Unlocking the Secrets of Salmon Skin Defense Against Winter Ulcers

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By Milthon Lujan

Images of Atlantic salmon skin 4 days post M. viscosa infection. Source: Karlsen et al., (2023), Aquaculture.
Images of Atlantic salmon skin 4 days post M. viscosa infection. Source: Karlsen et al., (2023), Aquaculture.

Winter ulcer outbreaks in farmed Atlantic salmon (Salmo salar) are common in the North Atlantic region, with numerous bacterial species isolated from fish with ulcers.

The skin of Atlantic salmon serves as the first line of defense against a formidable foe: Moritella viscosa, the bacterium responsible for the devastating winter ulcer. This disease causes skin lesions and death, resulting in significant losses in salmon aquaculture.


Scientists from Nofima, Pharmaq part of Zoetis, and M. Sechenov Institute of Evolutionary Physiology and Biochemistry have diligently sought effective means to combat this disease. Recent research sheds light on the intricate defense mechanisms of the skin and the potential of vaccination strategies.

Winter Ulcer

According to the study, outbreaks of ulcers below 8°C seawater temperature, known as classic winter ulcers, are often associated with an infection by the bacterium M. viscosa. The disease manifests with symptoms of superficial skin lesions that evolve into larger chronic skin ulcers and degenerative changes in the underlying muscle tissue, followed by terminal septicemia.

Vaccination: A Shield Against Moritella viscosa

The study reveals that vaccination offers a promising approach to protect salmon against Moritella viscosa. Vaccinated fish exhibited significantly lower mortality rates and skin ulceration compared to their non-vaccinated counterparts.

However, the degree of protection varied depending on the specific strain used in the vaccine formulation, highlighting the need for vaccines tailored to various strains.

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Even the vaccine cannot keep all fish healthy. Christian René Karlsen, senior scientist at Nofima, speaks of a high development of ulcers even in vaccinated fish. In a recent trial mixing fish with ulcers with healthy fish, 15% of the fish suffered deep ulcers, and 25% had superficial wounds.

“The vaccine is not perfect. That’s why we have moved forward,” he stated.

Battlefield: Skin Layers and Immune Response

Delving into the microscopic battlefield, researchers revealed that Moritella viscosa initiates its attack by colonizing the surface of the scales. To understand the salmon’s response, scientists analyzed the skin tissue of vaccinated and challenged fish.

Interestingly, the repertoire of antibodies and the migration of B cells from lymphoid organs to the skin were not affected by vaccination or bacterial challenge.


“It is the organ that first receives notice that the surrounding environment is changing. The skin feels all the environmental changes that affect the fish,” says Karlsen.

Skin Defense: Transcriptomic Insights

Using a powerful tool called microarray, researchers examined the transcriptional response of the skin to vaccination and bacterial challenge. The results revealed a complex symphony of changes in gene expression. The skin responded with:

  • Activation of essential structural genes to maintain its integrity.
  • Stimulation of metabolic pathways to fuel the defense response.
  • Regulation of developmental processes for optimal wound healing.

Interestingly, the immune responses triggered by dead Moritella viscosa (vaccine) and live bacteria were remarkably similar, suggesting that the skin’s initial response might be generic rather than pathogen-specific.

Hidden Battlefield: Skin Layers and Immune Evasion

By combining transcriptomic data with imaging analysis, researchers discovered striking differences in infected skin layers between vaccinated and non-vaccinated groups. In vaccinated salmon, Moritella viscosa seemed to be trapped within the epidermis, the outermost layer of the skin. Conversely, in non-vaccinated fish, bacteria managed to penetrate deeper into the dermis, the inner layer.

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In addition to observing the effects of different Moritella viscosa strains, scientists have learned how the bacterium begins to harm the fish. “It sticks to the scale’s surface. Then it multiplies and forms a colony,” says Karlsen.

Scales are not the outermost part of the salmon. “The scales are covered in mucus. The bacteria manage to get between this layer of mucus and the scales. Then they can create large wounds that can even reach the muscle tissue,” he explains.

Furthermore, the study revealed a potential immune evasion mechanism employed by Moritella viscosa. In the dermis of exposed fish, the expression of genes associated with lymphocytes and antiviral responses decreased. This suggests that bacteria might suppress the activity of immune cells in this crucial layer, allowing for deeper colonization.

Innate Humoral Response: A Swift Defense System

The research also suggests that intraperitoneal vaccination could stimulate the innate humoral response, a vital component of the immune system. This preactivation allows for a faster and more robust response in the dermis when Moritella viscosa attempts its invasion.

Conclusion: Toward Effective Protection


This innovative research provides valuable insights into the intricate battle between salmon and Moritella viscosa within the skin layers. Understanding skin defense mechanisms and immune responses paves the way for developing more effective vaccination strategies and disease management approaches.

“This study revealed that using the M. viscosa variant in the vaccination formula can enhance vaccination against winter ulcers. Specific antibody levels are unknown, and our Ig-seq could not show the effect of vaccination or exposure to M. viscosa on the antibody repertoire and B cell trafficking from lymphoid organs to the skin,” concluded the study.

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By unraveling the secrets of the salmon’s defense system, we can advance in the fight against winter ulcers and ensure the health of this species.

The study was funded by the Research Council of Norway through the CtrlAQUA SFI project.

Christian Karlsen
Osloveien 1, 1430 Aas, Norway
Email: Christian.karlsen@nofima.no

Reference (Open Access):
Christian Karlsen, Elisabeth Ytteborg, Anette Furevik, Lene Sveen, Siv Tunheim, Sergey Afanasyev, Monica Gausdal Tingbø, Aleksei Krasnov. 2023. Moritella viscosa early infection and transcriptional responses of intraperitoneal vaccinated and unvaccinated Atlantic salmon, Aquaculture, Volume 572, 2023, 739531, ISSN 0044-8486, Read the full article.

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