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Trout immune response 'may be better during the day'

Rainbow trout were seen to have a stronger immune response during the day.
Rainbow trout were seen to have a stronger immune response during the day.

Scientists have discovered that there could be an increase in the immune competence of rainbow trout during the day, compared to the night, something that would be mainly mediated by myeloid cells.


Studies have provided evidence of a connection between circadian rhythms and the immune system, as several key parameters have been described as being dependent on the time of day.

Some parameters include: the number of circulating hematopoietic cells and leukocytes; the level of hormones and cytokines; phagocytic activity; cytotoxic activity of natural killer cells; antigen presentation, proliferation, and humoral immune responses in lymphocytes. In addition, circadian rhythms have been shown to influence susceptibility to infection and the course of disease.

Although most of these findings were described in mammals, researchers from Germany and the Czech Republic conducted a study to elucidate whether this also occurs in rainbow trout.

After studying the cellular composition and phagocytic activity of circulating leukocytes and in the head kidney, the researchers evaluated the ability of fish to respond to a bacterial stimulus, as well as changes in serum antimicrobial activity.

Increase in immune competition

“Our results suggest an increase in immune competition during the day, which is manifested by a greater presence of myeloid cells in the circulation; increased general phagocytic activity; and greater capacity of the sera to inhibit the growth of Aeromonas salmonicida”, they pointed out.

Their results showed that myeloid cells were the main population influenced by the time of day, while B cells and thrombocytes did not vary significantly.

“Although we observed the highest number of myeloid cells in the blood during the early hours of the morning, we evidenced an inverse trend in the head kidney, which suggests that the myeloid cells go to the deposit niches with the onset of the dark phase,” stated the authors regarding the distribution of cells during the day.

Both results, according to the scientists, could be correlated with food absorption and increased activity of fish during the day. However, they were unable to determine the exact mechanisms influencing these differences and how they can be manipulated with artificial changes in daylight, feeding times, or possible infections.

In conclusion, they pointed out that their data suggest a connection between diurnal rhythms and the immune response of rainbow trout, highlighting “the relevance of rhythmicity and its influence on experimental work in the field of fish immunology”.

Read the full study titled “Dawn to Dusk: Diurnal Rhythm of the Immune Response in Rainbow Trout (Oncorhynchus Mykiss)” here.