Submitted to: General and Comparative Endocrinology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/13/2004
Publication Date: 5/15/2005
Citation: Small, B.C., Bilodeau, A.L. 2005. Effects of cortisol and stress on channel catfish (Ictalurus punctatus) pathogen susceptibility and lysozyme activity following exposure to Edwardsiella ictaluri. General and Comparative Endocrinology 142:255-261. Interpretive Summary: Periods of stress in farm-raised catfish are often associated with disease outbreaks. Enteric septicemia of catfish (ESC) is the most prevalent disease affecting farm-raised catfish in the United States, and is responsible for as much as 50% of total losses to catfish farmers each year. The objective of this study was to determine whether cortisol, the primary stress hormone in catfish, is responsible for increased susceptibility of catfish to ESC. Unstressed catfish, stressed catfish, and cortisol-fed catfish were all exposed to Edwardsiella ictaluri, the bacterial agent causing ESC. Stressed catfish had higher mortality rates and associated disease factors, while cortisol administration, in the absence of stress, had no effect on ESC susceptibility. These results show that cortisol is not responsible for increased ESC susceptibility in catfish, and that other mechanisms should be investigated. Identifying the mechanisms by which stress increases susceptibility to ESC will lead to improved management techniques to reduce ESC outbreaks on commercial catfish operations.
Technical Abstract: 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 channel catfish (Ictalurus punctatus), the role of cortisol in the absence of stress in affecting channel catfish pathogen susceptibility is unclear. The effects of short-term stress and exogenous cortisol administration (in the absence of stress) 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) 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. These data suggest that cortisol, in the absence of stress, is not immunosuppressive relative to susceptibility of channel catfish to ESC.