Lack of nervous necrosis virus (NNV) neutralizing antibodies in convalescent sevenband grouper Hyporthodus septemfasciatus after NNV infection.

Viral nervous necrosis (VNN) is caused by nervous necrosis viruses (NNVs) belonging to genus Betanodavirus (Nodaviridae). It is one of the most serious diseases in aquaculture industry worldwide. In the present study, the kinetics of NNV-infectivity and NNV-specific antibodies in convalescent sevenband grouper Hyporthodus septemfasciatus after NNV infection was determined. When fish were infected with NNV at 17.5 °C, and reared for 84 days at natural seawater temperature (increasing rate: approximately 0.1 °C/day), NNV infectivity peaked on day 14 with 10 TCID/g at the highest, and declined to below the detection limit. When convalescent fish were reared at 27 °C, and re-infected with NNV at 10 or 10 TCID/fish, no mortality was observed although NNV multiplied up to 10 and 10 TCID/g at the highest, respectively, suggesting NNV-specific immune response. It also revealed that convalescent fish were re-infected by NNV although NNV multiplication was strongly regulated. Interestingly, NNV-specific antibodies were detectable in 20% and ≥80% of convalescent fish before and after re-infection with NNV, respectively. However, no NNV-neutralizing activity was detected before and after re-infection in almost all of the convalescent fish. Therefore, NNV-neutralizing antibodies might not be necessary for the protection of convalescent fish against NNV re-infection after previous NNV infection.