Water temperature and immunization period required to establish immunity against the viral hemorrhagic septicemia virus vaccine in olive flounder (Paralichthys olivaceus).

Water temperature is a crucial factor for adaptive immune responses in fish vaccinology, wherein the immune effects of vaccines are time × temperature (degree-days) dependent i.e. the higher the water temperature, the faster the fish develop protective immunity in response to the vaccine. In contrast, at low water temperature there happened to be reduced or no protective efficacy of vaccines as reported by several studies. Previously, we developed a squalene and aluminium hydroxide (Sq + Al) – based inactivated viral hemorrhagic septicemia virus (VHSV) vaccine, which showed good promises as a potential immunoprotective agent against VHSV infection in olive flounder (Paralichthys olivaceus) with >50% relative percent survival (RPS) at 3 to 40 weeks post-vaccination. In continuation, the present study investigated the role of the ambient water temperatures (at 10, 13, 15, and 20 °C) and immunization periods (10, 20, 30, and 40 days post-vaccination) on the development of protective efficacy of the developed (Sq + Al) – based vaccine in olive flounder using three in vivo experimental trials. The results showed fish immunized at 20 °C obtained significantly high protection starting at 10 dpv (200 degree-days) with a RPS of 86.67% (trial I), 66.67% (trial II), and 90.89% (trial III). Challenge tests at 20, 30, and 40 dpv also conferred a high level of protection when the fish were immunized at 20 °C. Moreover, immunization at 15 and 13 °C showed the best results, the RPS at 20, 30, and 40 dpv were 88.07–90.96% at 15 °C and 87.89–72.84% at 13 °C for trial I and II, respectively. Interestingly, olive flounder immunized at 10 °C also exhibited 20.00–80.02% RPS at 10–40 dpv, however in relative terms the protection was lower and slower as compared to immunization at 13, 15, and 20 °C. In addition, the study also displayed significant upregulation of various immune genes and higher specific antibody titers in the fish immunized both at 10 °C as well as 20 °C (trial III). Further, the vaccine groups showed comparatively high protection when 300 degree-days were reached in the respective groups irrespective of the temperature at which immunization was carried out. Thus, the present study indicates useful guidelines for sufficient time requirement of the (Sq + Al) – based inactivated VHSV vaccine to show its full protective efficacy against the virus in olive flounder. • Adjuvanted VHSV vaccine conferred protection at 300 degree days over 10–20 °C. • It provided significantly high protection from 10 dpv at 20 °C (200 degree days). • Vaccine efficacy increased with antibody titer at suboptimal water temperature (10 °C). [ABSTRACT FROM AUTHOR]