Impaired Neutrophil Defense Activities and Increased Susceptibility to Edwardsiellosis by Cortisol Implantation in Tilapia.

Abstract

In order to reveal the effect of cortisol on tilapia neutrophil defense activities in vivo, fish were given intraperitoneal injections of cortisol mixed with coconut oil as an “implant” (5 mg/0.5 mL/fish). Fish were injected with Escherichia coli into their swim bladders before 24 h prior to collecting neutrophils, which were then assessed for their chemotactic, phagocytic and respiratory burst activities. In addition, concentrations of plasma cortisol and glucose were measured. Cortisol-implanted fish showed considerably higher levels of cortisol and their neutrophil activities were significantly suppressed compared to the control group at 1 day post-implantation. Both groups of fish were challenged with Edwardsiella tarda (1 mg/fish). Cortisol-implanted fish started to die 6 days after the challenge, whereas no deaths were observed in the controls. After the challenge experiment, E. tarda was recovered from more than half of the cortisol-treated fish group and from the only one of the controls. Results from this study have demonstrated that elevated levels of cortisol in fish bodies, similar to that observed during stressful experiences, decrease neutrophil activities and increase susceptibility to bacterial pathogens.