EFFICACY OF A LIVE CRYPTOBIA-SALMOSITICA VACCINE, AND THE MECHANISM OF PROTECTION IN VACCINATED RAINBOW-TROUT, ONCORHYNCHUS-MYKISS, AGAINSTCRYPTOBIOSIS

An attenuated strain of Cryptobia salmositica was used as a live vacci ne to protect rainbow trout, Oncorhynchus mykiss, against cryptobiosis . Fish immunized with 1.0 X 10(4) or 5.5 X 10(4) attenuated C. salmosi tica per fish were protected 3 weeks after immunization; however, this period was reduced to 2 weeks if fish were vaccinated with 1.0 X 10(5 ) attenuated parasites per fish. Fish were still protected at 6, 12 an d 24 months after vaccination. Complement fixing antibodies in the blo od of immunized fish lysed C. salmositica under in vitro conditions. T he titre of complement fixing antibodies in vaccinated fish increased rapidly 1-2 weeks post-challenge. In vitro phagocytosis was enhanced b y antiserum and by activated macrophages from vaccinated fish. There w as also evidence of antibody independent and antibody dependent cell-m ediated cytotoxicity in vaccinated fish.