Motohiko Sano

The live attenuated vaccine P7‐P8 strain against herpesviral haematopoietic necrosis, which is caused by cyprinid herpesvirus 2 ( CyHV ‐2), exhibits high protective efficacy in goldfish at 25°C, the predominant temperature for this disease; however, the effect of water temperature during the vaccination period on efficacy has not been determined. In this study, an in vitro experiment revealed that the vaccine strain grew between 15 and 30°C in the goldfish cell line RyuF ‐2. Subsequent in vivo efficacy tests were conducted with vaccination temperatures ranging from 15 to 30°C. During the vaccination period, organs were sampled to determine the vaccine growth dynamics. Blood plasma was collected to assess anti‐CyHV ‐2 antibody titres. The protective efficacy of the vaccine at 15, 20, 25, and 30°C after subsequent virulent CyHV ‐2 challenge resulted in a relative percentage survival of 73.3%, 77.8%, 100%, and 77.8%, respectively, which indicated that the vaccine is effective over this temperature range. The vaccine virus load in the spleen was lowest at 15°C (10 3.7 DNA copies/mg) and highest at 25°C (10 6.5 DNA copies/mg). This indicates that the vaccine virus load over 10 4 DNA copies/mg may elicit sufficient acquired immunity. No significant differences in antibody titre were observed between groups, which suggests that cell‐mediated immunity can be fundamentally involved in protection.