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Bunyavirales is an order of negative-strand RNA viruses. It is the only order in the class Ellioviricetes. The name Bunyavirales derives from Bunyamwera, where the original type species Bunyamwera orthobunyavirus was first discovered. Ellioviricetes is named in honor of late virologist Richard M. Elliott for his early work on bunyaviruses.
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. In addition, there is a group of bunyaviruses whose replication is restricted to arthropods and is known as insect-specific bunyaviruses.
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 deer mice feces. Incidence of infection is closely linked to vector activity, for example, mosquito-borne viruses are more common in the summer.
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.
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). The antibody reaction plays an important role in decreasing levels of viremia.
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.
Bunyaviruses have tripartite genomes consisting of a 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).
Most of bunyaviruses have a negative-sense L and M segment. The S segment of the genus Phlebovirus, and both M and S segment of the genus Tospovirus are ambisense. 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.
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.
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.
There are currently 383 virus species recognised in this order, organized into the following 12 families:
Plants can host bunyaviruses from the families Tospoviridae and Fimoviridae (e.g. tomato, pigeonpea, melon, wheat, raspberry, redbud, and rose). Members of some families appear to be insect-specific, for example the phasmavirids, first isolated from phantom midges, and since identified in diverse insects including moths, wasps and bees, and other true flies.
Diseases in humans
Bunyaviruses that cause disease in humans include:
- California encephalitis virus, La Crosse encephalitis virus, Jamestown Canyon virus 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 mammals, domestic mammals Family: Phenuiviridae
- Bwamba Fever reservoir: monkeys vector: mosquitoes, amplifying hosts: donkeys Family: Peribunyaviridae
- Severe fever with thrombocytopenia syndrome
- 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 recombination and increasing the risk of outbreak. The bunyavirus that causes severe fever with thrombocytopenia syndrome can undergo recombination both by reassortment of genome segments and by intragenic homologous recombination. 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.
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.
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
2017: Bunyavirales order is created
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Bunya: from Bunyamwera, place in Uganda, where type virus was isolated.
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