Порядок РНК-вирусов Bunyavirales и их характеристики

Order of RNA viruses

Bunyavirales is an order of segmented negative strand RNA viruses with mainly tripartite genomes. Member viruses infect arthropods, plants, protozoans, and vertebrates. It is the only order in the class Ellioviricetes. 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 rodent feces. Incidence of infection is closely linked to vector activity, for example, mosquito borne viruses are more common in the summer. 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. 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.

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). Most 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.

Bunyaviruses infect arthropods, plants, protozoans, and vertebrates. and since identified in diverse insects including moths, wasps and bees, and other true flies.

There are 477 virus species recognised in this order. 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.