The Togaviridae are a family of viruses, including the following genera:
The Togaviridae family belong to group IV of the Baltimore classification of viruses. The genome is linear, single-stranded, positive sense RNA that is 10,000-12,000 nucleotides long. The 5'-terminus carries a methylated nucleotide cap and the 3'-terminus has a polyadenylated tail, therefore resembling cellular mRNA. The virus is enveloped and forms spherical particles (65-70 nm diameter), the capsid within is icosahedral, constructed of 240 monomers, having a triangulation number of 4. The receptors for binding are unknown, however the tropism is varied and it is known that the glycoprotein spikes act as attachment proteins. After virus attachment and entry into the cell, gene expression and replication takes place within the cytoplasm. The vector for Togaviridæ is primarily the mosquito, where replication of the virus occurs.
[edit] Genome replication
The non-structural proteins are encoded at the 5’ end, formed during the first of two characteristic rounds of translation. These proteins are originally translated as a polyprotein, which consequently undergo self cleavage, forming four non-structural proteins responsible for gene expression and replication. The formation of a sub-genomic fragment, encoding the structural proteins and a negative sense fragment, a template for further synthesis of positive sense RNA are the characteristic second phase of translation. Assembly takes place at the cell surface, where the virus buds from the cell, acquiring the envelope. The replication cycle is very fast, taking around 4 hours.
[edit] History
Initially the Togavirus family included what are now called the Flaviviruses, within the Alphavirus genus. The Flaviviruses were formed into their own family when sufficient differences with the Alphaviruses were noted.
- Early 19th century - Rubella is identified as a distinct disease
- 1941 - Western Equine Encephalitis epidemic is seen in the United States. It affects 300,000 horses and 3,336 humans.
- 1963 - Ross River virus, which causes epidemic polyarthritis (mostly seen in Australia), is isolated by Doherty and colleagues.
- 1964 - The last major epidemic of Rubella in the United States is seen. Approximately 20,000 infants are left with permanent damage following in-utero rubella exposure.
- 1971 - Last epidemic of Venezuelan equine encephalitis is seen in horses in southern Texas.
- 1991-92 - Most recent worldwide epidemic of rubella. Probably due to vaccine failures and missed vaccinations.
- 2006 - Major epidemic of the chikungunya virus in India with over 1.5 million cases reported.
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