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Betanodaviruses are pathogens of fish causing a disease called “viral nervous necrosis” and are named after the host fish from which they were isolated, followed by nervous necrosis virus (Munday et al., 2002). They infect more than 50 species of marine and freshwater fish. Five genetic lineages of betanodavirus have been identified.
Virions are non-enveloped, spherical in shape, 25–33 nm in diameter, and have icosahedral symmetry (T=3). Distinct surface protrusions are observed by electron microscopy of negatively stained preparations (Figure 1.Betanodavirus). Image reconstruction of virus-like particles of Malabaricus grouper nervous necrosis virus (MGNNV) indicates that the capsid protein (CP) of betanodaviruses has a two domain structure compared to the single domain structure of the CP of alphanodaviruses. The average diameter of the particle is 31 nm. In contrast with most alphanodaviruses, empty particles have been seen by electron microscopy of some preparations of betanodaviruses.
Virion buoyant density in CsCl of striped jack nervous necrosis virus (SJNNV) has not been reported but that of Dicentrarchus labrax encephalitis virus (DlEV), a related but not yet classified betanodavirus, is about 1.31–1.36 g cm−3. Virions of DlEV are stable between pH 2 and 9 and resistant to heating at 56 °C for 30 min. Infectivity is resistant to extraction of virions with chloroform.
The genome consists of two molecules of positive-sense ssRNA: RNA1 (Mr 1.01×106, 3.1 kb) and RNA2 (Mr 0.49×106, 1.4 kb). Both RNA molecules are encapsidated in the same virus particle, and both are required for infectivity. Both molecules are capped at their 5′-ends and lack poly(A) tails at their 3′-ends.
Betanodavirus capsids contain 180 copies of a single structural protein of 42 kDa. In contrast to alphanodaviruses, maturation cleavage of this protein is not observed.
Betanodaviruses replicate in the cytoplasm. Infected cells contain three ssRNAs: RNA1 (Mr 1.01×106; 3.1 kb); RNA2 (Mr 0.49×106; 1.4 kb) and a subgenomic (sg) RNA3 (Mr about 0.13×106; 0.4 kb) derived from RNA1. RNA3 is not packaged into virions. RNA1 encodes protein 1a (110 kDa), the RNA-dependent RNA polymerase (RdRP). RNA2 encodes protein 2a (42 kDa), the CP. The RNA3 of SJNNV encodes protein B2 and has a potent RNA silencing-suppression activity, as also observed for alphanodaviruses (Iwamoto et al., 2001).
Betanodaviruses are cross-reactive by immunoblot analysis using polyclonal antisera but differential reactivity is observed with monoclonal antibodies. Virus neutralization with polyclonal antisera divides four betanodaviruses into three serotypes; serotype A for SJNNV, serotype B for tiger puffer nervous necrosis virus (TPNNV), and serotype C for red-spotted grouper nervous necrosis virus (RGNNV) and barfin flounder nervous necrosis virus (BFNNV).
Isolates of all species of betanodaviruses have been isolated from larvae, juvenile or adult marine fish, in which they cause “viral nervous necrosis” or “viral encephalopathy and retinopathy” associated with behavioral abnormalities and high mortalities. SJNNV and TPNNV have a limited host range: striped jack for SJNNV and tiger puffer for TPNNV. In contrast, RGNNV and BFNNV have a wide range of host fish; RGNNV is isolated from warm-water fishes and BFNNV is isolated from cold-water fishes. These diseases cause significant problems for the marine aquaculture industry.
Betanodaviruses replicate in cultured cells from striped snakehead fish (SNN-1 and E-11) and other cells derived from fish such as groupers (GF-1), sea bass (SBL), turbot (TV-1), and gilthead sea bream (SAF-1). A low level of virus replication is observed in mammalian (COS-1 and HeLa) cells.
Betanodavirus antigens and/or CP genes are detected in eggs, larvae and ovaries of some inapparently infected hatchery-reared fish species, and the CP gene is frequently detected in a variety of wild fishes without any disease signs, indicating both horizontal and vertical modes of transmission of the virus.
The following criteria can be applied to the demarcation of species within the Betanodavirus genus:
Dicentrarchus labrax encephalitis virus
Melanogrammus aeglefinus nervous necrosis virus
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