measures 20.

Slovenia

• Incidence:

  • A survey of Slovenian farmed rainbow trout confirmed the presence of F. psychrophilum (51 isolates characterized) 4.

• Treatment & Management:

  • 86.3% of isolates showed reduced susceptibility to oxytetracycline and enrofloxacin, and 90.2% to oxolinic acid, while all isolates were wild-type for florfenicol and erythromycin 4.

Chile

• Incidence:

  • F. psychrophilum is described as the most important bacterial pathogen of freshwater farmed salmonids in Chile, with 125 isolates characterized from reared salmonids showing clinical signs of disease 2.
  • In 2025, flavobacteriosis accounted for 95.1% of freshwater mortality attributed to infectious causes in rainbow trout, 51.9% in coho salmon, and 24.9% in Atlantic salmon (where mycosis led with 60.2%). These shares refer to infectious mortality only, which is a minority of total freshwater mortality (8.4% in Atlantic salmon, where productive culling was the main recorded cause). F. psychrophilum was also the second most frequently detected pathogen in positive diagnostic reports from freshwater farms (17%), after piscine orthoreovirus (PRV, 36.9%) 25.

• Treatment & Management:

  • Flavobacteriosis was the leading reason for antimicrobial use in Chilean freshwater salmonid farming in 2025, accounting for 46.65% of freshwater use, ahead of renibacteriosis (BKD, 30.96%) and furunculosis (7.35%). Freshwater treatments relied on oxytetracycline (60.63%) and florfenicol (39.15%), mostly given in feed but also by immersion, and immersion treatments against flavobacteriosis drive the high use in the Los Ríos region. The freshwater phase accounted for only 1.17% of national antimicrobial use, and freshwater use fell by 34.1% compared with the previous year 26.
  • MIC-based epidemiological cut-off analysis found non-wild-type frequencies of 70% for oxytetracycline and 38–45% for the quinolones oxolinic acid, flumequine, and enrofloxacin, while only 2% of isolates (8% by disk diffusion) showed reduced susceptibility to florfenicol. This resistance pattern is broadly consistent with the Slovenian data above despite the geographic distance between the two surveys 2. An earlier study of 40 Chilean isolates, using a different interpretive cut-off, reported resistance in 90% to oxytetracycline, 92.5% to florfenicol, and 85% to oxolinic acid, and linked these levels to the large quantities of antimicrobials used to control F. psychrophilum outbreaks in Chilean farms 27.

Research and References

Latest Research Findings

Recent studies on Bacterial Cold-Water Disease / RTFS (2024–2026) have focused on host genetic resistance, an expanding host range, environmental drivers of disease, shedding and monitoring, antimicrobial resistance, and phage-derived alternatives to antibiotics:

1. "Flavobacterium psychrophilum as an Important Pathogen Associated with Overwintering Mortality Syndrome in Grass Carp (Ctenopharyngodon idella): Isolation, Characterization, and Pathogenicity" Authors: Song, H., Xue, M., Zhou, Y., et al. Reference: Animals, 2026; 16(10), 1465. Key findings: A highly pathogenic F. psychrophilum strain isolated from overwintering grass carp in Hubei, China, caused 15–75% mortality after dose-dependent intramuscular challenge, supporting its role in overwintering mortality syndrome. DOI: 10.3390/ani16101465

2. "Comparative genomics of crucian carp-isolated Flavobacterium psychrophilum: toward the foundations of overwintering mortality syndrome" Authors: Jiang, H., Zhao, Z., Zhao, Y., et al. Reference: Frontiers in Microbiology, 2026; 17, 1751566. Key findings: The first genome of a crucian carp (cyprinid) isolate retains the core F. psychrophilum genome and carries a distinct set of virulence-associated genes that may support infection in this non-salmonid host. DOI: 10.3389/fmicb.2026.1751566

3. "Environmental DNA/RNA for non-invasive early detection and monitoring of pathogen dynamics in Atlantic salmon (Salmo salar) recirculating aquaculture systems (RAS)" Authors: Krishna, D., Petersen, P. E., Dahl, M. M., et al. Reference: Aquaculture, 2026; 611, 743060. Key findings: In commercial pre-smolt RAS, F. psychrophilum was detected in water at higher and more stable levels than in gill swabs, and was the only pathogen found in all three RAS halls studied. DOI: 10.1016/j.aquaculture.2025.743060

4. "Remarkable heterogeneity revealed in the genetic architecture of resistance to a key bacterial pathogen in two commercial rainbow trout populations" Authors: Pouil, S., Rigaudeau, D., Lee, B.-H., et al. Reference: BMC Genomics, 2026; 27, 49. Key findings: Eleven QTLs for BCWD resistance with limited overlap between two French commercial populations; complement-system genes (C3, Cfb) lie within two of the QTL regions. DOI: 10.1186/s12864-025-12231-3

5. "Effects of Increased Water Salinity and Temperature on the Development of Rainbow Trout Fry Syndrome (RTFS) Caused by Flavobacterium psychrophilum" Authors: Donati, V. L., Lorenzen, N., & Madsen, L. Reference: Journal of Fish Diseases, 2026; 49(2), e70037. Key findings: Higher temperature accelerated RTFS onset, while higher salinity delayed onset and improved fry survival. DOI: 10.1111/jfd.70037

6. "The C-terminal domain of T9SS component protein SprA assists Flavobacterium psychrophilum bacteriophage endolysin Ely174 to lyse Gram-negative bacteria" Authors: Xie, S., Lu, Y., Li, X., et al. Reference: Applied and Environmental Microbiology, 2025; 91(11), e0189125. Key findings: The phage endolysin Ely174 lysed F. psychrophilum in vitro; fused with the C-terminal domain of the T9SS protein SprA, it lysed untreated F. psychrophilum cells without outer-membrane permeabilisers, though slowly and at high concentrations. DOI: 10.1128/aem.01891-25

7. "Genome-wide association analysis of resistance to bacterial cold-water disease in an important rainbow trout aquaculture breeding population" Authors: Santana, B., Palti, Y., Gao, G., et al. Reference: Frontiers in Genetics, 2025; 16, 1582138. Key findings: BCWD resistance behaved as an oligogenic trait; genomic information raised prediction accuracy from 0.27 to 0.36, with the largest association peaks on Omy25 and Omy8. DOI: 10.3389/fgene.2025.1582138

8. "Phenotypic and Genotypic Antimicrobial Resistance Profiles of Flavobacterium psychrophilum and Flavobacterium branchiophilum Isolated From Rainbow Trout (Oncorhynchus mykiss) in Slovenia" Authors: Pavlin, K., Papić, B., Zdovc, I., et al. Reference: Journal of Fish Diseases, 2025; 48(7), e14119. Key findings: F. psychrophilum showed high resistance to oxytetracycline and quinolones but full susceptibility to florfenicol. DOI: 10.1111/jfd.14119

9. "Varying Flavobacterium psychrophilum shedding dynamics in three bacterial coldwater disease-susceptible salmonid (Family Salmonidae) species" Authors: Knupp, C., Soto, E., & Loch, T. P. Reference: Microbiology Spectrum, 2024; 12(2), e0360123. Key findings: Dead salmonids shed F. psychrophilum at higher rates and for longer than live fish. DOI: 10.1128/spectrum.03601-23

Conclusion

F. psychrophilum causes substantial, recurring losses in cold-water salmonid hatcheries worldwide. The most effective management strategies currently available combine water-temperature and salinity management, prompt removal of dead and moribund fish to limit shedding, iodine-based egg disinfection to reduce egg-associated bacterial load, and judicious antibiotic use guided by local resistance surveillance. Florfenicol remains the most consistently effective option in recent surveys from geographically distant farming regions (Slovenia, Chile), even where resistance to oxytetracycline and quinolones is already widespread, although reduced florfenicol susceptibility has been reported in Chile and Canada. No commercially robust, broadly available vaccine yet exists, and none of the limited vaccines that do exist are suitable for the juvenile fry that bear the greatest disease burden, leaving antibiotics as the default response to most outbreaks. Genomic selection for BCWD resistance in rainbow trout offers a complementary, longer-term route, although resistance loci can differ between breeding populations.

Bacteriophage-based biocontrol, while still preliminary, merits continued investigation as a non-antibiotic intervention, particularly around the egg stage where vertical transmission can be intercepted. Documented outbreaks from Norway, Chile, and the Great Lakes point to the same practical lessons: epidemic clones spread through the trade in eggs, gametes, and live fish; several strains can co-circulate within a single farm, which complicates vaccine design; and sustained biosecurity can keep the disease to a handful of sites per year, as Norway's official surveillance shows. Country-level surveillance data nonetheless remains sparse outside of the few farming regions covered here.

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Last Modified: 2026-09-25

Other Bacterial Diseases

Bacterial Kidney Disease (BKD) Furunculosis (Aeromonas spp.) Moritella Viscosa Pasteurellosis Salmonid Rickettsial Septicaemia (SRS) Tenacibaculosis (Tenacibaculum spp) Vibriosis Winter Wounds Yersiniosis (enteric redmouth disease)

Citations:

[1] Loch, T. P., & Faisal, M. (2015). Emerging flavobacterial infections in fish: A review. Journal of Advanced Research, 6(3), 283–300. https://doi.org/10.1016/j.jare.2014.10.009

[2] Miranda, C. D., Smith, P., Rojas, R., Contreras-Lynch, S., & Alonso Vega, J. M. (2016). Antimicrobial susceptibility of Flavobacterium psychrophilum from Chilean salmon farms and their epidemiological cut-off values using agar dilution and disk diffusion methods. Frontiers in Microbiology, 7, 1880. https://doi.org/10.3389/fmicb.2016.01880

[3] Onuk, E. E., Ciftci, A., Findik, A., & Durmaz, Y. (2010). Development and evaluation of a multiplex PCR assay for simultaneous detection of Flavobacterium psychrophilum, Yersinia ruckeri and Aeromonas salmonicida subsp. salmonicida in culture fisheries. Journal of Veterinary Science, 11(3), 235–241. https://doi.org/10.4142/jvs.2010.11.3.235

[4] Pavlin, K., Papić, B., Zdovc, I., Knific, T., Gruntar, I., Sitar, R., Žele Vengušt, D., Seničar, M., Ocepek, M., & Švara, T. (2025). Phenotypic and genotypic antimicrobial resistance profiles of Flavobacterium psychrophilum and Flavobacterium branchiophilum isolated from rainbow trout (Oncorhynchus mykiss) in Slovenia. Journal of Fish Diseases, 48(7), e14119. https://doi.org/10.1111/jfd.14119

[5] Duchaud, E., Rochat, T., Habib, C., Barbier, P., Loux, V., Guérin, C., Dalsgaard, I., Madsen, L., Nilsen, H., Sundell, K., Wiklund, T., Strepparava, N., Wahli, T., Caburlotto, G., Manfrin, A., Wiens, G. D., Fujiwara-Nagata, E., Avendaño-Herrera, R., Bernardet, J.-F., & Nicolas, P. (2018). Genomic diversity and evolution of the fish pathogen Flavobacterium psychrophilum. Frontiers in Microbiology, 9, 138. https://doi.org/10.3389/fmicb.2018.00138

[6] Sundell, K., Landor, L., Nicolas, P., Jørgensen, J., Castillo, D., Middelboe, M., Dalsgaard, I., Donati, V. L., Madsen, L., & Wiklund, T. (2019). Phenotypic and genetic predictors of pathogenicity and virulence in Flavobacterium psychrophilum. Frontiers in Microbiology, 10, 1711. https://doi.org/10.3389/fmicb.2019.01711

[7] Pérez-Pascual, D., Rochat, T., Kerouault, B., Gómez, E., Neulat-Ripoll, F., Henry, C., Quillet, E., Guijarro, J. A., Bernardet, J. F., & Duchaud, E. (2017). More than gliding: involvement of GldD and GldG in the virulence of Flavobacterium psychrophilum. Frontiers in Microbiology, 8, 2168. https://doi.org/10.3389/fmicb.2017.02168

[8] Gómez, E., Méndez, J., Cascales, D., & Guijarro, J. A. (2014). Flavobacterium psychrophilum vaccine development: a difficult task. Microbial Biotechnology, 7(5), 414–423. https://doi.org/10.1111/1751-7915.12099

[9] Donati, V. L., Lorenzen, N., & Madsen, L. (2026). Effects of increased water salinity and temperature on the development of rainbow trout fry syndrome (RTFS) caused by Flavobacterium psychrophilum. Journal of Fish Diseases, 49(2), e70037. https://doi.org/10.1111/jfd.70037

[10] Knupp, C., Soto, E., & Loch, T. P. (2024). Varying Flavobacterium psychrophilum shedding dynamics in three bacterial coldwater disease-susceptible salmonid (family Salmonidae) species. Microbiology Spectrum, 12(2), e0360123. https://doi.org/10.1128/spectrum.03601-23

[11] Donati, V. L., Dalsgaard, I., Runtuvuori-Salmela, A., Kunttu, H., Jørgensen, J., Castillo, D., Sundberg, L.-R., Middelboe, M., & Madsen, L. (2021). Interactions between rainbow trout eyed eggs and Flavobacterium spp. using a bath challenge model: preliminary evaluation of bacteriophages as pathogen control agents. Microorganisms, 9(5), 971. https://doi.org/10.3390/microorganisms9050971

[12] Strepparava, N., Wahli, T., Segner, H., & Petrini, O. (2014). Detection and quantification of Flavobacterium psychrophilum in water and fish tissue samples by quantitative real time PCR. BMC Microbiology, 14, 105. https://doi.org/10.1186/1471-2180-14-105

[13] Hoare, R., Ngo, T. P. H., Bartie, K. L., & Adams, A. (2017). Efficacy of a polyvalent immersion vaccine against Flavobacterium psychrophilum and evaluation of immune response to vaccination in rainbow trout fry (Onchorynchus mykiss L.). Veterinary Research, 48, 43. https://doi.org/10.1186/s13567-017-0448-z

[14] Santana, B., Palti, Y., Gao, G., Tripathi, V., Martin, K. E., & Fragomeni, B. O. (2025). Genome-wide association analysis of resistance to bacterial cold-water disease in an important rainbow trout aquaculture breeding population. Frontiers in Genetics, 16, 1582138. https://doi.org/10.3389/fgene.2025.1582138

[15] Pouil, S., Rigaudeau, D., Lee, B.-H., Segret, E., Desgranges, A., D'Ambrosio, J., François, Y., Boudinot, P., Duchaud, E., Phocas, F., & Rochat, T. (2026). Remarkable heterogeneity revealed in the genetic architecture of resistance to a key bacterial pathogen in two commercial rainbow trout populations. BMC Genomics, 27, 49. https://doi.org/10.1186/s12864-025-12231-3

[16] Song, H., Xue, M., Zhou, Y., Liu, W., Liu, W., Jiang, N., Li, Y., Meng, Y., Ren, X., & Fan, Y. (2026). Flavobacterium psychrophilum as an important pathogen associated with overwintering mortality syndrome in grass carp (Ctenopharyngodon idella): Isolation, characterization, and pathogenicity. Animals, 16(10), 1465. https://doi.org/10.3390/ani16101465

[17] Jiang, H., Zhao, Z., Zhao, Y., Hao, K., Li, R., Xia, A., & Xue, H. (2026). Comparative genomics of crucian carp-isolated Flavobacterium psychrophilum: toward the foundations of overwintering mortality syndrome. Frontiers in Microbiology, 17, 1751566. https://doi.org/10.3389/fmicb.2026.1751566

[18] Krishna, D., Petersen, P. E., Dahl, M. M., Egholm, I., von Gersdorff Jørgensen, L., & Christiansen, D. H. (2026). Environmental DNA/RNA for non-invasive early detection and monitoring of pathogen dynamics in Atlantic salmon (Salmo salar) recirculating aquaculture systems (RAS). Aquaculture, 611, 743060. https://doi.org/10.1016/j.aquaculture.2025.743060

[19] Xie, S., Lu, Y., Li, X., Xu, Y., Cao, X., & Chen, J. (2025). The C-terminal domain of T9SS component protein SprA assists Flavobacterium psychrophilum bacteriophage endolysin Ely174 to lyse Gram-negative bacteria. Applied and Environmental Microbiology, 91(11), e0189125. https://doi.org/10.1128/aem.01891-25

[20] Moldal, T., Wiik-Nielsen, J., Oliveira, V. H. S., Svendsen, J. C., & Sommerset, I. (Eds.). (2026). Fiskehelserapporten 2025 [Norwegian Fish Health Report 2025] (Veterinærinstituttets rapportserie nr. 5a/2026; Chapter 8.1, Flavobakteriose, by H. K. Nilsen). Norwegian Veterinary Institute. https://www.vetinst.no/rapporter-og-publikasjoner/rapporter/2026/fiskehelserapporten-2025

[21] Nilsen, H., Johansen, R., Colquhoun, D. J., Kaada, I., Bottolfsen, K., Vågnes, Ø., & Olsen, A. B. (2011). Flavobacterium psychrophilum associated with septicaemia and necrotic myositis in Atlantic salmon Salmo salar: a case report. Diseases of Aquatic Organisms, 97(1), 37–46. https://doi.org/10.3354/dao02390

[22] Nilsen, H., Sundell, K., Duchaud, E., Nicolas, P., Dalsgaard, I., Madsen, L., Aspán, A., Jansson, E., Colquhoun, D. J., & Wiklund, T. (2014). Multilocus sequence typing identifies epidemic clones of Flavobacterium psychrophilum in Nordic countries. Applied and Environmental Microbiology, 80(9), 2728–2736. https://doi.org/10.1128/AEM.04233-13

[23] Ilardi, P., Valdes, S., Rivera, J., Irgang, R., & Avendaño-Herrera, R. (2023). Co-occurrence of heterogeneous Flavobacterium psychrophilum isolates within the same Chilean farm and during the same infectious outbreak. Journal of Fish Diseases, 46(10), 1085–1096. https://doi.org/10.1111/jfd.13828

[24] Van Vliet, D., Loch, T. P., & Faisal, M. (2015). Flavobacterium psychrophilum infections in salmonid broodstock and hatchery-propagated stocks of the Great Lakes Basin. Journal of Aquatic Animal Health, 27(4), 192–202. https://doi.org/10.1080/08997659.2015.1088488

[25] Servicio Nacional de Pesca y Acuicultura (SERNAPESCA). (2026a). Informe con antecedentes sanitarios de agua dulce y mar, año 2025. Subdirección de Acuicultura, Departamento de Salud Animal. https://www.sernapesca.cl/app/uploads/2026/03/Informe-Situacion-Sanitaria-Salmonicultura-Ano-2025.pdf

[26] Servicio Nacional de Pesca y Acuicultura (SERNAPESCA). (2026b). Informe sobre uso de antimicrobianos en la salmonicultura nacional, año 2025. Subdirección de Acuicultura, Departamento de Salud Animal. https://www.sernapesca.cl/app/uploads/2026/03/Informe-de-uso-de-antimicrobianos-ano-2025.pdf

[27] Henríquez-Núñez, H., Evrard, O., Kronvall, G., & Avendaño-Herrera, R. (2012). Antimicrobial susceptibility and plasmid profiles of Flavobacterium psychrophilum strains isolated in Chile. Aquaculture, 354–355, 38–44. https://doi.org/10.1016/j.aquaculture.2012.04.034

[28] Godoy, M. (2023, August 28). Flavobacteriosis en salmón coho (Oncorhynchus kisutch), en estuario: patología macroscópica. Marcos Godoy. https://marcosgodoy.com/flavobacteriosis-en-salmon-coho-oncorhynchus-kisutch-en-estuario-patologia-macroscopica/

[29] Moreira, M., Schrama, D., Farinha, A. P., Cerqueira, M., Raposo de Magalhães, C., Carrilho, R., & Rodrigues, P. (2021). Fish pathology research and diagnosis in aquaculture of farmed fish; a proteomics perspective. Animals, 11(1), 125. https://doi.org/10.3390/ani11010125

[30] Avila, B. W., Winkelman, D. L., & Fetherman, E. R. (2022). Dual resistance to Flavobacterium psychrophilum and Myxobolus cerebralis in rainbow trout (Oncorhynchus mykiss, Walbaum). Journal of Fish Diseases, 45(6), 801–813. https://doi.org/10.1111/jfd.13605

[31] Kumar, G., Menanteau-Ledouble, S., Saleh, M., & El-Matbouli, M. (2015). Yersinia ruckeri, the causative agent of enteric redmouth disease in fish. Veterinary Research, 46, 103. https://doi.org/10.1186/s13567-015-0238-4

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