The salmon's skin cells have many special functions. They are migratory cells that can quickly cover small wounds. They can also absorb bacteria and destroy them through a process called phagocytosis.
Mucous cells produce a special mucous with antimicrobial and mechanical properties that covers the skin and acts as a protective barrier against both infection agents and mechanical wear and tear.
The salmon's skin is covered by a layer of hard shell. This acts as armour and is also an important mineral store for the fish in the same way as the skeleton is.
If the skin barrier is ruptured, the immune cells deep down in the skin will get ready to attack any infection agents. The immune status of the fish will be crucial with respect to how many immune cells there are and how effective they are.
Over recent years, mechanical damage to the skin has become more widespread.
This can partially be explained by the increased use of non-medicinal mechanical methods to control salmon lice, which means that the fish have to be pumped from the cages and into well-boats for treatment.
In addition, there has been increased wear and tear to the skin and mucous layer due to the use of hydrogen peroxide as a delousing agent. If these treatments are performed at low temperatures, this can increase the chances for the development of wounds as all the biological processes in salmon, including wound healing, occur more slowly at low temperatures. Mechanical wounds can act as entrance points for bacterial wound infections.
Bacterial wound infections
The salmon's skin is exposed to bacterial infections that can lead to very serious wounds.
Different bacteria are associated with wound problems, and both the classic winter bacterium Moritella Viscosa and the bacteria Tenacibaculum ssp and Aliivibrio Wodanis are frequently isolated from wound infections in salmonids. These are often observed together in wounds, and there is a lot of evidence that the composition will be key for the degree of severity. Which problems arise vary to such a degree that we talk about different "syndromes", classic winter wounds, and atypical bacteria wounds.