THE EFFECT OF DIETARY VITAMIN-C ON THE IMMUNE-RESPONSE OF THE ATLANTIC SALMON (SALMO-SALAR L)

Atlantic salmon parr were provided with diets depleted of (0.05 g/kg) or supplemented with normal (0.31 g/kg) and high (2.75 g/kg) levels of vitamin C. After 26 weeks, endogenous liver levels in each dietary group correlated well with dietary vitamin C input. Serum protein levels and differential leucocyte Counts were unaffected by vitamin C nutrition. Of the non-specific cellular defences examined, respiratory burst (superoxide anion) activity and erythrophagocytosis were unaffected by dietary vitamin C levels. Serum complement activity, however, was significantly reduced in the depleted fish, and high-dose fish exhibited enhanced complement activity compared with levels in fish fed normal diets. Specific humoral responses after immunisation with either Aeromonas salmonicida or human gamma globulin, and lymphokine (MAF) production functioned independently of dietary vitamin C levels. On bath challenge with a virulent strain of A. salmonicida, vitamin C-depleted fish exhibited a significantly increased mortality rate when compared to the other dietary groups.