Linking Stress to the Increased Susceptibility of Channel Catfish to Enteric Septicemia Using Cortisol

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Abstract
Although stress is known to increase the susceptibility of channel catfish to disease, this has not been correlated to a clinical measure of stress like plasma cortisol. By exposing healthy, unstressed catfish fingerlings (70–85 mm) to six concentrations of Edwardsiella ictaluri, we determined that the dose at which 30% of the fingerlings would die (LD30) was 1.4 log colony‐forming units (CFU) per liter. Twenty‐five small fingerling catfish per tank were confined to 12.5‐cm × 15‐cm × 10‐cm plastic nets for 30 or 60 min, while unstressed fish remained free‐swimming. Blood samples were taken for cortisol analysis, and the LD30 dose of the pathogen was administered at the conclusion of the stress period. Unstressed fish were sampled and challenged similarly. Infection rates were monitored for 21 d. A second challenge was conducted with larger fingerlings (114–168 mm) in the same manner. In the experiment with small fingerlings, cortisol concentrations (mean ± SD) for unstressed fish (23.6 ± 6.02 ng/mL), fish stressed 30 min (77.7 ± 19.16 ng/mL), and fish stressed 60 min (115.4 ± 26.65 ng/mL) differed significantly (P < 0.05) from each other. Significant increases in mortality occurred in conjunction with increases in cortisol (unstressed: 22.5 ± 4.2%, 30 min: 47.5 ± 5.2%, 60 min: 81.7 ± 6.8%). For large fingerlings, cortisol concentrations also differed between unstressed fish (31.44 ± 17.9 ng/mL) and stressed fish (30 min: 103.1 ± 20.7 ng/mL, 60 min: 109.6 ± 24.9 ng/mL). Although cortisol did not differ notably between the 30‐ and 60‐min stress groups, mortalities in the large fingerlings increased significantly with level of stress (unstressed: 25.8 ± 7.4%, 30 min: 62.5 ± 10.4%, 60 min: 100 ± 0.0%). This study demonstrates the relationship between cortisol and the increased susceptibility of channel catfish to enteric septicemia of catfish.