Effects of cortisol and stress on the immune system in Atlantic salmon (Salmo salar L)

A suppressive effect of cortisol on LPS mitogenesis was demonstrated on peripheral blood cells from Atlantic salmon (Salmo salar L.) in vitro, and the suppression was dose and time dependent. When cortisol was injected intraperitoneally into fish, there was a rapid increase in the level of cortisol in the blood which was, however, effectively cleared from the circulation within a few hours. The number of cells in the various populations of blood leucocytes was counted after cortisol administration demonstrating a marked increase in the relative number of thrombocytes, while the number of granulocytes, monocytes and lymphocytes was relatively constant. Qualitatively, however, the lymphocytes seemed to be affected by cortisol, since the number of Ig positive cells were dramatically downregulated during the first days after cortisol injection. A significant reduction in the mitogenic response of blood lymphocytes to LPS was also observed and the response gradually returned to normal levels over the next four days. When Atlantic salmon were ex:posed to repeated handling stress, there was no significant difference in the total immunological status compared to non-stressed fish as judged by antibody production and protection against Aeromonas salmonicida infection. (C) 1996 Academic Press Limited