LAUSR

Abstract As a consequence of nervous necrosis virus (NNV) infection in the East Asian aquaculture industry, sevenband grouper Hyporthodus septemfasciatus in particular has been severely impacted, and is a species that incurs large economic losses. Recently, it has been observed that sevenband grouper in some aquaculture farms do not die even after challenge with NNV. To investigate further, sevenband grouper obtained from an aquaculture farm were intramuscularly inoculated with NNV at 104.3 and 106.3 TCID50/fish (doses that are 100-fold and 10,000-fold the LD50) at eight day intervals, in order to analyze the in vivo multiplication states of NNV, the induction of NNV-specific antibodies and induction of NNV-neutralizing activity. No mortality was observed in the fish challenged once and/or twice with NNV. Multiplication of NNV was observed in 57.1% and 21.4% of fish after the first and second injections, respectively. Although NNV-specific antibodies were reliably detected from one out of eight fish before NNV injection, antibody levels were almost universally elevated following each NNV injection. After the second injection, 85.7% fish were positive for NNV-specific antibodies. Interestingly, almost no NNV-neutralizing activity was detected in any fish sera. These responses are consistent with those of fish that have convalesced from NNV infection. Therefore, we conclude that the fish used in these experiments had experienced mild NNV infections without mass mortality in the past, and naturally convalesced in an aquaculture farm setting.