Efficacy of the inactivated nervous necrosis virus vaccine against viral nervous necrosis in pond-reared orange-spotted grouper Epinephelus coioides

The field efficacy of the formalin-inactivated nervous necrosis virus (NNV) against viral nervous necrosis (VNN) in orange-spotted grouper (Epinephelus coioides) reared in floating net cages in earthen pond was investigated. Seroneutralization assay conducted on the sera of vaccinated fish exhibited the occurrence of neutralizing antibody titers from Day 30 (mean titer 1:1792±701) to Day 150 (1:704±351) with the highest titer observed at Day 60 (1:6656±3435) post-vaccination. Because mortality attributed to VNN was not encountered during the pond experiment, intramuscular challenges of vaccinated and unvaccinated (L-15 injected) fish with NNV (10^6.5 TCID₅₀/fish) were conducted in indoor tanks at Day 30 (Mean body weight [MBW]: vaccinated [21±3.4 g]; unvaccinated [20.6±1 g) and Day 120 (MBW: vaccinated [178±27 g]; unvaccinated [176±19 g) postvaccination, respectively, to demonstrate the in vivo efficacy of the inactivated vaccine. Nil and 25 % mortality rate were obtained in vaccinated and control fish, respectively, challenged with NNV at Day 30 post-vaccination. On the contrary, nil mortality were obtained in both groups challenged with NNV at Day 120 post-vaccination. Although nil mortality was obtained in NNV-challenged unvaccinated fish, 30 % of the fish manifested dark coloration of the skin and abnormal swimming behavior that commenced and disappeared at Day 3 and Day 7 post-NNV challenge, respectively, suggesting an age/ weight-dependent resistance to the disease. Our current data illustrate that single vaccination with inactivated vaccine could mount the production of protective antibodies and concomitant conferment of protection against VNN in groupers especially during the early phase of grow-out culture in earthen ponds where they are highly susceptible to the disease.