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Haemorrhagic smolt syndrome (HSS)

Overview​

What is Haemorrhagic Smolt Syndrome?​

Haemorrhagic smolt syndrome (HSS), also known as haemorrhagic diathesis (HD), is a bleeding disorder affecting farmed Atlantic salmon (Salmo salar) during the late freshwater stage, often around the smoltification process, and can lead to mortality and production losses 12. It is characterised by haemorrhages in internal organs and skeletal muscle, often accompanied by anaemia 1. The exact cause remains unknown: an infectious etiology has not been established; however, disturbances in osmoregulation associated with smoltification have been proposed as a primary contributing mechanism 23.

Clinical Signs of Haemorrhagic Smolt Syndrome​

Common Signs​

  • Physical Signs:

    • Haemorrhages in musculature, peritoneum and internal organs: Petechial (spot-like) and ecchymotic (larger patches) hemorrhages throughout the visceral fat, peritoneum, gastrointestinal tract, liver, swim bladder, heart, and skeletal muscle 45
    • Skin haemorrhages: Petechial bleeding under the belly and at the fin bases 13.
    • Anaemia: Pale gills indicative of reduced red blood cell count 12
    • Exophthalmia: Protruding or bulging eyes can sometimes occur 5
    • Swollen Abdomen (ascites): Abdominal distension due to fluid accumulation 3.
    • A pale or yellowish liver 2
    • Dark skin colouration 5

  • Behavioural Changes:

    • Lethargy: Reduced activity and slow swimming 5.
    • Abnormal Swimming Patterns: Surface swimming and loss of equalibrium 3.

Progression of Symptoms The progression of clinical signs in HSS has not been clearly described, but the reported timing of onset and resolution is outlined below.

  • Onset: The onset of clinical signs appear in freshwater pre-smolts and smolts, primarily around the smoltification period, but has also been observed in freshwater tanks mixed with seawater 3.

  • Mortality & Resolution: Mortality is generally low (usually <5%) 5. Symptoms typically resolve spontaneously within a few weeks following transfer to seawater or introduction of salinity 3.

  • Impact on Fish Health: Haemorrhagic smolt syndrome (HSS) represents a major fish welfare concern and is consistently ranked in Nowegian official surveys as one of the most important causes of mortality and reduced welfare among Atlantic salmon in freshwater hatcheries 1. Mortality is generally low, although substantial losses can occur in severe cases. HSS has also been associated with impaired antibody responses, including lower levels of vaccine-specific antibodies in affected fish, suggesting that the syndrome may compromise humoral immunity 2

Causes of Haemorrhagic Smolt Syndrome​

Etiology

  • Unknown etiology: The exact cause of Haemorrhagic Smolt Syndrome (HSS) remains unknown. In 2003, electron microscopy by Nylund et al. revealed two types of virus-like particles in HSS-affected tissues, leading researchers to suspect a viral origin 6. However, clinical diagnostic screening and experimental transmission trials indicate that it is a non-infectious disorder 2 5. No bacterial or viral pathogen has been established as the causative agent to this day.
  • Potential contributing factors: The prevailing scientific hypothesis links HSS to physiological strain and an inability to properly balance ion regulation during smoltification. This osmoregulatory imbalance is thought to cause vascular fragility and leakage of blood cells (diathesis) into surrounding tissues 1235. Further research is needed to determine what underlying mechanisms trigger the syndrome and how they lead to haemorrhage.
  • Links to Nephrocalcinosis: Nephrocalcinosis (kidney calcification) is often reported as an additional finding in fish with HSS. Although the two are distinct conditions, their co-occurrence has raised questions about possible shared contributing factors or interactions. These relationships remain unclear, but their frequent co-occurrence may suggest they are driven by shared environmental and operational stressors in freshwater hatcheries—such as osmoregulatory strain during smoltification and water quality issues 1

Risk Factors

  • Smoltification-related physiological disturbances: HSS occurs primarily around smoltification, and disturbances in physiological adaptation and osmoregulation have been proposed as contributing factors. The role of specific smoltification protocols, including photoperiod manipulation, in the development of HSS has not been clearly established.

  • Large-smolt production: HSS is often reported during the production of large smolts. Inadequate adaptation to the physiological changes associated with smoltification has been proposed as a contributing factor, although the underlying mechanisms remain unclear 7.

  • Sudden Water Temperature Drops: A temperature drop followed by prolonged exposure to very low freshwater temperatures (0.5–2 °C) was associated with increased mortality in one field case involving pre-existing HSS and concurrent nephrocalcinosis 2.

Diagnosis​

Diagnostic Methods

  • Clinical Examination: HSS is suspected based on the production history, timing around smoltification, and findings such as anaemia and widespread haemorrhages in internal organs and skeletal muscle. These findings are not specific to HSS and should be investigated through laboratory examination.

  • Laboratory Tests:

    • Histopathology: Microscopic examination of tissues is required for a definitive diagnosis. Bleeding into the renal tubules is a characteristic finding, together with haemorrhages in other tissue 2.
    • PCR & Pathogen Exclusion: Diagnostic RT-PCR screening for bacterial and viral pathogens is conducted not to confirm HSS, but to rule out bacterial or viral infections that produce identical systemic bleeding patterns, particularly viral haemorrhagic septicaemia (VHS), infectious haematopoietic necrosis (IHN) and infectious salmon anaemia (ISA) 2.
    • Haematology (Blood Examination): Assessment of haematocrit levels to evaluate the severity of anaemia. Affected salmon exhibit a dramatic drop in haematocrit (in one case falling from a normal ~35% down to 1.4–5%), indicating severe acute haemorrhagic anaemia 5.

Differential Diagnosis

  • Distinguishing HSS from Other Diseases: It is crucial to differentiate HSS from diseases with similar symptoms. The anaemia and widespread haemorrhages associated with HSS can resemble those seen in Infectious Salmon Anemia (ISA), viral haemorrhagic septicaemia (VHS) and infectious haematopoietic necrosis (IHN) 2. Bacterial septicaemia, particularly Yersiniosis (enteric redmouth disease), should also be considered because it can cause overlapping findings, including internal haemorrhages. Laboratory investigations are needed to distinguish these conditions, with pathogen-specific testing and bacteriological examination selected according to the suspected cause.

Treatment and Prevention​

Treatment Options

  • Current Treatments:
    • There are no current treatments against HSS. Management of HSS focuses on supporting physiological adaptation around smoltification.
  • Management:
    • Seawater Transfer: The most effective measure against HSS is transferring the affected fish group to seawater. Once in seawater, clinical symptoms, internal hemorrhaging, and mortality typically cease spontaneously within a few weeks. However, fish with extensive haemorrhages may still die shortly after transfer and transfer should therefore not be presented as a guaranteed treatment 3.
    • Salinity Management in Freshwater: Mixing seawater into freshwater tanks or administering salt-enriched feed may be considered under the guidance of fish health personnel. The choice of measure should take account of the fish’s smoltification status and overall health 3.

Preventive Measures

  • Farm Management Practices:
    • Stable Environmental Conditions: Maintain stable, appropriate rearing conditions during the freshwater phase, particularly as fish approach smoltification 7.
    • Clear Smoltification Signals: Provide appropriate light, temperature and salinity regimes to support smoltification 3.
    • Size-graded groups: Maintain reasonably uniform size groups during freshwater production 3.

Case Studies​

Real-World Examples

  • Notable Outbreaks:
    • No data currently available. Want to edit this section? Start here.

Data Insights​

Disease Impact by Country​

Norway​

  • In the annual Norwegian Veterinary Institute surveys (Fiskehelserapporten), fish health personnel and inspectors have consistently ranked HSS as the #1 cause of mortality among Atlantic salmon pre-smolts in land-based smolt facilities throughout the 2021–2025 period 8.
  • Because HSS is a non-notifiable disorder, exact national case totals are not captured in government registries. However, diagnostic records from the Norwegian Veterinary Institute and Fish Vet Group confirmed HSS in at least 59 land-based facilities in 2019, with the actual nationwide figure presumed higher as data from other private laboratories were not included 1.
  • HSS is very frequently diagnosed alongside Nephrocalcinosis (kidney calcification) as a secondary concurrent finding in Norwegian land-based smolt facilities 1.

No data currently available on other countries. Want to edit this section? Start here

Research and References​

Latest Research Findings​

Recent studies on Haemorrhagic Smolt Syndrome in salmonids have focused on various aspects of the disease, including its cause, preventive measure, immune consequences and pathology. Here are some notable recent research findings:

  1. “Consequences of Haemorrhagic Smolt Syndrome (HSS) for the Immune Status of Atlantic salmon (Salmo salar L.) (Case Study)”
    Authors: A. Krasnov, I. Sommerset, T. Søfteland, S. Afanasyev, P. Boysen, H. Lund.
    Reference: A. Krasnov, et al. (2020). Consequences of Haemorrhagic Smolt Syndrome (HSS) for the Immune Status of Atlantic salmon (Salmo salar L.) (Case Study). Biology, 9(1), 1.
    Link to study

  2. “Nefrokalsinose og hemoragisk smoltsyndrom – årsakssamanhengar og moglegheiter for førebygging” Authors: I. Sommerset, A. B. Olsen, A. Nilsen, M. Gjessing, et al.
    Reference: I. Sommerset, et al. (2020). Nefrokalsinose og hemoragisk smoltsyndrom – årsakssamanhengar og moglegheiter for førebygging. Norsk veterinærtidsskrift, (5).
    Link to article

Conclusion​

Haemorrhagic smolt syndrome (HSS), also known as haemorrhagic diathesis, remains an important health and welfare concern in Atlantic salmon freshwater hatcheries. Though it's full cause is not properly known, recent research indicates it's most likely a non-infectious disorder rooted in osmoregulatory dysfunction rather than a contagious pathogen. Current management recommendations focus on maintaining stable environmental conditions and supporting coordinated smoltification through appropriate environmental signals. Further research is needed to clarify the underlying mechanisms, identify specific risk factors, and evaluate effective prevention and management strategies.

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Last Modified: 2026-11-11

Tags: #HemorrhagicDiathesis, #Salmonids, #Salmon, #AtlanticSalmon, #Trout, #RainbowTrout, #Norway, #environmentalconditions, #HSS, #HemorrhagicSmoltSyndrome

Other Environmental & Physical Conditions​

Gas Bubble Disease Nephrocalcinosis

Citations: [1] Sommerset, I., Olsen, A. B., Nilsen, A., Gjessing, M., Kraugerud, M., Lie, K.-I., Hosfeld, C. D., Fivelstad, S., Mayer, I., Weltzien, F. A., Remø, S. C., Folkedal, O., Pedersen, A. Ø., Fjelldal, P. G., & Waagbø, R. (2020). Nefrokalsinose og hemoragisk smoltsyndrom – årsakssamanhengar og moglegheiter for førebygging. Norsk veterinærtidsskrift, (5). https://nvt.vetnett.no/journal/2020/5/m-571/Nefrokalsinose_og_hemoragisk_smoltsyndrom_-_%C3%A5rsakssamanhengar_og_moglegheiter_for_f%C3%B8rebygging

[2] Krasnov, A., Sommerset, I., Søfteland, T., Afanasyev, S., Boysen, P., & Lund, H. (2020). Consequences of haemorrhagic smolt syndrome (HSS) for the immune status of Atlantic salmon (Salmo salar L.) (case study). Biology, 9(1) https://doi.org/10.3390/biology9010001

[3] 1. Åkerblå AS. (n.d.). HSS – hemorragisk smolt syndrom. Akvademiet. https://akvademiet.no/fiskesykdommer/settefiskrelatert-avgang/hss-hemorragisk-smolt-syndrom/. Accessed November 2026

[4] Sandoval, C. (2019, February 27). Haemorrhagic smolt syndrome (HSS) – Gross pathology. Fish Pathology. https://fishhistopathology.com/?p=833. Accessed November 2026

[5] Rodger, H. D., & Richards, R. H. (1998). Haemorrhagic smolt syndrome: A severe anaemic condition in farmed salmon in Scotland. Veterinary Record, 142(20), 538–541 https://doi.org/10.1136/vr.142.20.538

[6] Nylund, A., Plarre, H., Hodneland, K., Devold, M., Aspehaug, V., Aarseth, M., Koren, C., & Watanabe, K. (2003). Haemorrhagic smolt syndrome (HSS) in Norway: Pathology and associated virus-like particles. Diseases of Aquatic Organisms, 54(1), 15–27. https://doi.org/10.3354/dao054015

[7] Norges miljø- og biovitenskapelige universitet. (n.d.). Produksjonslidelser. Havbrukslære. https://www.havbrukslare.no/produksjonslidelser#Hemoragisk-smoltsyndrom. Accessed November 2026

[8] Moldal, T., Wiik-Nielsen, J., Oliveira, V. H. S., Svendsen, J. C., & Sommerset, I. (2026). Fiskehelserapporten 2025 (Rapport 5a/2026). Veterinærinstituttet. https://www.vetinst.no/rapporter-og-publikasjoner/rapporter/2026/fiskehelserapporten-2025