Improving salmon vaccines against winter ulcers

Researchers have new knowledge about the skin of the salmon and vaccines, which may be useful facing the coming winter.

Moritella viscosa is one of the types of bacteria that causes ulcers on the salmon. Here, Christian Karlsen grows two varieties of the bacteria in the lab. Credits: Nofima
December 6, 2023

Salmon can get ulcers in winter with cold seawater due to a bacterium called Moritella viscosa. Currently, there are vaccines available against this bacteria but the problem is that it has many different strains. Nofima researchers have new knowledge about the skin of the salmon and vaccines, which may be useful facing the coming winter.

The vaccine works much better if the correct strain is used and not even the vaccine can keep all the fish healthy. Senior scientist, Christian René Karlsen, mentioned high ulcer development even in fish that have been vaccinated. In a recent trial where fish with ulcers were mixed with healthy fish, 15% of the fish suffered deep ulcers and 25% suffered superficial wounds. “The vaccine isn’t perfect. That is why we moved on,” Karlsen said.

In addition to looking at the effects of the different Moritella viscosa strains, the scientists have learned how the bacterium begins to harm the fish. “It sticks to the surface of the scales. It then multiplies and forms a colony,” said Karlsen.

The scales are not the outermost part of the salmon. “The scales are covered in mucus. The bacterium manages to get in between this layer of mucus and the scales. They can then create large wounds that can even reach muscle tissue,” Karlsen said.

When a fish is vaccinated, it has the first bacteria on its scales. In unvaccinated fish, the bacteria go deeper into the skin earlier. Luckily, there are now several vaccines against different Moritella viscosa strains on the market.

This research has been conducted in collaboration with the vaccine company Pharmaq, and financed by the Research Council of Norway.


Christian Karlsen, Elisabeth Ytteborg, Anette Furevik, Lene Sveen, Siv Tunheim, Sergey Afanasyev, Monica Gausdal Tingbø, Aleksei Krasnov. Moritella viscosa early infection and transcriptional responses of intraperitoneal vaccinated and unvaccinated Atlantic salmon. Aquaculture, Volume 572, 2023, 739531.