Cortisol can increase the susceptibility of brown trout, Salmo trutta L., to disease without reducing the white blood cell count

Chronic elevation of plasma cortisol levels in the brown trout by means of slow‐release, intraperitoneal implants increased the susceptibility of the fish to disease. Elevation from a mean basal level of 1–4ng ml −1 to ∼10ngml −1 for a period of 2–4 weeks was sufficient to increase the mortality rate due to furunculosis, Saprolegnia infection and bacterial fin‐rot. This level of plasma cortisol is well within that capable of being produced by the fish under conditions of chronic stress. The increase in susceptibility to disease was not accompanied by a reduction in the number of circulating lymphocytes and it is suggested that under certain conditions chronically‐elevated cortisol levels may be more sensitive, predictive indicators of reduced disease‐resistance than are changes in blood cell counts.