Immune Response and Resistance of Channel Catfish to Edwardsiella ictaluri Challenge when Fed Various Dietary Levels of Zinc Methionine and Zinc Sulfate

Abstract

The effects of dietary levels of zinc from zinc methionine (ZnM) and zinc sulfate ZnSO₄ on nonspecific immune responses and disease resistance of juvenile channel catfish Ictalurus punctatus (mean initial weight of 3.3 g) were determined. Fish in triplicate aquaria were fed to satiation twice daily for 16 weeks with an egg white basal diet without zinc supplementation or supplemented with 5, 20, and 60 mg zinc/kg as ZnM or 20 and 60 mg zinc/kg as ZnSO₄. Relative percentages of peripheral blood lymphocytes, thrombocytes, and macrophages were not affected by dietary zinc. The relative percentages of neutrophils significantly increased in fish fed 20 and 60 mg Zn/kg as ZnM. Chemotactic responses of macrophages were significantly higher for fish fed the three ZnM-supplemented diets and the 60-mg ZnSO₄. Dietary zinc, however, had no influence on the phagocytic activity of macrophages for Edwardsiella ictaluri. Likewise, neither the source nor the level of dietary zinc provided protection to channel catfish against E. ictaluri. The severity of E. ictaluri infection (number of colony-forming units/g trunk kidney) and the percentage offish that were culture negative for E. ictaluri (15 d postchallenge) were also unaffected by dietary zinc.