Distribution of nervous necrosis virus (NNV) in infected sevenband grouper, Hyporthodus septemfasciatus by intramuscular injection or immersion challenge

Nervous necrosis virus (NNV) is one of the most serious fish pathogens which causes damage in >120 species of cultured marine fishes worldwide including sevenband grouper (Hyporthodus septemfasciatus). The present study was undertaken to compare NNV genomic segments RNA1 and RNA2 through qRT-PCR and determine viral infectivity in eight organs of dead or survived sevenband grouper after intramuscular (IM) injection or immersion challenge by NNV. IM injection with NNV at 100.5, 101.5, 102.5 and 103.5 TCID50/100 μL/fish resulted in cumulative mortality of 0%, 20%, 40% and 100%, respectively. Immersion challenge with NNV at 101.5, 103.5 and 105.5 TCID50/mL resulted in cumulative mortality of 0%, 40% and 60%, respectively. The minimum NNV concentration pathogenic to fish by IM injection was approximately at least 10-fold lower compared to that by immersion challenge route. The ratio of the RNA1 segment and infective particles in dead fish was higher than RNA2/TCID50, which indicates that RNA1 was overproduced than RNA2 segment. This implies that RNA2 segment is very efficient in packaging infectious particles. Brain and eye were found to have the highest NNV titer and copy number. The gill, spleen and kidney were detected higher than the other non-nervous tissues, suggesting that these tissues can be used for NNV diagnostics although they do not seem to represent the infective tissues. In survived fish, NNV was under the detection limit although NNV copy number was detected, which indicates that although NNV could get into the body, it failed to produce complete virion.