Evidence for and characterization of nervous necrosis virus infection in Pacific cod (Gadus macrocephalus)

• Published in: Archives of virology Vol. 160; no. 9; pp. 2237 - 2248
• Main Authors: Mao, Ming-Guang, Wen, Shi-Hui, Perálvarez-Marín, Alex, Li, Hua, Jiang, Jie-Lan, Jiang, Zhi-Qiang, Li, Xing, Sun, Hang, Lü, Hui-Qian
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 01.09.2015; Springer Nature B.V
• Subjects: amino acid sequences; Animal Structures - virology; Animals; Bacteria; Betanodavirus; Biomedical and Life Sciences; Biomedicine; breeding; Capsid Proteins - chemistry; Capsid Proteins - genetics; China; Cloning; Cluster Analysis; coat proteins; cod (fish); diagnostic techniques; eggs; electron microscopy; epitopes; Escherichia coli; Escherichia coli - genetics; farmers; Fish Diseases - virology; flounder; Gadiformes - virology; Gadus macrocephalus; Gene Expression; genotype; gonads; Infections; Infectious Diseases; larvae; Marine; Medical Microbiology; Microscopy; Microscopy, Electron, Transmission; Models, Molecular; Molecular Sequence Data; Molecular Weight; Mortality; Nodaviridae - genetics; Nodaviridae - isolation & purification; Nodaviridae - ultrastructure; Open Reading Frames; Original Article; Phylogeny; Pleuronectiformes; Protein Conformation; Proteins; quantitative polymerase chain reaction; Real-Time Polymerase Chain Reaction; RNA; RNA Virus Infections - veterinary; RNA Virus Infections - virology; sequence alignment; Sequence Analysis, DNA; Sequence Homology, Amino Acid; temperature; Verasper moseri; Viral Load; Virology; Viruses; China; Japan; China, People's Rep; Nervous Necrosis Virus; Coat Protein; Capsid Protein Gene; Viral Copy Number; Capsid Protein
• ISSN: 0304-8608; 1432-8798
• DOI: 10.1007/s00705-015-2484-1

A mortality rate higher than 90 % was observed in a larva-rearing facility for Pacific cod, Gadus macrocephalus, in China. Larvae showing clinical signs of infection were collected. Initial suspicion of nervous necrosis virus (NNV) infection was confirmed by sequencing, absolute quantification real-time PCR (A-qPCR), and electron microscopy. The nucleotide sequence of RNA2 was 1,375 bases long (GenBank no. KM576685), coding for a single ORF corresponding to the capsid protein from residues 21 to 1034. Phylogenetic analysis of the capsid protein sequence showed that PCNNV belongs to the barfin flounder NNV (BFNVV) genotype. An amino acid sequence alignment revealed 39 differences between the cold- and warm-resistant viral groups, suggesting that PCNNV evolved under temperature selection. The 3-D structure of the predicted capsid protein was modeled to identify potential epitopes, and the gene was expressed in Escherichia coli, yielding a protein with a molecular mass of 55 kDa. During PCNNV outbreaks, the viral copy number was found to reach 10⁷ per ng of total RNA, which could be considered the lethal copy number of NNV in cod. The gonads, eggs, fertilized eggs and asymptomatic cod fry were all positive for PCNNV, indicating viral vertical transmission as the main source of the viral load. The amount of virus in the apparent healthy fry or survivors seemed to decrease gradually with development. These results might lead to efficient diagnostic methods to help farmers select NNV-free broodfish for cod breeding.