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Gyrodactylus salaris

Overview​

What is Gyrodactylus salaris?​

Gyrodactylus salaris is a 1.5mm freshwater ectoparasitic flatworm (monogenean) first described by Malmberg in 1957 14. It is a microparasite (reproduce on the host and short generation time) that primarily infects the skin, fins, and gills of Atlantic salmon (Salmo salar) 2, 19 It also affects, though with varying susceptibility; rainbow trout (Oncorhynchus mykiss), Brown trout (Salmo trutta), Artic Charr (Salvelinus alpinus), and Grayling (Thymallus thymallus)17 . The parasite originated in the Baltic Sea region, where it is widespread across the Baltic Sea catchment area 3 G. salaris was first detected in Norway in 1975 4, introduced via imported Baltic salmon from Sweden 5. It has since been detected in 53 rivers and 39 farms, occurring across 23 countries in Europe 5. The parasite has been estimated to have reduced Norwegian wild Atlantic salmon populations by 86%, representing one of the worst ecological disasters to affect the species 4 5. Over NOK 1.5 billion has been spent on eradication efforts and a National Control Plan 5. The UK is currently recognised as G. salaris-free. The dominant paradigm holds that Norwegian (East Atlantic) salmon are highly susceptible due to a lack of co-evolutionary experience with the parasite, while Baltic salmon stocks show greater resistance 6. Rainbow trout and certain Baltic salmon strains commonly carry subclinical infections, which can persist for 90 days or more, making them one of the primary ways the infection is spread; the movement of subclinical infected stock between farms and countries 9.

Clinical Signs of Gyrodactylus salaris​

Common Signs​

  • Physical Signs:
    • Fin erosion and increased mucus production — infected fish develop eroded fins and excess mucus, giving them a grey colouration 7. Additionally, dorsal and pectoral fins may have a whitish appearance due to thickening of the epidermis 17
    • Behavioural Changes: Infected fish exhibit flashing and restlessness as a response to skin irritation 7
    • Eye accumulation: In advanced cases, parasites accumulate on and around the eyes and can be seen as a cloudy film over the eyes 7. They can be seen with the naked eye if present on the outside, looking like white small elongated pin heads 15.
    • Severe population decline: Severe infections cause high mortality, particularly in Atlantic salmon 8

Progression of Symptoms

  • Early Stages: Subtle behavioural changes such as flashing, alongside the onset of fin erosion and increased mucus production 7
  • Advanced Stages: Parasite populations proliferate rapidly on susceptible hosts. In severe cases, fish death can occur before the parasite population begins to decline 6
  • Impact on Fish Health: G. salaris causes severe epidemics with high morbidity rates, particularly in Atlantic salmon. The hyperviviparous reproductive cycle of the parasite allows for rapid population growth on susceptible hosts 4

Causes of Gyrodactylus salaris​

Etiology

  • Causative Agent: G. salaris is a monogenean ectoparasite with a hyperviviparous reproductive cycle, enabling rapid population growth on susceptible hosts 4. Atlantic salmon strains from Norway and the UK lack innate resistance to the parasite, while Baltic strains are able to regulate infection 4.

  • Transmission Methods:

    • Direct fish-to-fish contact: The parasite spreads directly between fish hosts with no free-living life stages and no requirement for an intermediate host 2. It was introduced to Norway on at least three separate occasions, likely via salmonid stock movements 7.

    • Anthropogenic translocation: Further spread has occurred through the transport of infected fish between farms and countries 9. Infected fish transport equipment was implicated in a separate epidemic in Russia’s Keret’ River, which began around 1992 and caused a 200-fold reduction in salmon parr density within four years 10

    • Brackish-water dispersal: Farm escapees and migration of infected salmon through fjords — where low surface salinity allows the parasite to survive briefly in brackish water — have contributed to spread between river systems 5

  • Role of Rainbow Trout as a Reservoir Host: Rainbow trout are considered an important reservoir host for Gyrodactylus salaris because they are generally less susceptible to severe disease than Atlantic salmon and can maintain infections for extended periods, including more than 90 days in experimental observations 11, 9. Brown trout and rainbow trout may also carry infections asymptomatically, allowing the parasite to persist undetected for long periods. 7.

  • Environmental Factors: Temperature range is from 2.5 to 13°C, with parasites living longer at lower temperatures but number of offspring increasing with temperature (peaking at 6.5-13°C) 16. Norway’s temperature range appears particularly optimal for G. salaris population growth 4. Seasonality and fish size can play a role, as in the case of Arctic Charr, where G. salaris was found on larger (>28cm) and only in the autumn 18.

  • Species Affected: Atlantic salmon Salmo salar), Rainbow trout (Oncorhynchus mykiss), Brown trout (Salmo trutta), Artic Charr (Salvelinus alpinus), brook trout (Salvelinus fontinalis) and Grayling (Thymallus thymallus) 17 .

Diagnosis​

Diagnostic Methods

  • Traditional Methods: Conventional diagnosis involves electrofishing, euthanasia, and manual microscopic examination — a costly and lethal process (6,981 fish were killed during surveillance in 2016 alone) 11.
  • Morphological Identification: A major diagnostic challenge is discriminating G. salaris from the morphologically similar but benign G. thymalli, as morphology alone cannot reliably distinguish the two species 8. Molecular confirmation is therefore required 8.
  • Molecular Methods:
    • The ITS region is nearly identical between G. salaris and G. thymalli; COI and IGS markers are required for reliable discrimination 9
    • ITS2 sequencing (ribosomal internal transcribed spacer 2) is used for species-level identification 8
    • Environmental DNA (eDNA) water sampling eliminates the labour intensity and lethal impact of conventional surveillance based on catching and killing sample fish 11.

Treatment and Prevention​

Treatments​

  • Rotenone: The primary river treatment is rotenone 5 2 6 9. As an extreme measure that kills all fish — and therefore eliminates the parasite, which cannot survive without a host — rivers are treated in two consecutive years before being restocked with uninfected fish. This approach has been used in Norway to reduce the number of infected rivers to 8 in the Drammen area 2.
  • Aluminium sulphate (AlS) method: Continuous application for approximately 14 days at low pH selectively kills the parasite while allowing salmon to survive. This method was applied in River Lærdalselva during 2011–2012 5 2.
  • Chlorine treatment: A chlorine-based method was trialled successfully in two western Norwegian rivers during 2022–2023, including a full-scale trial in River Driva 5 2.
  • Heat treatment: Exposure to water at 40°C for 5–50 minutes is sufficient to kill the parasite — hot tap water is considered a viable practical option 3.

Prevention​

  • Physical barriers: Installation of barriers to prevent upstream fish migration from infected areas 3.
  • Equipment disinfection: 1% Virkon S for 15 minutes has been confirmed effective; even 0.1% kills the parasite in under 10 minutes 3.
  • Biosecurity controls: Preventing the transport of infected fish between farms and countries remains the most critical measure for limiting further spread 9.

Case Studies​

Notable Cases​

Norway​

  • The 2022 national surveillance programme examined 2,273 Atlantic salmon from 71 rivers and 3,216 Atlantic salmon and rainbow trout from 90 farms and hatcheries; G. salaris was not detected in any sample. The Skibotn infection region — comprising Skibotnvassdraget, Signaldalelva, and Kitdalselva — was formally declared free from infection in 2022.2.

    • Lærdalselva, western Norway: Successfully treated using the aluminium sulphate (AlS) method, demonstrating the viability of selective parasite elimination without total fish kill 5
    • Drammenselva: The most complex remaining case, posing particular challenges due to its large water flow, high fish species diversity (more than 20 species), and a broad brackish estuary zone where infected juvenile salmon have been found 5

  • As of 31 December 2024, G. salaris is confirmed present in six Norwegian river systems, all within the Drammen infection region:

    1. Drammenselva (012.Z) — Buskerud
    2. Lierelva (011.Z) — Buskerud
    3. Ebbestadelva (012.2Z) — Buskerud
    4. Sandeelva / Vesleelva (013.Z) — Vestfold og Telemark
    5. Selvikvassdraget (013.1Z) — Vestfold og Telemark
    6. Bergerelva (012.3Z) — Buskerud (newly detected in 2024) 13

  • No farms or hatcheries tested positive in 2024. A total of 3,566 Atlantic salmon and rainbow trout from 97 farms across Norway were examined, all returning negative results 13. Eradication of the Drammen region is planned to be carried out over a four-year period from 2025 to 2028 — the last remaining infected region in Norway 14.

Sweden​

  • G. salaris is believed to be naturally endemic in Baltic-draining rivers, where it is not pathogenic
  • On the west coast (Kattegat/Skagerak draining rivers), however, high infection intensities and significant population impacts have been recorded.
    • In the Säveån River (1998), salmon parr density declined by 90%, with a mean intensity of approximately 1,700 parasites per fish.
    • Unique divergent haplotypes identified in Göta älv and Lake Vänern likely represent an ancient, isolated lineage 9.

Romania​

  • A 2008 survey of 543 salmonids from 12 farms and one river across western and central Romania found G. salaris in 8 of the 12 farms sampled, infecting rainbow trout, brown trout, and brook trout.
  • G. salaris was detected on brook trout at the Văliug-Semenic farm, which kept its own brood stock and had no rainbow trout present.
  • G. truttae was recorded on brook trout at one farm and on wild brown trout in River Mare — both representing first records for Romania 8.

Italy​

  • G. salaris was first reported in 2009 from rainbow trout farms in central and northern Italy; however, examination of archived material showed that the parasite had been present since at least 2000.
  • The parasite was found in seven regions, with 81.5% of samples positive for Gyrodactylus spp. No significant mortalities were observed, indicating subclinical infection 9.

Finland​

  • G. salaris was first detected on Baltic salmon in Finland in 1984 and officially reported in 1987. An 8% mortality was recorded in one-year-old Baltic salmon at Ossauskoski farm.
  • Parts of Finland — including the Tenojoki and Näätämönjoki catchments — have since been declared G. salaris-free under EC Decision 2010/221/EU 9.

Russia (Keret’ River)​

  • Infection prevalence in the Keret’ River approached 100%, with a mean intensity of approximately 300 parasites per fish, linked to a severe decline in salmon parr populations.
  • The origin of the outbreak was traced via mitochondrial DNA analysis to the Vyg (White Sea) hatchery, with introduction estimated to have occurred between 1986 and 1989 9.

Data Insights​

Norway:​

  • G. salaris presence was confirmed via eDNA, with an infection prevalence of 85.7% in sampled salmon and a mean intensity of 83 parasites per fish 11
    • G. salaris eDNA copies ranged from 560 to 371,440 copies per litre across Lierelva samples
    • Atlantic salmon eDNA was stable across all four sampling sites (average 8,948 copies/litre ± SD 945), suggesting consistent eDNA emission rates
    • Rainbow trout eDNA showed a clear dilution gradient with distance from the farm source (7,848,000 copies/litre at the farm vs. 22 copies/litre 25 km downstream)
    • Droplet digital PCR (ddPCR) detected rainbow trout eDNA at concentrations as low as 22 copies/litre, demonstrating high analytical sensitivity 11

Latest Research and References​

eDNA Monitoring​

  • "Catching the fish with the worm: a case study on eDNA detection of the monogenean parasite Gyrodactylus salaris and two of its hosts, Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss)" Reference: Rusch, J. C., Hansen, H., Strand, D. A., Markussen, T., Hytterød, S., & Vrålstad, T. (2018). Parasites & Vectors, 11, 333. Key Findings: The Lierelva study represents the first application of eDNA monitoring to a monogenean fish parasite (G. salaris), establishing proof of concept for non-lethal surveillance at a landscape scale 11 Limitations: eDNA assays cannot currently distinguish between G. salaris and G. thymalli due to their identical ITS sequences; mitochondrial cytochrome oxidase gene assays have been proposed as a future solution. Further research is also needed to establish detection limits at low parasite loads and to correlate eDNA signal with actual infection intensity 11

Conclusion​

Gyrodactylus salaris remains one of the most significant parasite threats to wild Atlantic salmon populations and has spread to multiple countries through fish movements and waterborne transmission. While eradication efforts have historically relied on chemical treatments and strict biosecurity measures, recent research has focused on improved surveillance, environmental DNA (eDNA) monitoring, and more targeted management approaches. Continued international collaboration, early detection, and effective containment strategies remain essential to limiting the parasite's impact and protecting vulnerable wild salmon populations.

Tags: #GyrodactylusSalaris, #Gyrodactylus, #Monogenean, #Salmon, #AtlanticSalmon, #RainbowTrout, #ParasiticDiseases, #Ectoparasite, #OIENotifiable, #Norway

Other Parasitic Diseases​

Sea Lice Amoebic Gill Disease (AGD) Proliferative Gill Disease

Citations:

[1] Malmberg, G. (1957). First description of Gyrodactylus salaris. Cited in: Paladini, G., et al. (2021). Geographical distribution of Gyrodactylus salaris Malmberg, 1957 (Monogenea, Gyrodactylidae). Parasites & Vectors, 14, 34. https://doi.org/10.1186/s13071-020-04504-5 (https://pubmed.ncbi.nlm.nih.gov/33422145/)

[2] Hansen, H., Fornes, G. J., Mohammad, S. N., Børresen, J. H., Amundsen, M. M. and Welde, H. I. The surveillance programme for Gyrodactylus salaris in Atlantic salmon and rainbow trout in Norway 2022. Surveillance program report 20. Veterinærinstituttet 2023. https://www.vetinst.no/overvaking/gyrodactylus-salaris-overv%C3%A5kningsprogram/_/attachment/inline/05aad634-e74d-495d-b83b-bbf856178cb9:3ed8d5dc75067786b28053f44c7be3b2047a41c8/2023_20_GS%20Atlantic%20salmon%20and%20rainbow%20trout%202022.pdf

[3] Koski, P., Anttila, P. & Kuusela, J. Killing of Gyrodactylus salaris by heat and chemical disinfection. Acta Vet Scand 58, 21 (2015). https://doi.org/10.1186/s13028-016-0202-y

[4] Denholm, S. J., Norman, R. A., Hoyle, A. S., Shinn, A. P., & Taylor, N. G. (2013). Reproductive trade-offs may moderate the impact of Gyrodactylus salaris in warmer climates. PloS one, 8(10), e78909. https://doi.org/10.1371/journal.pone.0078909 https://pubmed.ncbi.nlm.nih.gov/24205349/

[5] Mo, T. A. (2024). The battle against the introduced pathogenic monogenean Gyrodactylus salaris in Norwegian Atlantic salmon rivers and fish farms. Journal of Fish Diseases, 47(9), e13981. https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/jfd.13981

[6] Ramírez, R., Bakke, T.A. & Harris, P.D. Population regulation in Gyrodactylus salaris – Atlantic salmon (Salmo salar L.) interactions: testing the paradigm. Parasites Vectors 8, 392 (2015). https://doi.org/10.1186/s13071-015-0981-4

[7] Paladini, G., Hansen, H., Williams, C.F. et al. Reservoir hosts for Gyrodactylus salaris may play a more significant role in epidemics than previously thought. Parasites Vectors 7, 576 (2014) https://doi.org/10.1186/s13071-014-0576-5

[8] Hansen, H., Cojocaru, CD. & Mo, T.A. Infections with Gyrodactylus spp. (Monogenea) in Romanian fish farms: Gyrodactylus salaris Malmberg, 1957 extends its range. Parasites Vectors 9, 444 (2016) https://doi.org/10.1186/s13071-016-1727-7

[9] Paladini, G., Shinn, A.P., Taylor, N.G.H. et al. Geographical distribution of Gyrodactylus salaris Malmberg, 1957 (Monogenea, Gyrodactylidae). Parasites Vectors 14, 34 (2021). https://doi.org/10.1186/s13071-020-04504-5

[10] Mieszkowska, A., Górniak, M., Jurczak-Kurek, A., & Ziętara, M. S. (2018). Revision of Gyrodactylus salaris phylogeny inspired by new evidence for Eemian crossing between lineages living on grayling in Baltic and White sea basins. PeerJ, 6, e5167. https://doi.org/10.7717/peerj.5167

[11] Rusch, J. C., Hansen, H., Strand, D. A., Markussen, T., Hytterød, S., & Vrålstad, T. (2018). Catching the fish with the worm: a case study on eDNA detection of the monogenean parasite Gyrodactylus salaris and two of its hosts, Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss). Parasites & Vectors, 11, 333. https://doi.org/10.1186/s13071-018-2916-3 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5987472/)

[13] Norwegian Veterinary Institute. (2025). The surveillance programme for Gyrodactylus salaris in Atlantic salmon and rainbow trout in Norway 2024. Report 16/2025. https://www.vetinst.no/overvaking/gyrodactylus-salaris-overvakningsprogram/_/attachment/inline/e1a504a6-a65f-4b04-9388-087e17c23e71:ed43b8756e6103e4c97092e3a7af3d902d7fb3de/2025_16_OK_AS%20and%20RT%202024.pdf Accessed June 2026

[14] NASCO. (May 2025). Report on Gyrodactylus salaris Roadmap in 2024 – Norway. NEA(25)11. https://nasco.int/wp-content/uploads/2025/05/NEA2511_Report-on-Gyrodactylus-Salaris-Roadmap-in-2024.pdf Accessed June 2026

[15] Torstein Helleve. (2015, March) Newspaper. "Avlivar mytar om lakseparasitt". Forskning.no. https://www.forskning.no/fiskehelse-fisk-partner/avlivar-mytar-om-lakseparasitt/507375 Accessed June, 2026

[16] Jansen, P. A., & Bakke, T. A. (1991). Temperature-dependent reproduction and survival of Gyrodactylus salaris Malmberg, 1957 (Platyhelminthes: Monogenea) on Atlantic salmon (Salmo salar L.). Parasitology, 102 Pt 1, 105–112. https://doi.org/10.1017/s0031182000060406

[17] Australian Government Dept. of Agriculture, Fisheries and Forestry. "# Aquatic Animal Diseases Significant to Australia: Identification Field Guide- Infection with Gyrodactylus salaris". Page last updated Dec 2025 (https://www.agriculture.gov.au/sites/default/files/documents/infection-gyrodactylus-salaris.pdf) Accessed June, 2026.

[18] Mo, T. A., Hansen, H., & Hytterød, S. (2023). Occurrence and seasonality of Gyrodactylus salaris and G. salmonis (Monogenea) on Arctic char (Salvelinus alpinus (L.)) in the Fustvatnet lake, Northern Norway. Journal of Fish Diseases, 46, 395–403. https://doi.org/10.1111/jfd.13752 https://onlinelibrary.wiley.com/doi/full/10.1111/jfd.13752

[19] Chapter authors: Phil Harris, Lutz Bachmann and Tor Bakke. Editors: Øystein Aas, Anders Klemetsen, Sigurd Einum, Jostein Skurdal. Publisher: John Wiley & Sons, (2011). Book Title: Atlantic Salmon Ecology. "Chapter 9: The Parasites and Pathogens of the Atlantic Salmon: Lessons from Gyrodactylus salaris" (https://books.google.no/books?hl=en&lr=&id=9lMZnUdUGZUC&oi=fnd&pg=PA221&dq=Gyrodactylus+salaris+brown+trout&ots=63OdufUAuF&sig=y4TNkUdCgxu112eAVWpD3V193lM&redir_esc=y#v=onepage&q=Gyrodactylus%20salaris%20brown%20trout&f=false)