An Avirulent Largemouth Bass Birnavirus Vaccine Candidate Protects Largemouth Bass against Birnavirus Infection

Background: Largemouth bass birnavirus (LBBV) disease outbreaks in largemouth bass fingerlings lead to high mortality in China. Therefore, the development of immersion immunization strategies is paramount. Methods: An avirulent LBBV strain was screened using a fish challenge assay. The proliferation dynamics of the avirulent strain were determined in vitro and in vivo. The efficacy of the avirulent vaccine was evaluated using immune gene expression, viral load, and a virus challenge, and the safety was also assessed using a reversion to virulence test. Results: An avirulent virus strain, designated as largemouth bass birnavirus Guangdong Sanshui (LBBV-GDSS-20180701), was selected from five fish birnavirus isolates. The proliferation peak titer was 109.01 TCID50/mL at 24 hpi in CPB cells and the peak viral load was 2.5 x 104 copies/mg at 4 dpi in the head kidneys and spleens of largemouth bass. The largemouth bass that were immersed within an avirulent vaccine or injected with an inactivated vaccine were protected from the virulent LBBV challenge with a relative percent survival (RPS) of 75% or 42.9%, respectively. The expression levels of IL-12, MHCI, MHCII, CD8, CD4, and IgM in the avirulent group were significantly upregulated at a partial time point compared to the inactivated vaccine group. Moreover, the viral load in the avirulent vaccine group was significantly lower than those in the inactivated vaccine group and control group using real-time PCR. Conclusions: LBBV-GDSS-20180701 is a potential live vaccine candidate against LBBV disease.