Flaviviridae

Flaviviridae
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Name	Flaviviridae
Caption	Schematic of a typical flavivirus virion structure
Taxon	Flaviviridae

Flaviviridae is a family of positive-sense, single-stranded RNA viruses primarily transmitted by arthropod vectors. This medically significant family includes major human pathogens responsible for global diseases such as dengue fever, Zika virus infection, and yellow fever. The family is named after the prototype yellow fever virus, with "flavi" deriving from the Latin for yellow. Research on these viruses is a major focus of institutions like the World Health Organization and the Centers for Disease Control and Prevention.

Taxonomy and classification

The family is divided into four genera: Flavivirus, Pestivirus, Hepacivirus, and Pegivirus. The genus Flavivirus contains the majority of arthropod-borne viruses, including the West Nile virus and the Japanese encephalitis virus. In contrast, members of the genera Pestivirus, such as bovine viral diarrhea virus, and Hepacivirus, most notably hepatitis C virus, are not vector-borne and primarily cause disease in animals and humans, respectively. The classification is based on genetic and antigenic relationships established through research at bodies like the International Committee on Taxonomy of Viruses.

Structure and genome

Virions are approximately 50 nm in diameter, enveloped, and have an icosahedral symmetry. The envelope contains two proteins: the membrane protein and the envelope protein, which is critical for host cell receptor binding. The genome consists of a single linear RNA molecule of roughly 9.6 to 12.3 kilobases, encoding a single long polyprotein that is co- and post-translationally cleaved by both host and viral proteases, including the NS3 protease. This process produces structural proteins like the capsid protein and non-structural proteins essential for replication.

Replication cycle

The cycle begins with virion attachment to host cell receptors, such as DC-SIGN on dendritic cells, followed by clathrin-mediated endocytosis. After uncoating in the acidic endosome, the genomic RNA is released into the cytoplasm and serves directly as mRNA for translation. Replication occurs on virus-induced membranes derived from the endoplasmic reticulum, involving the viral RNA-dependent RNA polymerase and other non-structural proteins like NS5. Assembly of new virions takes place in the endoplasmic reticulum, with maturation occurring through the trans-Golgi network before exocytosis.

Pathogenesis and diseases

These viruses cause a wide spectrum of diseases, ranging from febrile illness to severe hemorrhagic fever, encephalitis, and chronic hepatitis. Dengue virus can cause dengue hemorrhagic fever and dengue shock syndrome, while yellow fever virus targets the liver, causing jaundice. Hepatitis C virus establishes chronic infections leading to cirrhosis and hepatocellular carcinoma. Neurotropic viruses like the West Nile virus and tick-borne encephalitis virus can invade the central nervous system, causing meningitis or encephalitis, as documented in outbreaks across regions like the Mississippi River basin and Siberia.

Diagnosis and prevention

Diagnosis typically involves serological assays like ELISA to detect virus-specific IgM and IgG antibodies or molecular techniques such as reverse transcription polymerase chain reaction. Prevention strategies are centered on vector control, public health surveillance, and vaccination. Licensed vaccines exist for yellow fever, Japanese encephalitis, and tick-borne encephalitis, while a vaccine for dengue fever (Dengvaxia) has restricted use. The development of a vaccine for Zika virus is an active area of research at organizations like the National Institutes of Health.

Evolution and epidemiology

These viruses exhibit high evolutionary rates due to error-prone replication, leading to significant genetic diversity and the emergence of new variants. Their distribution is closely tied to the geographic range of their vectors, such as Aedes aegypti and Culex mosquitoes, and reservoir hosts. Factors like global warming, urbanization, and increased international travel, exemplified by events like the 2016 Summer Olympics, contribute to their spread. Major epidemics, such as the 2015-2016 Zika virus outbreak in the Americas and the ongoing global spread of hepatitis C virus, underscore their persistent public health threat.

Category:Virus families