Inactivated vaccine against viral hemorrhagic septicemia (VHS) emulsified with squalene and aluminum hydroxide adjuvant provides long term protection in olive flounder (Paralichthys olivaceus)

Abstract

Viral hemorrhagic septicemia (VHS) in olive flounder (Paralichthys olivaceus) remains an unsolved health problem in Korean aquaculture. Vaccination plays a significant role in modern aquaculture, and the duration of protection provided is of vital importance. Here, we have demonstrated the efficacy, duration of protection and safety of an inactivated vaccine emulsified with squalene (5%) and aluminum hydroxide (0.5%). The inactivated VHS vaccine provided a moderate protection of 37% and 47% relative percent survival (RPS) at 4 and 10 weeks post vaccination (wpv). Addition of squalene and aluminum hydroxide into inactivated VHS vaccine clearly enhanced the level of protection showing 58% and 83% RPS at 4 and 10wpv, respectively, indicating the need for adjuvants to enhance the efficacy. The vaccinated fish showed significant protection at 3, 6, 12, 18, 24, and 40wpv (except week 57) than non-vaccinated fish to an intraperitoneal challenge of 107.1TCID50/fish at 15°C, with RPS of 60%, 64%, 71%, 55%, 52% and 50% (45% at 57 week), respectively, covering the duration of natural outbreak. Fish challenged at 18wpv at 6°C showed 56% RPS and protection at a low temperature. The antibody titer was high at 3wpv with an OD of 1.08±0.13, but decreased gradually and was undetectable by 24wpv. The vaccine formulation was safe without injection site reactions, adhesions, or pigmentation observed at 6, 12, 18, or 24wpv. Inflammatory reactions were observed in the spleen intestine at 6 and 12wpv but were similar as control by 24wpv. These results confirm that this vaccine is efficient and safe for olive flounder and could offer an appropriate strategy to prevent VHS without causing side effects.