Efficacy of a live Cryptobia salmositica vaccine, and the mechanism of protection in vaccinated rainbow trout, Oncorhynchus mykiss, against cryptobiosis

An attenuated strain of Cryptobia salmositica was used as a live vaccine to protect rainbow trout, Oncorhynchus mykiss, against cryptobiosis. Fish immunized with 1.0 × 104 or 5.5 × 104 attenuated C. salmositica per fish were protected 3 weeks after immunization; however, this period was reduced to 2 weeks if fish were vaccinated with 1.0 × 105 attenuated parasites per fish. Fish were still protected at 6, 12 and 24 months after vaccination. Complement fixing antibodies in the blood of immunized fish lysed C. salmositica under in vitro conditions. The titre of complement fixing antibodies in vaccinated fish increased rapidly 1–2 weeks post-challenge. In vitro phagocytosis was enhanced by antiserum and by activated macrophages from vaccinated fish. There was also evidence of antibody independent and antibody dependent cell-mediated cytotoxicity in vaccinated fish.