Journal of Fish Diseases 1995, 18. 347-355 A comparison of the effect of silicone coated ascorbic acid and ascorbyl phosphate on the course of ichthyophthiriosis in rainbow trout, Oncorhynehus mykiss (Walbaum) T. WAHLI,'R.FRISCHKNECHT,'M. SCHMITT,'J. GABAUDAN,^ V. V E R L H A C ^ & W. M E I E R ' 'Fish Disease Laboratory, Institute of Animal Pathology, University of Berne. Berne, Switzerland, and ^CRNA, Societe Chimique Roche, St. Louis, France Abstract. 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') or ascorbyl phosphate (AP; 50/2000 mg AA equivalent kg feed"'). Fish infected with /. 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 ofthe general health status expressed as an improved nonspecific resistance and ability to cope with different stressors. In rainbow trout infected with /. 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. Introduction Vitamin C (ascorbic acid) is essential for the maintenance of many life processes. While domestic animals are able to synthesize vitamin C themselves (Homig, Glatthaar & Moser 1984), some vertebrate animals, including salmonid and ictalurid fish (Lall, Olivier, Weerakoon & Hines 1989) and also marine fish species (Saroglia, Scarano, Sciaraffia & Mazzatura 1991a, b), depend on the supply of this nutrient in the feed. F ish fed a vitamin-C deficient diet develop classic clinical signs of scurvy. This was described for rainbow trout, Oncorhynehus mykiss (Walbaum), (Kitamura, Ohara, Suwa, Nakagawa 1965; Halver, Ashley & Smith 1969; Meier & Wahli 1990), channel catfish, Ictaluruspunctatiis (Rafmesque), (Lovell 1973), coho salmon, Oncorhynehus kisutch (Walbaum), (Halver et al 1969) and sea bass, Dicentrarchus labrax L., (Saroglia et al. 1991b). Furthermore, vitamin C deficiency impairs wound healing (Ikeda 1992). In most cases, fish receiving supplemental dietary vitamin C were more resistant to experimental infections with viral, bacterial or parasitic pathogens than controls fed vitamin-C-deficient diets. Vitamin C significantly reduced mortality of rainbow trout challenged with the rhabdovirus causing infectious haematopoietic necrosis, IHN (Anggawati-Satyabudhy, Grant & Halver 1989). The mortality rate of catfish infected with Edwardsiella tarda or E. ictaluri was inversely coiTelated with the amount of vitamin C in the diet (Durve & Lovell 1982; Li & Lovell 1985). Similar results were obtained in rainbow trout challenged with Vibrio anguillarutn (Navarre & Halver 1989). In rainbow trout infected with the ciliate parasite Ichthyophthirius multifdiis, earlier experiments in our laboratory revealed a higher survival rate in fish fed high levels of vitamin C (Wahh, Streiff & Meier 1985; Wahh, Meier & Pfister 1986). ln contrast. Bell, Higgs & Traxler (1984) did not find any influence of dietary ascorbate, zinc and manganese on the development of bacterial kidney disease {Renibacterium salmoninarum) in sockeye Correspondence: Dr T. Wahli, Fish Disease Laboratory, Institute of Animal Pathology, University of Berne, Langgassstrasse 122, CH-3012 Berne, Switzerland. 347