Response to<i>Edwardsiella ictaluri</i>Challenge by Channel Catfish Fed Organic and Inorganic Sources of Selenium

Abstract

Fingerlings of channel catfish Ictalurus punctatus (average initial weight, 1.70 g) were fed a casein-based purified diet supplemented with 0, 0.02, 0.06, 0.20, or 0.40 mg of selenium/ kg from sodium selenite (Na2SeO3), selenomethionine (Se-M), or selenoyeast (Se-Y) for 9 weeks. Subsequently, fish were challenged by immersion with a virulent strain of Edwardsiella ictaluri (105.9 cells/mL), and mortalities were recorded during a 14-d period. Antibody titers of surviving fish were measured, and macrophage chemotactic activity was determined in fish from each treatment. Dietary concentration and source of selenium significantly influenced weight gain and immune responses of the fish. Dietary selenium concentrations for maximum weight gain were 0.20 mg/kg from Se-M and Se-Y and 0.40 mg/kg from Na2SeO3. Dietary selenium concentration for maximum survival from E. ictaluri challenge was 0.20 mg/kg for fish fed Se-M and 0.40 mg/kg for fish fed Se-Y and Na2SeO3. Antibody production generally increased as dietary concentration of selenium increased. Antibody titer was highest for fish fed Se-Y, intermediate for fish fed Se-M, and lowest for fish fed Na2SeO3. Macrophage chemotactic response was the same for fish fed the control (0 selenium) and Na2SeO3 diets and was significantly lower than responses by fish fed Se-M and Se-Y. This study indicates that channel catfish challenged with E. ictaluri are responsive to dietary selenium and that organic selenium sources have greater potency than inorganic sources.