A COMPARISON OF THE EFFECT OF SILICONE COATED ASCORBIC-ACID AND ASCORBYL PHOSPHATE ON THE COURSE OF ICHTHYOPHTHIRIOSIS IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS (WALBAUM)

The aim of the present study was to compare the effect of high and low doses of different vitamin C formulations on the course of ichthyophthiriosis (Ichthyophthirius multifiliis infection) in rainbow trout. After a depletion period, trout fingerlings were fed diets deficient in vitamin C(AA 0), or containing different levels of silicone coated ascorbic acid (AA; 50/200 mg kg feed(-1)) or ascorbyl phosphate (AP; 50/2000 mg AA equivalent kg feed(-1)). Fish infected with I. multifiliis demonstrated a significant reduction in mortality when fed high levels of AA or AP.A correlation between the parasite burden and the vitamin C content in the diet was not found. Specific immunity-measured as immobilization titre of sera-was not influenced by the level or the formulation of vitamin C in the diet. The most probable mechanism of high level doses of vitamin C is an increase of the general health status expressed as an improved nonspecific resistance and ability to cope with different stressors. In rainbow trout infected with I. multifiliis, the beneficial effect of ascorbyl phosphate is equal to that of silicone coated ascorbic acid, and therefore, the more stable AP may be a valuable alternative for use in fish diets.