106. Buoyant density in sucrose is 1.10-1.15 g/cm3; S20W is 140-150S. Virion infectivity is stable over a relatively broad pH range, but unstable at temperatures above 40°C. Solvents and detergents rapidly inactivate the viruses.
|
Type Species |
(BVDV-1) |
Relative to the other genera, pestiviruses encode two unique gene products. The first protein of the ORF, nonstructural protein Npro, possesses an autoproteolytic activity responsible for its release from the nascent polyprotein. One of the three viral envelope glycoproteins, Erns, possesses an intrinsic RNase activity.
Virions are 40-60 nm in diameter and spherical in shape (Fig. 3). The virion envelope has 10-12 nm ring-like subunits on its surface. The structure and symmetry of the core have not been characterized.
Physicochemical and Physical Properties
Virion Mr has not been precisely determined but can be estimated from the virus composition to be about 60 106. Buoyant density in sucrose is 1.10-1.15 g/cm3; S20W is 140-150S. Virion infectivity is stable over a relatively broad pH range, but unstable at temperatures above 40°C. Solvents and detergents rapidly inactivate the viruses.
Virions contain one positive sense molecule of ssRNA approximately 12.3 kb in size. Based on indirect data, the 5
-terminus is probably not capped; no poly(A) tract is present at the 3-end. Genomic RNA contains a single ORF spanning most of the viral genome. For some cytopathogenic strains of Bovine viral diarrhea virus (BVDV), a small and variable segment of host cell or viral nucleic acid is integrated into one particular region (NS2 or at the junction between NS2 and NS3) of the viral genome. Other cytopathogenic BVDV isolates contain viral gene duplications involving all or part of the Npro and NS3 protein coding regions, resulting in genomic RNA sized up to approximately 16.5 kb. In all these cases, the single large ORF is maintained. Finally, cytopathogenic viruses may also arise by deletion of large portions of their genomes. Such defective genomes may be rescued with intact helper viruses.
Virions are composed of four structural proteins: a basic nucleocapsid protein, C, and three envelope glycoproteins, Erns, E1 and E2. All three glycoproteins exist as intermolecular disulfide-linked complexes: Erns homodimers, E1-E2 heterodimers, and E2 homodimers. Erns possesses an intrinsic RNase activity and is a unique pestivirus gene product not found in the other genera of the family. Pestiviruses encode 7-8 nonstructural (NS) proteins. Npro, also a protein unique to the genus Pestivirus, is a proteinase that autocatalytically releases itself from the nascent polyprotein. Nonstructural protein p7 is presumed to have a role in virus maturation. NS2-3 is a multifunctional protein. The N-terminal 40% (NS2) is hydrophobic and contains a zinc finger motif suggesting a role in RNA binding or replication. The C-terminal 60% (NS3) acts as both serine proteinase involved in polyprotein processing and an RNA helicase/NTPase likely involved in RNA replication. NS2-3 is found in noncytopathogenic BVD viruses. In cytopathogenic BVD and Border disease virus (BDV), and in Classical swine fever virus (CSFV) isolates, NS2 and NS3 may be produced in addition to NS2-3. The NS4A protein acts as a cofactor to the NS3 proteinase activity. The role of NS4B is unknown. NS5A is the only phosphorylated pestivirus protein and presumably plays a yet to be identified role in RNA replication. NS5B possesses RNA-dependent RNA polymerase activity.
The viruses are enveloped, but no reports have described the lipid composition.
All virus envelope glycoproteins contain N-linked glycans.
Genome Organization and Replication
The genome RNA consists of a single large ORF encoding a polyprotein of about 4000 amino acids that is preceded by a 5-NCR of about 360-385 nts and followed by a 3-NCR of about 230 nts. The gene order is 5-Npro-C-Erns-E1-E2-p7-NS2-3(NS2-NS3)-NS4A-NS4B-NS5A-NS5B-3 (Fig. 4).
Pestivirus replication is initiated by receptor-mediated endocytosis involving one or more cell surface molecules and viral glycoproteins Erns and E2. After endocytosis and uncoating, the genome RNA serves as the mRNA; there are no subgenomic mRNA molecules. Translation initiation occurs by a cap-independent internal initiation mechanism involving an internal ribosome entry site (IRES) within the 5-NCR of the RNA. Polyprotein processing occurs co- and post-translationally by both host and viral proteinases. Nonstructural protein Npro, the first protein of the ORF, autoproteolytically cleaves itself from the nascent polyprotein by cleavage at the Npro/C site. Downstream cleavages that produce structural proteins C, Erns, E1 and E2 are likely mediated by cellular signal peptidase(s). Glycoprotein translocation to the endoplasmic reticulum probably occurs by an internal signal sequence, perhaps within the C protein. Cleavage between E2 and p7 is not complete, leading to two intracellular forms of E2 with different C-termini. Depending of the virus, NS2-3 remains intact (noncytopathic viruses) or is found together with its N- and C-terminal products NS2 and NS3 (cytopathic viruses). The mechanisms by which NS3 is generated involve an RNA recombinational event. Cytopathogenic BVD viruses have gene insertions, deletions, duplications or rearrangements that in some manner lead to NS3 production. The NS3 serine proteinase activity (and that of the NS2-3 protein of noncytopathic viruses) is responsible for all downstream polyprotein processing. NS4A facilitates cleavages by NS3 at the 4B/5A and 5A/5B sites.
RNA replication occurs in association with intracytoplasmic membranes, presumably in a replication complex composed of viral RNA and viral non-structural proteins. Replicative forms of pestiviral RNA have been detected. The ratio of positive-to-negative sense RNA in cells 12 hours post-infection is about 10. RNA synthesis is resistant to actinomycin D. Virus maturation is poorly understood. However, viral proteins are not found on the cell surface, suggesting that viruses mature in intracellular vesicles and are released by exocytosis. Considerable infectious virus remains cell-associated. Host cell RNA and protein synthesis continue throughout infection.
Pestiviruses are antigenically related and cross-reactive epitopes are present on all pestiviruses. Separate antigenic determinants defined by monoclonal antibodies (Mabs) have also been identified. Antigenic variation is particularly pronounced among BVDV isolates. The N-terminal portion of E2 contains an antigenically hypervariable region. Mab binding patterns are generally consistent with the genetic relatedness of viruses.
Infected animals mount potent antibody responses to two structural glycoproteins (Erns, E2) and to the NS3 protein, while antibody responses are extremely weak or nonexistent to all other virus-encoded polypeptides. Erns and E2 are able to induce protection independently. Mabs reactive with these proteins can neutralize virus infectivity. Antibodies to NS3 are not able to neutralize infectivity.
Pestiviruses infect pigs and ruminants, including cattle, sheep, goats and wild ruminants. There are no invertebrate hosts.
Transmission occurs by direct and indirect contact (e.g., nasal or urine secretion, contaminated food, etc.). Transplacental and congenital transmission occur in all host species.
Pestivirus infections may be subclinical or produce a range of clinical conditions including acute diarrhea, acute hemorrhagic syndrome, acute fatal disease and a wasting disease. Diaplacental infection can result in fetal death, congenital abnormalities, or lifelong persistent infection. Fatal mucosal disease occurs in cattle persistently infected with noncytopathogenic viruses. This coinfection in the persistently infected animal results in a fatal disease. Pestivirus infections of livestock are economically important worldwide.
Experimental infection models have not been established for bovine or ovine pestiviruses outside their natural mammalian hosts. However, CSFV can be adapted to propagate in rabbits.
Cells derived from natural host species (bovine, porcine, ovine) support virus replication. Most virus isolates are noncytopathogenic and can establish persistent infections in cell culture. Infectious noncytopathogenic BVDV is often present in bovine serum products used for cell culture. Cytopathogenic BVD viruses, which arise during persistent infections of animals with noncytopathogenic BVD viruses, exist that are capable of plaque formation and extensive cytopathology.
No hemagglutinating activity has been found associated with pestiviruses.
List of Species Demarcation Criteria in the Genus
The list of species demarcation criteria in the genus is:
|
Nucleotide sequence relatedness, | |
|
Serological relatedness, | |
|
Host of origin. |
Pestivirus species demarcation considers several parameters and their relationship to the type viruses of the currently recognized species (BVDV-1:NADL; BVDV-2:890; BDV:BD31; and CSFV:Alfort/187). Nucleotide sequence relatedness is an important criterion for pestivirus species demarcation. For example, the 5-NCR sequences among the four currently recognized type species viruses are over 15% divergent. In most cases the degree of homology within the 5-NCR will allow pestivirus species demarcation. However, in some cases the nucleotide sequence relatedness may be ambiguous and must be complemented with additional comparative analyses. Convalescent animal sera generated against a given pestivirus species (e.g., BVDV-1) generally show a several-fold higher neutralization titre against BVDV-1 viruses than against viruses from the other species. Finally, differences in host of origin and disease can assist in species identification.
For example, BVDV-1 and CSFV are considered different species because they differ from each other by (1) at least 25% at the nucleotide level (complete genomes), (2) at least ten-fold difference in neutralization titre in cross-neutralisation tests with polyclonal immune sera, and (3) host range-under natural conditions CSFV infects only pigs while BVDV-1 infects ruminants and pigs as well.
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:
|
Border disease virus |
(BDV) | |
|
BD31 |
[U70263] |
|
|
X818 |
[AF037405] |
|
|
Bovine viral diarrhea virus 1 |
(BVDV-1) | |
|
NADL |
[M31182] |
|
|
Osloss |
[M96687] |
|
|
SD-1 |
[M96751] |
|
|
CP7 |
[U63479] |
|
|
Bovine viral diarrhea virus 2 |
(BVDV-2) | |
|
strain 890 |
[U18059] |
|
|
C413 |
[AF002227] |
|
|
Classical swine fever virus (Hog cholera virus) |
(CSFV) | |
|
Alfort/187 |
[X87939] |
|
|
Alfort-Tübingen |
[J04358] |
|
|
Brescia |
[M31768] |
|
|
C strain |
[Z46258] |
Tentative Species in the Genus
Pestivirus of giraffe
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