Flavivirus

Flavivirus is a genus of viruses in the family Flaviviridae. This genus includes the West Nile virus, dengue virus, tick-borne encephalitis virus, yellow fever virus, Zika virus and several other viruses which may cause encephalitis, as well as insect-specific flaviviruses (ISFs) such as cell fusing agent virus (CFAV), Palm Creek virus (PCV), and Parramatta River virus (PaRV). Flaviviruses are named from the yellow fever virus, the type virus for the family; the word flavus means 'yellow' in Latin and the yellow fever in turn is named from its propensity to cause yellow jaundice in victims. Flaviviruses share several common aspects: common size (40–65 nm), symmetry (enveloped, icosahedral nucleocapsid), nucleic acid (positive-sense, single-stranded RNA around 10,000–11,000 bases), and appearance in the electron microscope. Most of these viruses are transmitted by the bite from an infected arthropod (mosquito or tick), and hence are classified as arboviruses. Human infections with most of these arboviruses are incidental, as humans are unable to replicate the virus to high enough titers to reinfect the arthropods needed to continue the virus lifecycle – humans are then a dead end host. The exceptions to this are the yellow fever, dengue, and zika viruses. These three viruses still require mosquito vectors, but are well-enough adapted to humans as to not necessarily depend upon animal hosts (although they continue to have important animal transmission routes, as well). Other virus transmission routes for arboviruses include handling infected animal carcasses, blood transfusion, child birth and consumption of unpasteurised milk products. Transmission from nonhuman vertebrates to humans without an intermediate vector arthropod is thought to be unlikely. For example, early tests with yellow fever showed that the disease is not contagious. The known non-arboviruses of the flavivirus family reproduce in either arthropods or vertebrates, but not both, with one odd member of the genus affecting a nematode. Viruses in Flavivirus are enveloped, with icosahedral and spherical geometries. The diameter is around 50 nm. Genomes are linear positive-sense RNA and non-segmented, around 10–11kb in length. Entry into the host cell is achieved by attachment of the viral envelope protein E to host receptors, which mediates clathrin-mediated endocytosis. Replication follows the positive stranded RNA virus replication model. Positive stranded RNA virus transcription is the method of transcription. Humans, mammals, mosquitoes, and ticks serve as the natural host. Transmission routes are zoonosis and bite.