Gyrodactylus salaris (GS)
How to spot Gyrodactylus salaris (GS), what to do if you suspect it and measures to prevent its spread.
Applies to England and Wales
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Details
Contents
- How to spot GS
- How GS is spread
- Preventing and controlling GS
- Further information on GS
- Legislation relating to GS
- Who to contact if you suspect GS
Gyrodactylus salaris (GS) is a small wormlike external fish parasite (or fluke) reaching up to 2mm in length, mainly affecting Atlantic salmon in freshwater. It can cause high levels of infection and mortality, particularly in juvenile life stages of salmon, and can be carried by other species of freshwater salmonid fish such as brown trout without causing observable signs of disease. This means it has potential to spread through waterways unnoticed with devastating consequences to wild salmon stocks. It attaches to the fins, gills and skin of fish with specially modified hooks and if detached from the fish is capable of surviving for several days in damp environments.
Species susceptible to GS include Artic char, Atlantic salmon, brook trout, brown trout, grayling, and rainbow trout. Other fish species living within the same water body could carry the parasite. More information on susceptible and vector species can be found here: Susceptible and vector species - GOV.UK (www.gov.uk)
Great Britain is free from GS, and it is a ‘listed’ (also referred to as ‘notifiable’) disease which you are legally obligated to report immediately to the Fish Health Inspectorate (FHI) at Cefas.
GS was first described in Sweden in the 1950’s and is believed to occur naturally in the eastern regions of the Baltic. It has been reported in Norway since the 1970’s where it has caused devastating losses in wild Atlantic salmon and has also been reported in several other countries in Europe, including Finland and Denmark. There has never been an outbreak of GS in the UK or Ireland.
It does not affect humans.
How to spot GS
Fish may show behaviour changes such as flashing, erratic swimming and lethargy.
Signs of the disease can include:
- increased mucus, thickening of the fins
- a white or grey appearance in heavily infected fish
- fin damage
- flashing and erratic swimming
- inactivity
How GS is spread
GS is transferred directly from fish to fish. Movements of infected fish both natural and due to human activity are the primary route of introduction and spread between rivers and farms. Fish migrating through brackish water can also spread the parasite between different river systems. However, GS cannot survive full strength seawater, therefore migratory fish returning from the sea will not introduce the parasite.
GS can also be spread through movements of contaminated water and equipment. Therefore, angling, husbandry, and transport equipment are potential risks for spreading GS, and equipment disinfection should be carried out thoroughly. Infected fish do not have to be sick or diseased to spread the parasite.
Preventing and controlling GS
Prevention is the best approach to controlling the disease. You can protect your fish by being aware of the signs of GS and other serious diseases and checking stock health regularly.
You must be sure any fish (or their eggs) introduced are GS free. Do not buy fish of unknown origin. Follow the rules on importing, exporting and moving fish.
Implement good husbandry and biosecurity practises, including surface disinfection of newly fertilised and eyed salmonid eggs using an appropriate method. For further information on biosecurity measures plans, guidance and templates: Biosecurity Measure Plans - Information and Templates.
Equipment (including angling equipment) that has been in contact with fish or the aquatic environment should be thoroughly cleaned and disinfected between sites, follow the Check, Clean, Dry biosecurity guidance. This is especially important if you have used your angling or water sports equipment abroad. Biosecurity Measure Plans - Information and Templates and Aquatic animal health and movements guides.
You must report it to the FHI immediately if you notice or become aware of fish showing any of the signs of GS. The FHI will investigate and apply disease control measures if required.
Further information on GS
World Organisation for Animal Health (WOAH) Aquatic Manual chapter GS.
Legislation relating to GS
Aquatic Animal Health (England and Wales) Regulations 2009
The Aquatic Animal Health (England and Wales) Regulations 2009
The listed diseases are in Regulation 1251/2008 as amended by the Animals, Aquatic Animal Health, Invasive Alien Species, Plant Propagating Material and Seeds (Amendment) (EU Exit) Regulations 2020, at Chapter 2, Annex 1A and Schedule 1 of The Aquatic Animal Health (England and Wales) Regulations 2009
Who to contact if you suspect GS
If you suspect or know of a listed disease of fish or shellfish in England and Wales, you must report it immediately to the Fish Health Inspectorate (FHI), Cefas. You must also tell the FHI immediately if you know or suspect that increased mortality has occurred.
Under the Aquatic Animal Health (England and Wales) Regulations 2009 it is a legal obligation to report suspicion of GS in any farmed or wild fish to the FHI. It is an offence under the Regulations to fail to inform the FHI of suspicion of GS.
- Email: fhi@cefas.gov.uk
- Telephone: 01305 206700 - Monday to Thursday - 9:00 am to 5:00 pm, Friday 9:00 am to 4:30 pm
- For emergencies out of hours telephone: 01305 206630
Fish Health Inspectorate
Centre for Environment, Fisheries and Aquaculture Science (Cefas)
Barrack Road, The Nothe
Weymouth
Dorset
DT4 8UB
Updates to this page
Last updated 11 September 2024 + show all updates
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Updated link to susceptible and vector species list.
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Vector and Susceptible species list updated from v1.1 to v1.2.
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Updated susceptible/vector species list.
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First published.