Ž .
of vitamin C was obtained only when high level 3000 mgrkg was used. Neither dietary levels of iron nor vitamin C or their interaction influenced survival of juvenile channel catfish against E.
ictaluri 14-day post challenge. However, the onset of mortality was earlier for fish fed the iron-deficient diet. q 2000 Published by Elsevier Science B.V.
Keywords: Iron; Ascorbic acid; Hematology; Immune response; Disease resistance
1. Introduction
Iron is an essential element for the functioning of organs and tissues of higher animals, including fish, because of its important role in oxygen transport and cellular
respiration. Iron is also one of the most important micronutrients in terms of its effect on Ž
immune system functions and host defense against infections Beisel, 1982; Bhaskaram, .
1988 . Fish can absorb soluble iron from the water across the gill membrane and Ž
. intestinal mucosa Roedar and Roedar, 1968 . However, feed is considered the major
Ž source of iron for fish due to low concentrations of soluble iron in natural waters NRC,
. 1993 . The total dietary iron requirement for optimum growth, feed efficiency, hemato-
logical values and immune response of juvenile channel catfish has been determined to Ž
. be about 30 mgrkg diet Gatlin and Wilson, 1986; Lim et al., 1996; Sealey et al., 1997 .
Chemotactic response of peritoneal macrophages to Edwardsiella ictaluri exoantigen Ž
was depressed in iron-deficient channel catfish Lim and Klesius, 1997; Sealey et al., .
1997 but this abnormality was remedied by feeding an iron-replete diet for 4 weeks Ž
. Lim and Klesius, 1997 . The effect of dietary iron on resistance of channel catfish to
Ž .
bacterial pathogens is unclear, although Sealey et al. 1997 suggested that high level of Ž
. dietary iron 180 mgrkg may lead to increased susceptibility.
Ascorbic acid or vitamin C is essential for most fishes, including channel catfish Ž
. NRC, 1993 . The requirement varies depending on the metabolic function and form of
vitamin C used. Using ascorbic acid as the dietary source, a level of 30 mgrdiet was adequate for optimum growth, but 60 mgrkg was needed for prevention of all
Ž deficiency signs Andrews and Murai, 1975; Lim and Lovell, 1978; Li and Lovell,
. Ž
. 1985 . When a stable form of vitamin C
L
-ascorbyl-2-polyphosphate was used, 11 to Ž
45 mgrkg diet was adequate for growth and prevention of deficiency signs El Naggar .
and Lovell, 1991; Robinson, 1992 . Evidence of the role of ascorbic acid in fish immunity and disease resistance is not consistent, although numerous studies have
shown that feeding fish ascorbic acid at levels higher than those required for growth and prevention of deficiency signs enhanced their immune response and resistance to
bacterial challenges.
Ascorbic acid is known to be involved in the metabolism of iron in animals including Ž
. fish Harper, 1975; Monsen, 1982; Hilton, 1989; NRC, 1993 . Ascorbic acid enhances
the absorption of iron from the intestine by reducing ferric iron to the ferrous state, a Ž
more soluble form that is readily absorbed El-Alwary et al., 1975; Harper, 1975; .
Ž .
Monsen, 1982 . Ascorbic acid is also involved with adenosine triphosphate ATP in the release and reduction of the ferric iron from ferritin and its subsequent incorporation
Ž with iron-binding protein, apoferritin and transferrin, into tissue ferritin Mazur et al.,
. 1960; Harper, 1975 .
No studies have been conducted on the combined influence of dietary levels of iron and ascorbic acid on the response of channel catfish. Thus, this study was conducted to
evaluate the interaction between dietary levels of iron and ascorbic acid on growth, hematology, immune response and resistance of channel catfish to E. ictaluri challenge.
2. Materials and methods
