The Function of the Stress Hormone Cortisol in Disease Susceptibility of Channel Catfish, Ictalurus punctatus

The purpose of these studies was to examine the roles that the stress hormone cortisol plays in disease susceptibility of channel catfish, Ictalurus punctatus, to enteric septicemia of catfish (ESC). ESC is a serious disease of the channel catfish industry with annual losses in the millions. Few treatment options such as antibiotics and vaccines are available, but efficacy of treatments has proven to be limited. Deeper investigations into the physiological causes of disease in fish that accompany commercial production are needed to promote understanding of disease treatment and prevention. Disease challenges using Edwardsiella ictaluri, the pathogenic bacterium that causes ESC, were used to examine the effects of stress and the associated hormone cortisol on disease susceptibility of channel catfish. Channel catfish were subjected to confinement stress of various lengths of time by being placed into net baskets. Blood was collected from fish for the analysis of blood plasma cortisol concentrations. Stressed fish were then subjected to cultures of E. ictaluri along with nonstressed control fish. In some experiments, fish received additional treatments such as hormone injections to help explore the effects of stress on disease susceptibility. Fish were then monitored for 21 days for mortality. Results demonstrated that cortisol was highly correlated with increases in disease susceptibility to ESC and that cortisol was an accurate predictor of increased risk of mortality in both small and large channel catfish fingerlings. A combination of both stress and bacterial pathogen concentration affected disease susceptibility of channel catfish, although stress and the concurrent physiologically high concentrations of cortisol were more highly correlated with disease susceptibility than bacterial pathogen load. The synthetic hormone, dexamethasone (Dex), is capable of suppressing cortisol concentrations to baseline/control concentrations, and adrenocorticotropic hormone (ACTH) injections can create cortisol concentration stress responses similar to confinement stress. Although Dex is capable of suppressing cortisol, its use as a stress preventative, especially for the reduction of disease susceptibility, is unfounded based on this study. Evaluation of the effect Dex and ACTH have on blood plasma mineral concentrations revealed that there were no differences in concentrations of sodium, potassium, and chloride in any treatment