Bunyaviridae

Hantavirus Nairovirus Orthobunyavirus Phlebovirus Tospovirus

Bunyaviridae is a family of negative-stranded RNA viruses. Though generally found in arthropods or rodents, certain viruses in this family occasionally infect humans.

Bunyaviridae are vector-borne viruses. With the exception of Hantaviruses, transmission occurs via an arthropod vector (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 Bunyaviruses, such as Crimean-Congo Hemorrhagic Fever virus, are associated with high levels of morbidity and mortality, consequently handling of these viruses must occur with a Biosafety level 4 laboratory.

Hanta virus or Hantavirus Hemorrhagic fever, common in Korea, Scandinavia, Russia, and the American southwest, is associated with high fever, lung edema and pulmonary failure. Mortality is around 55%.

The antibody reaction plays an important role in decreasing levels of virema.

The family Bunyaviridae contains the genera:
 * Genus Hantavirus; type species: Hantaan virus (Hantavirus pulmonary syndrome, Korean hemorrhagic fever)
 * Genus Nairovirus; type species: Dugbe virus
 * Genus Orthobunyavirus; type species: Bunyamwera virus
 * Genus Phlebovirus; type species: Rift Valley fever virus
 * Genus Tospovirus; type species: Tomato spotted wilt virus

Of these genera, all infect vertebrates except Tospoviruses, which only infect arthropods and plants.

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

Genome
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).

The L and M segment are negative sense. For the Genera of Phlebovirus and Tospovirus, the S segment is ambisense. Ambisense means that some of the proteins on the RNA strand are negative sense. The S segment codes for the viral nucleoprotein (N) in the negative sense and a nonstructural (NSs) protein in ambisense.

Total genome size ranges from 11-19 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 lengh of 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.

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