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Why stressed salmon are more vulnerable to disease

A new study shows how stress affects the blood cells of farmed salmon, making them less resistant to disease.

Two female researchers in lab coats examine graphs on a computer screen.
"If we can better understand the effects of stress and measure them using a simple blood test, we can prevent stress-related problems and ensure that the fish are more resilient against disease," says researcher Maria K. Dahle (on the right).
Mari Press Communications Adviser
Presented by: Presented by: Presented by: Norwegian Veterinary Institute
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Farmed salmon do not live a calm and tranquil life. Handling, transport, mechanical delousing, and vaccination expose the fish to prolonged stress. 

When stress becomes chronic, levels of the stress hormone cortisol remain high in the blood. New research now shows that this hormone changes the way the fish’s blood cells function.

“The study has shown us that stress weakens the blood cells’ ability to fight viruses," says Maria K. Dahle, a researcher at the Norwegian Veterinary Institute.

Defence genes are switched off

When cells are exposed to stress hormones, key defence genes are switched off. 

"This means that a stressed fish may become more susceptible to disease – possibly long after the cortisol level in the blood has returned to normal,” says Dahle.

The research team investigated how cortisol and a strong activator of the cortisol receptor, dexamethasone, affect genes in salmon red blood cells. 

It turns out that these cells, whose main role is to transport oxygen and help signal the presence of a viral infection, also respond strongly to stress hormones. 

Female researcher works at a lab bench with tubes and equipment.
Thomais Tsoulia measures cortisol levels in blood plasma of Atlantic salmon.

Some genes showed more than 100 times higher activity

More than 150 genes were altered in the cells, and some showed more than 100 times higher activity than normal.

One gene, fkbp5, remained highly active for a full two weeks after the cells were exposed to stress hormones. This makes it a strong candidate as a biomarker – a kind of ‘stress indicator’ in the fish. 

Another gene, ddit4, was also strongly activated and appeared to be an even better indicator of stress when the researchers tested blood from live fish with elevated cortisol levels. 

Going forward, the researchers aim to explore the long-term consequences of stress for the fish.

Early stress detection can improve animal welfare

The aquaculture industry loses large sums of money due to disease and mortality. 

"If we can better understand the effects of stress and measure them using a simple blood test, we can prevent stress-related problems and ensure that the fish are more resilient against disease," says Dahle.

This will contribute to more sustainable production and improved animal welfare.

“We also aim to continue researching the longer-term consequences of stress in fish. We're currently working on an additional scientific publication on the effects of acute stress on the red blood cells of farmed Atlantic salmon,” the researcher says.

Reference:

Tsoulia et al. Effects of glucocorticoid receptor activation on gene expression and antiviral responses in Atlantic salmon (Salmo salar L.) red blood cells, Veterinary Research, vol. 56, 2025. DOI: 10.1186/s13567-025-01605-w

About the research

Maria K. Dahle is the project leader of RED FLAG: Salmonid red blood cells – sensors of stress and infection, a project funded by the Research Council of Norway.

The research formed part of the PhD work of Thomais Tsoulia at the Norwegian Veterinary Institute.

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