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

Abstract

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 10(7) 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.

High prevalence of betanodavirus barfin flounder nervous necrosis virus as well as red‐spotted grouper nervous necrosis virus genotype in shellfish

“…However, the results support the premise that the prevalence tendency of BFNNV genotype is independent of the temperature of the sea surface, in contrast to RGNNV. Thus, it is possible that low viral concentrations in asymptomatic infected wild fishes like cod [genus Gadus (L.)], which is host species for BFNNV (Mao et al., ; OIE ) and which has been recently caught in large quantities in the waters near Korea (Jung et al., ), or some other endobiosis including molluscs and benthic organisms living in deep sea at consistently low water temperature condition would be one factor contributing to high prevalence of the psychrophilic group of betanodaviruses in shellfish, as BFNNV genotype caused the disease to spread to cultivated cold‐water fishes from wild (Johansen et al., ). On the other hand, all viruses included to each of the two genotypes were detected regardless of the shellfish species specificity and local relativity.…”

“…However, the results support the premise that the prevalence tendency of BFNNV genotype is independent of the temperature of the sea surface, in contrast to RGNNV. Thus, it is possible that low viral concentrations in asymptomatic infected wild fishes like cod [genus Gadus (L.)], which is host species for BFNNV (Mao et al, 2015;OIE 2016) were confirmed to be RGNNV rather than BFNNV (Table 4) (Byrne et al, 2008;Johansen & Sommer, 2001). Thus, these results might show that the RGNNV replication is preferred by the co-existence with another genotype in E-11 cells.…”

Isolation and Identification of a New Strain of Nervous Necrosis Virus from the Big-belly Seahorse Hippocampus Abdominalis in China

“…However, it is also reported that RNA1, encoding RdRp, protein B1 and B2, is the key contributing factor for the response to temperature [33], Ciulli et al also considered that RNA1 rather than RNA 2 plays a more important part in thermotolerance of NNV, although RNA2 was found to determine temperature sensitivity as well [34,43]. There is evidence that the RNA1 evolved rapidly than RNA2, indicating a strong selection to respond to various temperature ranges [15,45,46]. In our study, we only analysed the cp gene but not the complete sequence of RNA1 and RNA2.…”

Immune response of pacific cod larvae stimulated with PolyI:C

“…macrocephalus have been carried out for several years in the lab, Key laboratory of Mariculture & Stock Enhancement in North China′s Sea, Ministry of Agriculture and Rural Affairs, Dalian (Gu et al, 2020; Jiang et al, 2012; Mao, Chen, et al, 2018). However, high mortality during larvae stages is still a big problem (Mao, Jiang, et al, 2018; Mao, Wen, et al, 2015). In our previous study, kinds of immune‐related genes were examined in the early developmental stages using next‐generation sequencing technology (Mao, Li, et al, 2015).…”