Family: Bunyaviridae

Chapter Version: ICTV Ninth Report; 2009 Taxonomy Release

Virion properties

Morphology

Morphological properties vary among viruses in each of the five genera; however, virions generally are spherical or pleomorphic, 80–120 nm in diameter, and display surface glycoprotein projections of 5–10 nm which are embedded in a lipid bilayered envelope approximately 5 nm thick. Virion envelopes are usually derived from cellular Golgi membranes, or on occasion, from cell surface membranes. Viral ribonucleocapsids are 2–2.5 nm in diameter, 200–3000 nm in length, and usually (but not always) display helical symmetry. (See Figure 1.)

Physicochemical and physical properties

The virion Mr is 300×10⁶ to 400×10⁶ and has an S_20,W of 350–500. Virion buoyant densities in sucrose and CsCl are 1.16–1.18 and 1.20–1.21 g cm⁻³, respectively. Virions are sensitive to heat, lipid solvents, detergents and formaldehyde.

Nucleic acid

The viral genome comprises three unique molecules of negative or ambisense ssRNA, designated L (large), M (medium) and S (small), which total 11–19 kb (Table 1). The terminal nucleotides of each genome RNA segment are base-paired forming non-covalently closed, circular RNAs (and ribonucleocapsids). The terminal sequences of genome segments are conserved among viruses in each genus but are different from those of viruses in other genera. The genomic RNAs are not modified at their 5′ ends. The Mr of the genome ranges from 4.8×10⁶ to 8×10⁶ and this constitutes 1–2% of the virion by weight. Viral mRNAs are not polyadenylated and are truncated relative to the genomic RNAs at the 3′ termini. mRNAs have 5′-methylated caps and 10–18 non-templated nt at the 5′ end which are derived from host-cell mRNAs.

Table 1 Nucleotide lengths of selected completely sequenced genomes

Proteins

All viruses have four structural proteins, two external glycoproteins (Gn, Gc, named in accordance with their relative proximity to the amino or carboxy terminus of the polyprotein encoded by the M segment), a nucleocapsid protein (N) and a large (L) protein, an RNA-dependent RNA polymerase. Non-structural proteins are expressed from the S segments of some bunyaviruses, phleboviruses, tospoviruses and some hantaviruses, and from the M segments of bunyaviruses, nairoviruses, tospoviruses and some phleboviruses. Proteins encoded by each of the genome segments of viruses in each genus of the family are listed in Table 2.

Table 2 Deduced protein sizes (kDa)

Lipids

Virions contain 20–30% lipids by weight. Lipids are derived from the membranes where viruses mature and include phopholipids, sterols, fatty acids and glycolipids.

Carbohydrates

Virions contain 2–7% carbohydrate by weight. Asparagine-linked sugars on the Gn and Gc proteins are largely of the high mannose type when viruses are grown in vertebrate cells.

Genome organization and replication

The genome organization of the different genera is shown in Figure 2. For all viruses, the L, M and S genome segments encode, respectively, the viral RNA polymerase (L protein), envelope glycoproteins (Gn and Gc) and nucleocapsid protein (N) in the virus-complementary sense RNA. The L protein is encoded in the complementary mRNA. A single, continuous ORF in the M RNA encodes the glycoproteins, and the primary gene product is co-translationally cleaved (except for nairoviruses) to give mature Gn and Gc. Hantaviruses and Uukuniemi-like phleboviruses encode no additional proteins in their M genome segments. Orthobunyaviruses and other phleboviruses encode a nonstructural protein (NSm) in the virion-complementary sense RNA. Nairoviruses encode two proteins of unknown functions: a mucin-rich protein and glycoprotein GP38, which are the products of posttranslational cleavage of preGn. Tospoviruses encode a NSm protein in an ambisense ORF at the 5′ end of virion-sense RNA. Some orthobunyaviruses and some hantaviruses encode a nonstructural protein (NSs) in an overlapping ORF to that encoding N in the 3′-half of the virion-sense S RNA. There is no direct evidence that nairoviruses encode any additional proteins in their S genome segments. Phleboviruses and tospoviruses encode a NSs protein in an ambisense ORF in the 5′-half of virion-sense S RNA. The NSs proteins of orthobunyaviruses, phleboviruses, and hantaviruses have been shown to act as interferon antagonists, NSs of tospoviruses as an RNAi antagonist. The NSm of phleboviruses can act as an apoptosis antagonist.

All stages of replication occur in the cytoplasm. The principal stages of replication are:

• Attachment, mediated by an interaction of one or both of the integral viral envelope proteins with, as yet unidentified, host receptors.
• Entry and uncoating, by endocytosis of virions and fusion of viral membranes with endosomal membranes.
• Primary transcription: i.e., the synthesis of mRNA species complementary to the genome templates by the virion-associated polymerase using host cell-derived capped primers (Figure 3).
• Translation of primary L and S segment mRNAs by free ribosomes; translation of M segment mRNAs by membrane-bound ribosomes and primary glycosylation of nascent envelope proteins. Co-translational cleavage of a precursor to yield Gn and Gc, and for some viruses, NSm.
• Synthesis and encapsidation of antigenome RNA to serve as templates for genomic RNA or, in some cases, for sgRNA.
• Genome replication (Figure 3).
• Secondary transcription; i.e., the amplified synthesis of the mRNA species and ambisense transcription.
• Morphogenesis, including accumulation of Gn and Gc in the Golgi, terminal glycosylation, acquisition of modified host membranes, generally by budding into the Golgi cisternae; budding at the cell surface has been observed with isolates of Rift Valley fever virus (genus Phlebovirus) in rat hepatocytes and Sin Nombre virus (genus Hantavirus) in polarized epithelial cells. RNPs of tomato spotted wilt virus (genus Tospovirus) are enwrapped by entire Golgi stacks leading to doubly enveloped virus particles. These fuse together and also with ER and lead to the formation of large ER-derived vesicles containing large amounts of mature, singly enveloped tospovirus particles.
• Fusion of cytoplasmic vesicles with the plasma membrane (except for tospoviruses) and release of mature virions.

Antigenic properties

One or both of the envelope glycoproteins display hemagglutinating and neutralizing antigenic determinants. Complement-fixing antigenic determinants are principally associated with nucleocapsid protein.

Biological properties

Viruses in the genera Orthobunyavirus, Nairovirus and Phlebovirus are capable of alternately replicating in vertebrates and arthropods, and generally are cytolytic for their vertebrate hosts, but cause little or no cytopathogenicity in their invertebrate hosts. Different viruses are transmitted by mosquitoes, ticks, phlebotomine flies, and other arthropod vectors. Some viruses display a very narrow host range, especially for arthropod vectors. No arthropod vector has been demonstrated for hantaviruses. Tospoviruses are transmitted by thrips and are capable of replicating in both thrips and plants. Transovarial and venereal transmission have been demonstrated for some viruses in their arthropod vectors. Aerosol infection occurs in certain situations or is the principal means of transmission for some viruses, particularly hantaviruses. In some instances, avian host and/or vector movements may result in virus dissemination. Some viruses cause a reduction in host-cell protein synthesis in vertebrate cells. Hantaviruses cause no detectable reduction in host macromolecular synthesis and routinely establish persistent, non-cytolytic infections in susceptible mammalian host cells, a finding consistent with their non-pathogenic persistence in their natural rodent or insectivore hosts. In natural infections of mammals, viruses are often targeted to a particular organ or cell type. Some viruses induce cell fusion at low pH. Some members have ion-dependent hemagglutinating activity. Genetic reassortment has been demonstrated for certain members both in vitro and in vivo.

Genus Orthobunyavirus

Type species Bunyamwera virus

Distinguishing features

The consensus terminal nucleotide sequences of the L, M and S genome segments are UCAUCACAUG… at the 3′ end and AGUAGUGUGC… at the 5′ end. The N and NSs proteins are encoded in overlapping reading frames by the S RNA and are translated from the same complementary mRNA as the result of alternate AUG initiation codon usage. Both glycoproteins and an NSm protein of 15–18 kDa are encoded as a precursor polyprotein by the M RNA. Genetic reassortment has been demonstrated between viruses, belonging to the same species but not between viruses from different species.

Viruses are serologically unrelated to members of other genera. Most viruses are mosquito-transmitted though some (e.g. the Tete group) are tick-transmitted. Occasionally alternate arthropods such as culicoid flies and phlebotomines transmit orthobunyaviruses. Some viruses are transmitted transovarially and venereally in arthopods.

Electron micrograph of negatively stained particles of California encephalitis virus strain La Crosse virus. The bar represents 100 nm.

(Courtesy of D. H. L. Bishop.)

Species demarcation criteria in the genus

The demarcation of orthobunyavirus species has proven difficult due to the lack of biochemical characterization of most of the named virus isolates. Species are thus primarily defined by serological criteria (cross-neutralization and cross-hemagglutination-inhibition tests). The limited available data indicate that one bunyavirus species is unable to form a reassortant with another species. Where known the aa sequences of the N proteins differ by more than 10%.

List of species in the genus Orthobunyavirus

Species names are in italic script; names of isolates are in roman script. Vector type {}, sequence accession numbers [ ] and assigned abbreviations ( ) are also listed. N.D. = no data.

List of other related viruses which may be members of the genus Orthobunyavirus but have not been approved as species

Genus Hantavirus

Type species Hantaan virus

Distinguishing features

The consensus terminal nt sequences of the L, M and S genomic segments are AUCAUCAUCUG… at the 3′ end and UAGUAGUAUGC… at the 5′ end. Viruses are serologically unrelated to members of other genera. Certain hantaviruses are etiologic agents of hemorrhagic fever with renal syndrome or hantavirus (cardio)pulmonary syndrome (HPS, HCPS). The host range of hantaviruses includes rodents and insectivores, and genetically distinct hantaviruses are usually associated with a single host species. Human infection is incidental to viral maintenance and is almost always a dead end in the infection chain, with the exception of a human-to-human transmission of Andes virus. In contrast to other viruses in the family, hantaviruses are not transmitted by arthropods, and both rodent and human infections are acquired by aerosol exposure to infectious virus in rodent urine, faeces or saliva, and less frequently by rodent bite. Hantaviruses cause no detectable cytopathology in vertebrate cell cultures and cause persistent, non-pathogenic infections of rodents. Hantavirus infections in insectivores are not systematically studied yet.

Electron micrographs of negatively stained particles of isolates of Hantaan virus (left, courtesy of C.S. Schmaljohn) and Tula virus (right, courtesy of S. Butcher and J. Hepojöki). The bars represent 100 nm.

Species demarcation criteria in the genus

Species are usually found in unique ecological niches, i.e. in different primary rodent/insectivore reservoir species. Species exhibit at least 7% difference in aa identity on comparison of the complete glycoprotein precursor and nucleocapsid protein sequences (there are some exceptions presumably caused by historically recent host-switching events). Species show at least four-fold difference in two-way cross neutralization tests. Species do not naturally form reassortants with other species.

List of species in the genus Hantavirus

Species names are in italic script; names of isolates are in roman script. Vector type {}, sequence accession numbers [ ] and assigned abbreviations ( ) are also listed.

List of other related viruses which may be members of the genus Hantavirus but have not been approved as species

Genus Nairovirus

Type species Dugbe virus

Distinguishing features

Virions are morphologically similar to other members of the family with very small surface units that appear as a peripheral fringe 7 nm in length (Figure 6). The L RNA segment (12.2 kb) is considerably larger than the L segments of other members of the family. The consensus terminal nt sequences of the L, M and S segments are AGAGUUUCU… at the 3′ end and UCUCAAAGA… at the 5′ end. The S segment does not encode a nonstructural protein. The M segment encodes a single precursor polyprotein that is processed by cotranslational cleavage into precursors to both Gn and Gc. In addition to Gn, posttranslational cleavage of preGn yields also a mucin-rich product and a glycoprotein GP38. Posttranslational cleavage of preGc removes a polypeptide from its C-terminus and yields a mature Gc.

The L protein is predicted to be much larger than those of other members of the family but has yet to be identified. Viruses are serologically unrelated to members of other genera. Most viruses are transmitted by ticks: CCHFV, DUGV and SAKV species are transmitted mainly by ixodid ticks and DGKV, HUGV and QYBV species are transmitted mainly by argasid ticks. Some viruses are transmitted transovarially in arthropods.

Species demarcation criteria in the genus

The paucity of biochemical data dictates that nairovirus species are defined by serological reactivities. There are seven species recognized in the genus Nairovirus.

List of species in the genus Nairovirus

 Species names are in italic script; names of isolates are in roman script. Vector type {}, sequence accession numbers [ ] and assigned abbreviations ( ) are also listed.

List of other related viruses which may be members of the genus Nairovirus but have not been approved as species

None reported.

Genus Phlebovirus

Type species Rift Valley fever virus

Distinguishing features

The surface morphology of phleboviruses is distinct in having small round subunits with a central hole (Figure 7). The consensus terminal nucleotide sequences of the L, M and S segments are UGUGUUUC… at the 3′ end and ACACAAAG… at the 5′ end. The S RNA exhibits an ambisense coding strategy, i.e. it is transcribed by the virion RNA polymerase to a subgenomic virus-complementary sense mRNA that encodes the N protein and, from a full-length antigenome S RNA, to a subgenomic virus-sense mRNA that encodes a nonstructural (NSs) protein. The M segment of viruses in the sandfly fever group but not viruses in the Uukuniemi group has a preglycoprotein coding region that codes for a nonstructural protein(s) (NSm). The Gn and Gc glycoproteins were earlier referred to as G1 and G2 based on apparent size on gel electrophoresis. However, the similar sizes of the G1 and G2 proteins resulted in the different G1:G2 order in the M segments of different viruses. The further adoption of the Gn/Gc nomenclature is strongly encouraged so as to achieve more consistency across the Bunyaviridae family. Phleboviruses are antigenically unrelated to members of other genera, but cross-react serologically among themselves to different degrees. Sandfly fever group viruses are transmitted by phlebotomines, mosquitoes or ceratopogonids of the genus Culicoides; Uukuniemi group viruses are transmitted by ticks.

Species demarcation criteria in the genus

The lack of biochemical data for most phleboviruses dictates that the species are defined by the serological relationships, and are distinguishable by four-fold differences in two-way neutralization tests.

List of species in the genus Phlebovirus

 Species names are in italic script; names of isolates are in roman script. Vector type {}, sequence accession numbers [ ] and assigned abbreviations ( ) are also listed.

List of other related viruses which may be members of the genus Phlebovirus but have not been approved as species

Genus Tospovirus

Type species Tomato spotted wilt virus

Distinguishing features

Morphogenesis occurs in clusters in the cisternae of the endoplasmic reticulum of host cells. Nucleocapsid material may accumulate in the cytoplasm in dense masses; these masses may be composed of defective particles. The morphology of a tospovirus is shown in Figure 8. The consensus terminal sequences of the L, M and S genomic segments are UCUCGUUA… at the 3′ end and AGAGCAAU… at the 5′ end. Both the M and S segment RNAs of tospoviruses utilize an ambisense coding strategy. The virion glycoproteins Gn and Gc are encoded in the complementary-sense RNA of the M segment, and a nonstructural protein, NSm, is encoded in the genome-sense RNA. The S segment encodes the nucleocapsid protein in the complementary-sense RNA and a nonstructural protein, NSs, in the genome-sense RNA. The NSm protein represents the viral (cell-to-cell) movement protein, present in all plant-pathogenic viral taxa and essential for systemic infection of a plant. At the front of infection NSm assembles into tubular structures that penetrate through plasmodesmata thus facilitating nucleocapsids to translocate to the next cell. NSs represents the suppressor of (antiviral) RNAi and can bind both long dsRNA and short dsRNA (i.e. siRNA ans miRNA). In virulent isolates of tospoviruses NSs is highly expressed and may then form paracrystalline or filamentous inclusions in infected plant cells.

At least 13 species of thrips in the genera Frankliniella (9), Thrips (2), Scirtothrips (1) and Ceratothripoides (1) have been reported to transmit tospoviruses, and the Gn and/or Gc glycoproteins are involved in virus–vector interactions. Transmission can also be achieved through infected plant sap. For isolates of the type species Tomato spotted wilt virus, more than 925 plant species belonging to 70 botanical families are known to be susceptible whereas the other tospoviruses have much narrower host ranges.

Species demarcation criteria in the genus

Species are defined on the basis of their vector specificity, their plant host range, serological relationships of the N protein and on the criterion that their N protein sequence should show less than 90% aa identity with that of any other described tospovirus species.

List of species in the genus Tospovirus

Species names are in italic script; names of isolates are in roman script; names of synonyms are in roman script and parentheses. Vector type {}, sequence accession numbers [ ] and assigned abbreviations ( ) are also listed.

List of other related viruses which may be members of the genus Tospovirus but have not been approved as species

List of other related viruses which may be members of the family Bunyaviridae but have not been approved as species

There are seven groups (19 viruses) and 21 ungrouped viruses which have not been assigned to a recognized genus in the family Bunyaviridae. For most, no biochemical characterization of the viruses has been reported to determine their taxonomic status.

Phylogenetic relationships within the family

As documented above, the analogous genes and gene products of viruses in the different genera vary widely in size, and there is little obvious global similarity at either nt or aa level. Attempts to produce convincing alignments of either genome segments or structural proteins from which to generate phylogenetic trees have so far proved unsuccessful, with the exception of the putative polymerase domain of the L proteins. Such analysis suggests that viruses in the family Bunyaviridae fall into two major lineages, comprising orthobunyaviruses, hantaviruses and tospoviruses on one, and nairoviruses and phleboviruses in the other. The significant point to note is that L protein phylogeny does not segregate with the use of an ambisense coding strategy. (See Figure 9.)

Similarity with other taxa

The plant-infecting tenuiviruses show some similarities to members of the family Bunyaviridae, particularly the genus Phlebovirus. Tenuiviruses have a ssRNA genome comprising four or five segments that encode proteins using a negative or ambisense coding strategy. The tenuivirus RNA terminal sequences are conserved and the 3′ and 5′-sequences exhibit inverted complementarity; the conserved 3′-sequence, UGUGUUUCAG…, is similar to the consensus phlebovirus sequence. Tenuiviruses employ a cap-snatching mechanism to prime viral mRNA synthesis, similar to members of the family Bunyaviridae. Weak sequence homology has been noted between the Rice stripe virus 94 kDa protein and phlebovirus glycoproteins, and between tenuivirus nucleocapsid proteins and those of phleboviruses.

Derivation of names

Bunya: from Bunyamwera, place in Uganda, where type virus was isolated.

Hanta: from Hantaan, river in South Korea near where type virus was isolated.

Nairo: from Nairobi sheep disease, first reported disease caused by member virus.

Phlebo: refers to phlebotomine vectors of sandfly fever group viruses; Greek phlebos, “vein”.

Tospo: from tomato spotted wilt virus.

Further reading

Elliott, 1996 R.M. Elliott, The Bunyaviridae. In: R.M. Elliott, The Bunyaviridae. Plenum Press, New York1996.

Goldbach and Kuo, 1996 R. Goldbach, G. Kuo, Introduction (Tospoviruses and Thrips). Acta Horticult. 431 (1996) 21–26.

Karabatsos, 1985 N. Karabatsos, International Catalogue of Arboviruses Including Certain Other Viruses of Vertebrates. In: N. Karabatsos, International Catalogue of Arboviruses Including Certain Other Viruses of Vertebrates. American Society of Tropical Medicine and Hygiene, San Antonio, Texas1985.

Plyusnin, 2002 A. Plyusnin, Genetics of hantaviruses: implications to taxonomy. Arch. Virol. 147 (2002) 665–682.

Schmaljohn and Nichol, 2007 C.S. Schmaljohn, S. Nichol, D.M. Knipe, P. Howley, BunyaviridaeFields Virology. In: D.M. Knipe, P. Howley, Fields Virology. Lippincott, Williams and Wilkins, Philadelphia20071741–1789.

Schmaljohn and Nichol, 2001 Schmaljohn, C.S. and Nichol, S.T. (Eds.) (2001). Hantaviruses. Current Topics in Microbiology and Immunology 256. Berlin: Springer Verlag.

Contributed by

Plyusnin, A., Beaty, B.J., Elliott, R.M., Goldbach, R., Kormelink, R., Lundkvist, Å., Schmaljohn, C.S. and Tesh, R.B