Efficacy of heat-killed and formalin-killed vaccines againstTilapia tilapinevirusin juvenile Nile tilapia (Oreochromis niloticus)

AbstractTilapia tilapinevirus(also known as tilapia lake virus, TiLV) is considered to be a new threat to the global tilapia industry. The objective of this study was to develop simple cell culture-based heat-killed (HKV) and formalin-killed (FKV) vaccines for the prevention of disease caused by TiLV. The fish were immunized with 100 µL of either HKV or FKV by intraperitoneal injection with each vaccine containing 1.8 × 106TCID50inactivated virus. A booster vaccination was carried out at 21-day post vaccination (dpv) using the same protocol. The fish were then challenged with a lethal dose of TiLV at 28 dpv. The expression of five immune genes (IgM, IgD, IgT, CD4andCD8) in the head kidney and spleen of experimental fish was assessed at 14 and 21 dpv and again after the booster vaccination at 28 dpv. TiLV-specific IgM responses were measured by ELISA at the same time points. The results showed that both vaccines conferred significant protection, with relative percentage survival (RPS) of 71.3% and 79.6% for HKV and FKV, respectively. Significant up-regulation ofIgMandIgTwas observed in the head kidney of fish vaccinated with HKV at 21 dpv, whileIgM, IgDandCD4expression increased in the head kidney of fish receiving FKV at the same time point. After booster vaccination,IgTand CD8transcripts were significantly increased in the spleen of fish vaccinated with the HKV, but not with FKV. Both vaccines induced a specific IgM response in both serum and mucus. In summary, this study showed that both HKV and FKV are promising injectable vaccines for the prevention of disease caused by TiLV in Nile tilapia.