Photo: NOFIMA
NOFIMA: Understanding salmon skin
(NORWAY, 12/15/2023)
Last winter, many farmed salmon died of winter ulcers. Now the researchers have new knowledge about the skin of the salmon and vaccines, which may be useful facing the coming winter.
The result is improved vaccination against ulcers.
“We have documented what everyone thought,” says Senior Scientist Christian René Karlsen. He is talking about salmon that get ulcers on their skin caused by bacteria.
Skin first
Nofima scientists have been concerned about salmon skin for a long time. “It is the organ that first receives a notice that the surrounding environment is changing. The skin feels all the environmental changes that affect the fish,” says Karlsen.
Salmon can get ulcers in cold seawater. A bacterium called Moritella viscosa causes these winter ulcers.
It is possible to vaccinate against these winter ulcer bacteria, but the problem is that the bacterium has many different strains. The vaccine works much better if you develop it using the correct strain of bacteria.
<-- AB/PAS histological staining of Atlantic salmon skin from (A) fish in the V-Mv- group, which showed a normal skin morphology. The enlarged box in (B) shows a thin layer of epidermis covering the tip of the scale (arrow).
Improved vaccine
Not even the vaccine can keep all the fish healthy. Karlsen talks about 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 have moved on,” he says.
(C) Fish in the V+Mv- group showed a similar skin morphology to the V-Mv- group, but with more mucous cells (hollow arrows) in the outer epidermis. (D) Bacterially challenged fish in both the unvaccinated and vaccinated groups (V-Mv+/V+Mv+) showed thickening of the epidermis around the tip of the scales (arrow), purple mucous cells (hollow arrow), and curving of the scales. -->
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,” says 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,” he says.
When a fish is vaccinated, it has the first bacteria on its scales. In unvaccinated fish, the bacteria go deeper into the skin earlier. There are now several vaccines against different Moritella viscosa bacteria on the market.
The research has been conucted in collaboration with the vaccine company Pharmaq, and financed by the Research Council of Norway.
Source: NOFIMA
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