Effects of cortisol and stress on channel catfish (Ictalurus punctatus) pathogen susceptibility and lysozyme activity following exposure to Edwardsiella ictaluri

Periods of stress are often associated with disease outbreaks in cultured fish, and stress is often characterized by the secretion of cortisol. Although stress and cortisol secretion are highly correlated in fish, the role of cortisol in affecting channel catfish (Ictalurus punctatus) pathogen susceptibility is unclear. The effects of short-term stress and exogenous cortisol administration on channel catfish susceptibility to Edwardsiella ictaluri, the etiologic agent of enteric septicemia of catfish (ESC), were investigated. Channel catfish were exposed to virulent E. ictaluri following a standardized 30-min low-water stress or administration of dietary cortisol (100 mg/kg feed) and compared to a pathogen-challenged control group of catfish. Pathogen susceptibility increased in stressed catfish (43.3% mortality) when compared to cortisol-fed catfish (26.7%) and controls (26.7%). A greater (P < 0.05) percentage of stressed catfish (25.9%) tested positive for E. ictaluri relative to cortisol-fed catfish (13.0%) over the course of the study, however, average levels of circulating bacteria were not different (P > 0.05) among the treatments. Catfish challenged by the low-water stress event had elevated (P < 0.05) circulating levels of cortisol 1-day post-pathogen exposure and elevated (P < 0.05) lysozyme activity 4 and 14 days post-pathogen exposure when compared to cortisol-fed and control-challenged catfish. Cortisol concentrations were not correlated (P > 0.05) to either lysozyme activity or bacterial levels; however, lysozyme activity was positively correlated (P = 0.0197) to blood bacterial concentrations. These results implicate other stress factors or pathways, separate from or possibly in conjunction with cortisol, in the stress-associated immunosuppression of channel catfish as it relates to ESC susceptibility.