Aquaculture and Fisheries Management 1994, 25, 21-36 Influence of vitamin C on the immune system of salmonids V. VERLHAC & J. GABAUDAN F. Hoffmann-La Roche Ltd, Vitamin and Fine Chemical Research, Basel, Switzerland Abstract. Trials were run to determine the immunomodulatory effect of dietary vitamin C in rainbow trout. Oncorhynchus mykiss (Walbaum), and Atlanticsalmon,.ya/mo5a/or L. We first investigated whether depletion of ascorbic acid (AA) could depress the immune system. Subsequently, we focused our studies on the stimulatory effect of a high dietary dose of stable vitamin C derivatives (lOOOmg AA-equivalent per kg of feed) in order to determine their prophylactic role. Three trials were run with rainbow trout fed, for periods of time varying from 2 to 6 months, a high dietary dose of ascorbate-2-polyphosphate (AAPP) or L-ascorbate-2-monophosphate calcium salt (AAMP) equivalent to lOOOmg AA per kg of feed. The high dietary level of vitamin C was compared to 60mg AA-equivalent per kg of feed. Another experiment was conducted with Atlantic salmon pre-fed AAPP at 60mg/kg for 6 months followed by AAPP at lOOOmg/kg for 2 months. Various immune parameters were tested such as mitogen-induced proliferation, serum complement level, phagocytosis and natural cytotoxicity. Leucocyte and plasma AA contents were also determined. The immune response in vitamin-C deficient trout was not strongly different from that in trout fed the minimal growth requirement level. Serum complement level was not affected by vitamin C. A high dietary dose of vitamin C was found to increase trout and salmon leucocyte A A content, mitogen-induced proliferation of lymphocytes, and chemiluminescence response of leucocytes with a higher constancy, when the cells were stimulated with opsonized zymosan and with luminol as a substrate. Natural cytotoxicity was enhanced in trout fed a high dietary dose of vitamin C. The results of these immunity studies support supplementation levels of vitamin C which are much higher than the minimal requirement known to prevent deficiency symptoms and promote normal growth. Further investigations on the time-dose response are needed to define quantitatively the high dietary dose of vitamin C. Introduction The immune system plays a critical role in the protection of organisms against infectious diseases, which are major causes of mortality in fish culture, where the spread of disease is extremely rapid. Therapeutic means such as antibiotics exist and are efficient, but their use in fish encounters environmental problems. Although vaccines are available in aquaculture, some infectious diseases still cause heavy losses. Some parasitical infections remain difficult to treat. A possibility to reduce the incidence of high mortality would be a prophylactic treatment giving animals an improved immune status, thus enabling them to better resist diseases. Besides its essentiality for growth, vitamin C has been demonstrated to play an important role in the functioning of the immune system when supplied at dietary levels higher than standard ones. This is consistent with the high ascorbate concentration found in leucocytes and its rapid consumption during infection and phagocytosis. Vitamin C seems to be Correspondence: V. Verlhac, F. Hoffmann-La Roche Ltd, Vitamin and Fine Chemical Research, CH-4002 Basel. Switzerland. 21