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 o n peripheral blood cells from Atlantic salmon (Salmo salar L.) in vitr o, and the suppression was dose and time dependent. When cortisol was injected intraperitoneally into fish, there was a rapid increase in th e level of cortisol in the blood which was, however, effectively clear ed from the circulation within a few hours. The number of cells in the various populations of blood leucocytes was counted after cortisol ad ministration demonstrating a marked increase in the relative number of thrombocytes, while the number of granulocytes, monocytes and lymphoc ytes was relatively constant. Qualitatively, however, the lymphocytes seemed to be affected by cortisol, since the number of Ig positive cel ls were dramatically downregulated during the first days after cortiso l injection. A significant reduction in the mitogenic response of bloo d lymphocytes to LPS was also observed and the response gradually retu rned to normal levels over the next four days. When Atlantic salmon we re ex:posed to repeated handling stress, there was no significant diff erence in the total immunological status compared to non-stressed fish as judged by antibody production and protection against Aeromonas sal monicida infection. (C) 1996 Academic Press Limited