Bunyavirales


Bunyavirales is an order of negative-sense single-stranded RNA viruses. It is the only order in the class Ellioviricetes. It was formerly known as Bunyaviridae family of viruses. 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.
In 2017, the ICTV reclassified the family Bunyaviridae as Bunyavirales, a taxonomic shift from a family of viruses to an order of viruses. The body made these decisions in a 2016 convening in Budapest. Primary reasons for this alteration revolve around these observations: approximately half of viruses in the former Bunyaviridae were at the time unassigned to a genus; novel viruses discovered that were characteristic of and clustered around Bunyaviridae based on phylogenetic analyses had bi-segmented genomes ; and plant viruses also lacking tri-segmentation were previously known to be "bunya-like" yet were not properly assigned to the family Bunyaviridae based upon the past taxonomic classifications. All five genera formerly in the family Bunyaviridae are now novel viral families, some of which have been combined. These new families include: Hantaviridae, Feraviridae, Fimoviridae, Jonviridae, Nairoviridae, Peribunyaviridae, Phasmaviridae, Phenuiviridae, and Tospoviridae.
This order of viruses belong to the fifth group of the Baltimore classification, the so-called negative-sense single-stranded ribonucleic acid ssRNA. They have enveloped RNA viruses. Though generally found in arthropods or rodents, certain viruses in this order occasionally infect humans. Some of them also infect plants.
A majority of bunyaviruses are vector-borne. With the exception of Hantaviruses and Arenaviruses, all viruses in the Bunyavirales order are transmitted by arthropods. 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 must occur with a Biosafety level 4 laboratory. They are also the cause of severe fever with thrombocytopenia syndrome.
Hantavirus is another medically-important member of the family Hantaviridae. It is found worldwide, and relatively common in Korea, broader Scandinavia, Russia, western North America and parts of South America. It is 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, up to 15% in Old World hantaviruses, and as little as 0.1% in Puumala virus. The antibody reaction plays an important role in decreasing levels of viremia.

Virology

Classification

There are currently 383 virus species recognised in this order, organized into the following 12 families:
Plants can host bunyaviruses from the Tospoviridae and Fimoviridae families.
Members of some families appear to be insect-specific, e.g. the phasmavirids, first isolated from phantom midges, and since identified in diverse insects including moths, wasps and bees, and other true flies.

Structure

Bunyavirus morphology is somewhat similar to that of the Paramyxoviridae family; Bunyavirales form enveloped, spherical virions with diameters of 90–100 nm. These viruses contain no matrix proteins.

Genome

Bunyaviridae have tripartite genomes consisting of a large, medium, and small 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.
Most of the Bunyaviridae have 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 in the negative sense and a nonstructural protein in the positive sense. The ambisense M segment codes for glycoprotein in the negative sense and a nonstructural protein in the positive sense.
Total genome size ranges from 10.5 to 22.7 kbp.

Replication

This ambisense arrangement 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.
Bunyavirus RNA replicates 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.

Diseases in humans

Bunyaviruses that cause disease in humans include:
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

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 should be conducted in BSL-2, Rift Valley Fever virus research is conducted in BSL-3, Congo-Crimean Hemorrhagic Fever virus research be conducted in BSL-4 laboratories, per CDC.

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