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Bunyavirales
	Crimean-Congo hemorrhagic fever virus (CCHFV) virion and replication cycle
Virus classification
(unranked):	Virus
Realm:	Riboviria
Kingdom:	Orthornavirae
Phylum:	Negarnaviricota
Subphylum:	Polyploviricotina
Class:	Ellioviricetes
Order:	Bunyavirales
Families
	Arenaviridae; Cruliviridae; Fimoviridae; Hantaviridae; Leishbunyaviridae; Mypoviridae; Nairoviridae; Peribunyaviridae; Phasmaviridae; Phenuiviridae; Tospoviridae; Wupedeviridae;

Bunyavirales is an order of segmented negative-strand RNA viruses with mainly tripartite genomes. Member viruses infect arthropods, plants, protozoans, and vertebrates.^[2] It is the only order in the class Ellioviricetes.^[1] The name Bunyavirales derives from Bunyamwera,^[3] where the original type species Bunyamwera orthobunyavirus was first discovered.^[4] Ellioviricetes is named in honor of late virologist Richard M. Elliott for his early work on bunyaviruses.^[5]

Bunyaviruses belong to the fifth group of the Baltimore classification system, which includes viruses with a negative-sense, single-stranded RNA genome. They have an enveloped, spherical virion. Though generally found in arthropods or rodents, certain viruses in this order occasionally infect humans. Some of them also infect plants.^[6] In addition, there is a group of bunyaviruses whose replication is restricted to arthropods and is known as insect-specific bunyaviruses.^[7]

A majority of bunyaviruses are vector-borne. With the exception of Hantaviruses and Arenaviruses, all viruses in the Bunyavirales order are transmitted by arthropods (mosquitos, tick, or sandfly). Hantaviruses are transmitted through contact with rodent feces. Incidence of infection is closely linked to vector activity, for example, mosquito-borne viruses are more common in the summer.^[6]

Human infections with certain members of Bunyavirales, such as Crimean-Congo hemorrhagic fever orthonairovirus, are associated with high levels of morbidity and mortality, consequently handling of these viruses is done in biosafety level 4 laboratories. They are also the cause of severe fever with thrombocytopenia syndrome.^[8]

Hantaviruses are another medically important member of the order Bunyvirales. They are found worldwide, and are relatively common in Korea, Scandinavia (including Finland), Russia, western North America and parts of South America. Hantavirus infections are associated with high fever, lung edema, and pulmonary failure. The mortality rate varies significantly depending on the form, being up to 50% in New World hantaviruses (the Americas), up to 15% in Old World hantaviruses (Asia and Europe), and as little as 0.1% in Puumala virus (mostly Scandinavia).^[9] The antibody reaction plays an important role in decreasing levels of viremia.

Virology

Structure

Bunyavirus morphology is somewhat similar to that of the Paramyxoviridae family; Bunyavirales form enveloped, spherical virions with diameters of 80–120 nm. These viruses contain no matrix proteins.^[10] Instead, the viral surface glycoproteins which form a continuous layer on the virion surface are thought to play a role in the formation of new virions by budding from a cell membrane.^[11]

Genome

Bunyaviruses have bi- or tripartite genomes consisting of a large (L) and small(s), or large (L), medium (M), and small (S) RNA segment. These RNA segments are single-stranded, and exist in a helical formation within the virion. Besides, they exhibit a pseudo-circular structure due to each segment's complementary ends. The L segment encodes the RNA-dependent RNA polymerase, necessary for viral RNA replication and mRNA synthesis. The M segment encodes the viral glycoproteins, which project from the viral surface and aid the virus in attaching to and entering the host cell. The S segment encodes the nucleocapsid protein (N).^[12]

Most bunyaviruses have a negative-sense L and M segment. The S segment of the genus Phlebovirus,^[13] and both M and S segment of the genus Tospovirus are ambisense.^[14] Ambisense means that some of the genes on the RNA strand are negative sense and others are positive sense. The ambisense S segment codes for the viral nucleoprotein (N) in the negative sense and a nonstructural protein (NSs) in the positive sense. The ambisense M segment codes for glycoprotein (GP) in the negative sense and a nonstructural protein (NSm) in the positive sense.^[14]

The total genome size ranges from 10.5 to 22.7 kbp.^[15]

Life cycle

The ambisense genome requires two rounds of transcription to be carried out. First, the negative-sense RNA is transcribed to produce mRNA and a full-length replicative intermediate. From this intermediate, a subgenomic mRNA encoding the small segment nonstructural protein is produced while the polymerase produced following the first round of transcription can now replicate the full-length RNA to produce viral genomes.^{[citation needed]}

Bunyaviruses replicate in the cytoplasm, while the viral proteins transit through the ER and Golgi apparatus. Mature virions bud from the Golgi apparatus into vesicles which are transported to the cell surface.^{[citation needed]}

Transmission

Bunyaviruses infect arthropods, plants, protozoans, and vertebrates.^[2] Plants can host bunyaviruses from the families Tospoviridae and Fimoviridae (e.g. tomato, pigeonpea, melon, wheat, raspberry, redbud, and rose). Members of some families are insect-specific, for example the phasmavirids, first isolated from phantom midges,^[16] and since identified in diverse insects including moths, wasps and bees, and other true flies.

Taxonomy

There are 477 virus species recognised in this order.^[1] The phylogenetic tree diagram provides a full list of member species and the hosts which they infect.^[2] The order is organized into the following 12 families:^[1]

Diseases in humans

Bunyaviruses that cause disease in humans include:

California encephalitis virus, Jamestown Canyon virus, La Crosse encephalitis virus, Oropouche orthobunyavirus, and Snowshoe hare virus (vector: mosquitoes; family: Peribunyaviridae);
Hantavirus reservoir: small mammals or rodents (vector: aerosolized excreta from these mammals; family: Hantaviridae);
Crimean–Congo hemorrhagic fever reservoir and (vector: ticks; amplifying hosts and vector: small mammals, domestic mammals; family: Nairoviridae);
Rift Valley fever (reservoir: bats; vector: mosquitoes; amplifying hosts: small and domestic mammals; family: Phenuiviridae);
Bwamba Fever (reservoir: monkeys; vector: mosquitoes; amplifying hosts: donkeys; family: Peribunyaviridae);
Severe fever with thrombocytopenia syndrome (vector: ticks);
Lassa fever and Argentine hemorrhagic fever (reservoir: rodents; vector: aerosolized excreta from these mammals; family: Arenaviridae).

Bunyaviruses have segmented genomes, making them capable of rapid reassortment and increasing the risk of outbreak.^[17]^[18] The bunyavirus that causes severe fever with thrombocytopenia syndrome can undergo recombination both by reassortment of genome segments and by intragenic homologous recombination.^[19]^[20] Bunyaviridae are transmitted by hematophagous arthropods including mosquitoes, midges, flies, and ticks. The viral incubation period is about 48 hours. Symptomatic infection typically causes non-specific flu-like symptoms with fever lasting for about three days. Because of their non-specific symptoms, Bunyavirus infections are frequently mistaken for other illnesses. For example, Bwamba fever is often mistaken for malaria.^[21]

Prevention

Prevention depends on the reservoir, amplifying hosts and how the viruses are transmitted, i.e. the vector, whether ticks or mosquitoes and which animals are involved. Preventive measures include general hygiene, limiting contact with vector saliva, urine, feces, or bedding. There is no licensed vaccine for bunyaviruses. As precautions Cache Valley virus and Hantavirus research are conducted in BSL-2 (or higher), Rift Valley Fever virus research is conducted in BSL-3 (or higher), Congo-Crimean Hemorrhagic Fever virus research is conducted in BSL-4 laboratories.^{[citation needed]}

Timeline

1940s: Crimean–Congo hemorrhagic fever is discovered in Russia

1951: 3,000 cases of Hantavirus were reported in South Korea in 1951, a time when UN forces were fighting on the 38th parallel during the Korean War

1956: Cache Valley virus isolated in Culiseta inornata mosquitoes in Utah

1960: La Crosse virus was first recognized in a fatal case of encephalitis in La Crosse, Wisconsin

1977: Rift Valley Fever virus caused approximately 200,000 cases and 598 deaths in Egypt

2017: Bunyavirales order is created

References

^ ^a ^b ^c ^d "Virus Taxonomy: 2020 Release". International Committee on Taxonomy of Viruses (ICTV). March 2021. Retrieved 19 May 2021.
^ ^a ^b ^c Herath, Venura; Romay, Gustavo; Urrutia, Cesar D.; Verchot, Jeanmarie (September 2020). "Family Level Phylogenies Reveal Relationships of Plant Viruses within the Order Bunyavirales". Viruses. 12 (9): 1010. doi:10.3390/v12091010. PMC 7551631. PMID 32927652.
^ "ICTV 9th Report (2011) Bunyaviridae". International Committee on Taxonomy of Viruses (ICTV). Retrieved 31 January 2019. Bunya: from Bunyamwera, place in Uganda, where type virus was isolated.
^ Smithburn, K. C.; Haddow, A. J.; Mahaffy, A. F. (March 1946). "A Neurotropic Virus Isolated from Aedes Mosquitoes Caught in the Semliki Forest". The American Journal of Tropical Medicine and Hygiene. s1-26 (2): 189–208. doi:10.4269/ajtmh.1946.s1-26.189. ISSN 1476-1645. OCLC 677158400. PMID 21020339.
^ Wolf, Yuri; Krupovic, Mart; Zhang, Yong Zhen; Maes, Piet; Dolja, Valerian; Koonin, Eugene V.; Kuhn, Jens H. "Megataxonomy of negative-sense RNA viruses" (docx). International Committee on Taxonomy of Viruses (ICTV). Retrieved 12 January 2019.^{[dead link‍]}
^ ^a ^b Plyusnin, A; Elliott, RM, eds. (2011). Bunyaviridae: Molecular and Cellular Biology. Caister Academic Press. ISBN 978-1-904455-90-5.
^ Elrefaey, Ahmed ME; Abdelnabi, Rana; Rosales Rosas, Ana Lucia; Wang, Lanjiao; Basu, Sanjay; Delang, Leen (September 2020). "Understanding the Mechanisms Underlying Host Restriction of Insect-Specific Viruses". Viruses. 12 (9): 964. doi:10.3390/v12090964. PMC 7552076. PMID 32878245.
^ Yu XJ, Liang MF, Zhang SY, et al. (April 2011). "Fever with thrombocytopenia associated with a novel bunyavirus in China". N. Engl. J. Med. 364 (16): 1523–32. doi:10.1056/NEJMoa1010095. PMC 3113718. PMID 21410387.
^ Walter Muranyi; Udo Bahr; Martin Zeier; Fokko J. van der Woude (2005). "Hantavirus Infection". Journal of the American Society of Nephrology. 16 (12): 3669–3679. doi:10.1681/ASN.2005050561. PMID 16267154.
^ "Bunyaviridae - Negative Sense RNA Viruses - Negative Sense RNA Viruses (2011)". International Committee on Taxonomy of Viruses (ICTV). Retrieved 2020-09-08.
^ Huiskonen JT, Hepojoki J, Laurinmäki P, Vaheri A, Lankinen H, Butcher SJ; et al. (2010). "Electron cryotomography of Tula hantavirus suggests a unique assembly paradigm for enveloped viruses". J Virol. 84 (10): 4889–97. doi:10.1128/JVI.00057-10. PMC 2863824. PMID 20219926.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Ariza, A.; Tanner, S. J.; Walter, C. T.; Dent, K. C.; Shepherd, D. A.; Wu, W.; Matthews, S. V.; Hiscox, J. A.; Green, T. J. (2013-06-01). "Nucleocapsid protein structures from orthobunyaviruses reveal insight into ribonucleoprotein architecture and RNA polymerization". Nucleic Acids Research. 41 (11): 5912–5926. doi:10.1093/nar/gkt268. ISSN 0305-1048. PMC 3675483. PMID 23595147.
^ Elliott, Richard M; Brennan, Benjamin (April 2014). "Emerging phleboviruses". Current Opinion in Virology. 5 (100): 50–57. doi:10.1016/j.coviro.2014.01.011. PMC 4031632. PMID 24607799.
^ ^a ^b Lima, R. N.; De Oliveira, A. S.; Leastro, M. O.; Blawid, R.; Nagata, T.; Resende, R. O.; Melo, F. L. (7 July 2016). "The complete genome of the tospovirus Zucchini lethal chlorosis virus". Virology Journal. 13 (1): 123. doi:10.1186/s12985-016-0577-4. PMC 4936248. PMID 27388209.
^ "00.011. Bunyaviridae". ICTVdB—The Universal Virus Database, version 4. 2006. Retrieved 2009-01-01.
^ Ballinger, MJ; Bruenn, JA; Hay, J; Czechowski, D; Taylor, DJ (2014). "Discovery and evolution of bunyavirids in arctic phantom midges and ancient bunyavirid-like sequences in insect genomes". J Virol. 88 (16): 8783–94. doi:10.1128/JVI.00531-14. PMC 4136290. PMID 24850747.
^ Briese, Thomas; Calisher, Charles H.; Higgs, Stephen (November 2013), "Viruses of the family Bunyaviridae: Are all available isolates reassortants?", Virology, 446 (1–2): 207–216, doi:10.1016/j.virol.2013.07.030, hdl:2097/17679, PMID 24074583
^ Horne, Kate McElroy; Vanlandingham, Dana L. (2014-11-13). "Bunyavirus-Vector Interactions". Viruses. 6 (11): 4373–4397. doi:10.3390/v6114373. ISSN 1999-4915. PMC 4246228. PMID 25402172.
^ Lv Q, Zhang H, Tian L, Zhang R, Zhang Z, Li J, Tong Y, Fan H, Carr MJ, Shi W. Novel sub-lineages, recombinants and reassortants of severe fever with thrombocytopenia syndrome virus. Ticks Tick Borne Dis. 2017 Mar;8(3):385-390. doi: 10.1016/j.ttbdis.2016.12.015. Epub 2017 Jan 3. PMID 28117273
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