Protective Efficacy of a Formalin-inactivated Vaccine against Viral Hemorrhagic Septicemia in Japanese Flounder Paralichthys olivaceus

Abstract

We examined the effectiveness of a formalin-inactivated vaccine against viral hemorrhagic septicemia (VHS) in Japanese flounder Paralichthys olivaceus by the pathogen challenge. Japanese flounder (average weight, 10 g) kept at 12°C, 20°C and 28°C were vaccinated by intraperitoneal injection of a formalin-inactivated VHS virus (VHSV) and maintained at each temperature for 21, 19 and 17 days post vaccination (dpv), respectively. The fish vaccinated at 20°C and 28°C were acclimated to 12°C, and then all vaccinated fish were challenged with VHSV at 12°C at 21 dpv. Fish that received the vaccine at 20°C showed relative percent survival (RPS) of 48%, compared with 0% for those given the vaccine at 12°C and 28°C. In other experiment, efficient protection against VHSV lasted at least 7 mo in Japanese flounder (average weight, 100 g) vaccinated at 20°C and maintained at ambient temperature, where high RPS values (71-100%) were obtained after viral challenges at 18, 77, 98 and 213 dpv. We obtained promising results that a single immunization with the formalin-inactivated vaccine at the physiologically optimal temperature (20°C) provided significant protective and long-lasting immunity to Japanese flounder against VHS.