The small parasite Spironucleus salmonicida is causing concern for salmon farmers in Northern Norway.

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  Institute of Marine Research, Norway (edited by Nofima)

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The parasite that causes systemic spironucleosis in salmon has been detected in fish farms. The disease has been found in farmed salmon in Northern Norway on several occasions, most recently in 2022.

Spironucleosis can cause mortality in fish, but can also have a detrimental effect on the quality of the fish, resulting in large quantities being discarded upon slaughter. To prevent the spread of the parasite, it has often been necessary to slaughter all infected fish populations, which makes the parasite a major threat to the aquaculture industry.

Millions in funding from FHF

In 2022, the parasite and the disease were detected in several fish farms in Northern Norway, and the parasite was also detected for the first time in a smolt production facility. There has also been an increase in the number of infected fish as well as a possible spread between neighbouring net-pens at sea.

“The aim of the SpiroFri project is to gain increased knowledge about the parasite so that the industry can prevent the pathogen from entering and spreading in smolt production facilities, and therefore prevent it from appearing in farmed fish that have been transferred to the sea”, says project manager and research director Bjørn Olav Kvamme at the Institute of Marine Research.

The project has received NOK 7 million in support from the Norwegian Seafood Research Fund, with a total budget of NOK 8.9 million. It is a collaborative project between the Institute of Marine Research, Nofima, the Norwegian Veterinary Institute, Pure Salmon Kaldnes and Grieg Seafood.

Parasites living in the intestine

Several species of Spironucleus can be found in Norwegian salmonids, and these tiny, single-celled organisms normally live in the intestine.

“The special thing about this parasite is that it leaves the intestine where it typically resides, and enters the bloodstream,” says Egil Karlsbakk, research scientist and scientific responsible at the Institute of Marine Research, adding:

“No one knows why and how it penetrates the intestinal wall. When it enters the bloodstream, it can spread to all parts of the fish, which is why we say the infection is systemic. It can enter the brain and form abscesses in the internal organs and the muscle.”

Infection trials at the Institute of Marine Research

Karlsbakk leads the infection trials that will be carried out at IMR. Salmon, sea trout, Arctic sea char and lumpfish will all be part of the research project. A pilot infection trial led by Nofima has already been carried out with salmon in seawater at the Aquaculture Research Station in Tromsø.

At IMR, marine scientists will investigate the role these fish species play in the spread of infection, and if infection from fish farming facilities can have a detrimental effect on those species as well.

“It turns out that lumpfish used as cleaner fish in the net pens have been infected, which is new information – i.e. the fact that the parasite can also infect fish that are not salmonids.  We are therefore concerned that it can also spread to other species,” says Karlsbakk and adds:

“Our trials are currently focused on how to infect the fish.” 

Report anticipated this summer

Farmed salmon with the disease systemic spironucleosis caused by theparasite Spironucleus salmonicida. Abscesses (granulomas) are formed in musculature and internal organs in infected fish, shown here in the liver. Photo: Roy-Inge Hansen, Nofima

The new project is divided into three work packages: a review of current findings and data on the parasite; investigations of biosecurity measures and consequences of infection in hatcheries; and infection trials that investigate possible routes of infection among the fish.

“There is an urgent need for more knowledge about this parasite and its relatives, so we expect to have the first knowledge summary in Norwegian as early as this summer,” says Karlsbakk, adding:

“When it is ready, we will start our own infection trials here in Bergen. The scientists at Nofima in Tromsø will look at possible means to disinfect the intake water so that the parasite does not enter or spread in the smolt facility.

“The parasite has been detected in wild salmonids living in freshwater that is used as a water source for hatcheries, which therefore represents a potential source of infection that we must try to control,” says Lill-Heidi Johansen, scientist and project manager at Nofima.

The results from the project will be key to preventing and dealing with problems and losses caused by the parasite, as well as preventing the spread of infection.

“There is still much we do not know about the parasite, and it is a very good thing that FHF were able to provide funding for this research so quickly”, says Kvamme, adding: 

“We are also very pleased that important research institutions and industry partners are collaborating on this project. Both industry stakeholders and support from public administration will contribute to developing evidence-based interventions to tackle this challenge.”

Facts: Spironucleus salmonicida

  • The Spironucleus species are single-celled microparasites that are commonly found in fish.
  • They normally live in the intestine, and are harmless.
  • They live in oxygen-poor environments; high oxygen levels are harmful to them. 
  • Some species cross over into the bloodstream and thereby infect the entire host animal.
  • Those species can be serious pathogens.

Facts: The spironucleosis disease

  • Systemic spironucleosis is a rare parasitic disease in farmed fish, which is caused by the flagellate Spironucleus salmonicida
  • “Systemic” means that the parasite spreads to all parts of the fish’s body – skin, internal organs, and musculature, where it forms abscesses and ulcers. 
  • Microscopic samples of the contents of the abscesses will typically be teeming with highly mobile flagellates of about 10 µm in size. 
  • Mortality can be high, but even in apparently healthy fish, muscle abscesses can occur that make the fish unsuitable for human consumption.

About SpiroFri:

  • The SpiroFri project is a collaboration between the Institute of Marine Research, Nofima, the Norwegian Veterinary Institute, Pure Salmon Kaldnes and Grieg Seafood, and is led by IMR.
  • The project has received NOK 7 million in funding from the Norwegian Seafood Research Fund (FHF: 901831), and has a total budget of NOK 8.9 million. 
  • The project will run from January 2023 to December 2024.

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