Genus Novirhabdovirus

Type Species	Infectious hematopoietic necrosis virus	(IHNV)

Distinguishing Features

This genus comprises one of the two subgroups of rhabdoviruses known to infect aquatic hosts. Members of the other subgroup, represented by Spring viremia of carp virus, are tentatively assigned to the Vesiculovirus genus based on available sequence data.

The replication temperature range and thermal inactivation temperatures for these viruses are typically lower than those of other rhabdoviruses, due to the aquatic poikilotherm nature of the host species for this genus. Optimum virus replication temperatures range from 15-28°C, depending roughly on the ambient water temperature in the geographic range of each virus.

Novirhabdoviruses have five major structural proteins, designated L (Mr 150-225 10³), G (Mr 63-80 10³), N (Mr 38-47 10³), P (Mr 22-26 10³, formerly designated M1), and M (Mr 17-22 10³, formerly designated M2). In addition to the structural proteins, novirhabdoviruses encode a small, sixth, non-virion protein designated NV (Mr 12-14 10³), which is expressed at variable levels in infected cells but is not detectable in purified virions. The function of the NV protein is not known, but the preservation of an open reading frame in numerous diverse virus species and strains indicates a significant biological role. The NV protein amino acid sequences are significantly less conserved between virus species than sequences of the other structural proteins, such that there is no significant amino acid sequence similarity between the NV proteins of Infectious hematopoietic necrosis virus (IHNV) and Viral hemorrhagic septicemia virus (VHSV).

The genomic RNA is approximately 11.1  kb, with six genes in the order 3-N-P-M-G-NV-L-5. For IHNV it is known that the genome contains a leader region of approximately 60  nts preceding the transcription start of the N gene, and a trailer of about 100  nts following the transcription termination of the L gene. Genes begin with either the conserved putative transcription start signal 3-CCRWG (vRNA sense, most often 3-CCGUG), or with the signal 3-UUGU, which is also found upstream of the translation initiation site in nearly all genes. Transcription terminates at the signal 3-UCURUC(U)₇, and non-transcribed intergenic regions are single nucleotides, G or A (vRNA sense).

Biological Properties

Novirhabdoviruses infect numerous species of fishes. The natural host ranges of individual viruses are relatively broad, often infecting several species as diverse as salmonids and herring. In nature and in artificial environments novirhabdoviruses can be transmitted horizontally, from fish to fish, by waterborne virus. Egg-associated transmission has also been clearly demonstrated by several cases in which the spread of virus to new geographic regions has occurred with transport of contaminated eggs. It is increasingly apparent that wild fish can serve as reservoirs of virus. The existence of invertebrate reservoirs or vectors of virus has been postulated but their importance is uncertain. Similarly, the potential for a carrier state in survivors of IHNV infections has been demonstrated, but the frequency and significance of this phenomenon is still under investigation.

The geographic distribution of novirhabdoviruses is broad. IHNV is enzootic to western North America, but inadvertent transport of the virus with contaminated eggs and infected fish has resulted in spread and establishment of IHNV in western Europe, Korea, Taiwan, Japan, and mainland China. VHSV is enzootic to western Europe, but more recently several North American strains have been described, and the virus has been found in marine fish in the northeastern Pacific Ocean, as well as the Baltic and North Seas. Hirame rhabdovirus (HIRRV) is at present only isolated in Japan.

Species in the Novirhabdovirus genus cause disease in cultured fish hosts, resulting in significant economic losses to the aquaculture industry. Both IHNV and VHSV have been well documented as severe pathogens of cultured salmonids since the 1950s, often resulting in losses of 50-100%. IHNV has also been reported to cause epizootics in wild salmonid populations. IHNV and VHSV both cause hemorrhagic diseases, with petechial hemorrhages evident both externally and internally. Major degenerative changes and necrosis in the kidneys and hematopoietic tissue are evident upon histopathological examination, and are believed to be the actual cause of mortality.

List of Species Demarcation Criteria in the Genus

Species within the genus have been distinguished serologically on the basis of cross-neutralization with polyclonal rabbit antisera. In general, strains within a species are neutralized by a single polyclonal antiserum. Thus, IHNV and HIRRV each comprise single serotypes, and VHSV has one major serotype with a small number of associated strains. Viruses from different species do not show cross-neutralization, but in some cases there is a low level of cross-reaction with specific proteins in western blot analyses. Nucleotide sequence data are available for most genes of these viruses, and will undoubtedly contribute to the distinction of viral species in the future. For strains within a virus species the nucleotide sequence divergence values range from 3-4% for IHNV G and NV genes, to a maximum of 18% for the G genes of European and North American VHSV. N protein amino acid sequence identity between IHNV and VHSV is approximately 34%.

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:

Species in the Genus

Hirame rhabdovirus	[D45422, U24073, U47847]	(HIRRV)
Infectious hematopoietic necrosis virus	[L40883, M16023, U47846, U50401, U50402, X89213]	(IHNV)
Viral hemorrhagic septicemia virus (Egtved virus)	[D00687, U02624, U02630, U03502, U03503, U28745, U28746, X59241]	(VHSV)

Tentative Species in the Genus

Eel virus B12	(EEV-B12)
Eel virus C26	(EEV-C26)
Snakehead rhabdovirus	(SHRV)