A molecular phylogenetic analysis of 25 isolates of fish nodaviruses, the causative agents of viral nervous necrosis of marine fish, was performed based on the nucleotide sequences (427 bases) of the coat protein gene. These fish nodaviruses were classified into four clusters: tiger puffer nervous necrosis virus, striped jack nervous necrosis virus, berfin flounder nervous necrosis virus, and red-spotted grouper nervous necrosis virus.
Striped jack nervous necrosis virus (SJNNV), a nodavirus, is the causative agent of viral nervous necrosis (VNN) in larval striped jack fish. In the present study, the SJNNV coat protein gene was sequenced and compared with that of four known insect nodaviruses and with four other fish nodaviruses causing VNN. The SJNNV coat protein gene was 1410 bases in length and contained a single ORF of 1023 bases encoding a protein of 340 amino acids. The sequence similarities between the coat protein gene of SJNNV and four insect nodaviruses were 28.6 % or less at the nucleotide level and 10.6% or less at the amino acid level. A portion of the coat protein gene from four additional fish VNN viruses was amplified by PCR using primers designed for SJNNV and the amplified fragments (870-876 bases) were sequenced. The sequence similarities among SJNNV and the four VNN viruses were 75-8 % or greater at the nucleotide level and 80.9 % or greater at the amino acid level. In the fish nodaviruses a highly conserved region of 134 amino acids with sequence similarity of 92.5 % or greater was detected. This conserved sequence was not found in the coat protein of insect nodaviruses. These results indicate that the fish nodaviruses that cause VNN are closely related to each other but are quite different from insect nodaviruses.
Mass mortalities of sevenband grouper Epinephelus septemfasciatus reared in Oita Prefecture were investigated. The outbreaks of the disease occurred in summer seasons (July to October; water characterized by upside down swimming and inflation of swimbladder. Histopathologically, necrosis and vacuolation in the central nervous tissues and retina were observed. Although bacteria or parasites associated with the disease were not detected from affected fish and no CPE was observed on RTG-2, BF -2, and EPC cell lines, spherical virus particles, 28 nm in diameter, were found abundantly in the cytoplasm of affected nerve cells of most of the affected fish. The virus was identified as a fish nodavirus (nervous necrosis virus), based on the results of fluorescent antibody technique test with anti-SJNNV rabbit serum and polymerase chain reaction test with primers for amplification of SJNNV coat protein gene. These results suggest that the mortalities of sevenband grouper were at least partly due to viral nervous necrosis (VNN). This is the first report describing VNN of sevenband grouper and the first case of VNN in the adult fish.
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