Influence of the Dietary Level of Iron from Iron Methionine and Iron Sulfate on Immune Response and Resistance of Channel Catfish to Edwardsiella ictaluri

Channel catfish Ictalurus punctatus fingerlings were fed purified diets supplemented with iron at levels of 0, 20, 60, and 180 mgkg from iron sulfate (FeS) or 5, 10, 20, 60, and 180 mgkg from iron methionine (FeM) in triplicate tanks for 8 wk. Fish were then divided into two groups and subjected to different assays to measure disease resistance and individual immune functions. Representative fish from each dietary treatment were challenged by bacterial immersion with virulent Edwardsiella ictaluri, and mortality due to enteric septicemia was recorded. Other fish were immunized with 0.2-mL formalin-killed E. ictaluri and boosted 21 d post-immunization. Antibody response was determined by FAST-ELISA. Chemiluminescent and chemotaxis assays were performed using peritoneal macrophages. Supplementation of the diet with various levels of iron from FeS or FeM did not significantly affect antibody production. Chemotactic migration by macrophages was depressed in iron-deficient fish and a level of 60 mgkg from either FeS or FeM provided the highest chemotactic indexes. A deficiency of dietary iron was found to increase mortality of channel caffish due to enteric septicemia of catfish (ESC). However, more studies should be conducted to better understand the effects of sources and levels of dietary iron on immune responses and disease resistance in channel caffish. Culture of channel catfish Ictalurus punctatus resulted in gross farm revenues of 326 million dollars in 1993 (Aquaculture Situation and Outlook 1994). An additional 2030 million dollars is estimated to be lost annually due to disease problems caused by Edwardsiella icfaluri (Plumb and Vinitnantharat 1993). Edwardsiella ictaluri was described by Hawke (1979) as the causative agent of the disease, enteric septicemia of catfish (ESC). Current methods of decreasing loss due to bacterial pathogens are limited to costly vaccines and antibacterial compounds administered through medicated feed. A more effective and economical method of control is needed to reduce losses. The National Research Council (NRC) summarized the known minimum dietary requirements for most essential nutrients for maximum performance of channel catfish I Corresponding author. (NRC 1993). However, studies have shown that increasing levels of some nutrients above minimum requirements improved immune responses and disease resistance in channel catfish. Durve and Love11 (1982) showed that increased levels of vitamin C reduced mortalities of channel catfish infected with E. fardu. Wise et al. (1993) showed that levels of vitamin E above the established minimum requirement improved phagocytosis of E. ictaluri. An improved understanding of the relationship between level and source of iron, and the immune response of channel catfish against E. ictaluri could provide a more effective method of decreasing losses due to ESC. The minimum dietary iron requirement for optimum growth, feed efficiency, and blood values in non-diseased channel catfish has been determined to be 30-mg Fekg in purified diets (NRC 1993). Iron deficiency in channel catfish has been characterized by suppressed hematocrit, hemoglobin, Q Copyright by the World Aquaculture Society 1997