Togaviridae is a family of RNA viruses. Humans, mammals, birds, and mosquitoes serve as natural hosts. Currently, 31 species are placed in this family in a single genus, Alphavirus. Diseases associated with alphaviruses include arthritis and encephalitis. 
The Togaviridae family belongs to group IV of the Baltimore classification of viruses. The genome is linear, nonsegmented, 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.
|Genus||Structure||Symmetry||Capsid||Genomic arrangement||Genomic segmentation|
Entry into the host cell is achieved by attachment of the viral E glycoprotein to host receptors, which mediates clathrin-mediated endocytosis. The receptors for binding are unknown, but the tropism is varied and the glycoprotein petal-like spikes are known to act as attachment proteins. After virus attachment and entry into the cell, gene expression and replication takes place within the cytoplasm.
Replication follows the positive-stranded RNA virus replication model. Positive-stranded RNA virus transcription is the method of transcription. Translation takes place by viral initiation, and suppression of termination. The vector for the Togaviridae is primarily the mosquito, where replication of the virus occurs. The family Togaviridae is classified into Old World and New World viruses based on geographical distribution, although a few transoceanic crossings likely have occurred. Human, mammals, marsupials, birds, and mosquitoes serve as the natural host. Transmission routes are zoonosis, bite, and respiratory.
|Genus||Host details||Tissue tropism||Entry details||Release details||Replication site||Assembly site||Transmission|
|Alphavirus||Humans; mammals; marsupials; birds; mosquitoes||None||Clathrin-mediated endocytosis||Secretion||Cytoplasm||Cytoplasm||Zoonosis: arthropod bite|
The nonstructural 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 nonstructural proteins responsible for gene expression and replication. The formation of P1234 occurs due to a stop codon read-through, which has a 10% to 20% chance of occurrence. Autoproteolytic activity of P2 enables cis cleavage of P1234 between P3 and P4, forming a P123 polyprotein and a nsP4 protein. The nsP4 protein is an RNA-dependent RNA polymerase and promotes further negative-strand RNA synthesis. This leads to the accumulation of P123 over time and once the concentration is high enough, all the other proteins are cleaved by the trans proteolytic activity of P2. Finally, the N-terminal of nsP4 is able to act as a scaffold and forms a complex with nsP1, nsP2, and nsP3 that then causes plus strand synthesis.
The formation of a subgenomic(dna)fragment, encoding the structural proteins and a negative-sense fragment, a template for further synthesis of positive-sense RNA, is 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.
Initially, the Togaviridae 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 due to the development of sequencing. Rubella virus was formerly included in the family Togaviridae in its own genus Rubivirus, but is now classified in its own family Matonaviridae
Note that 'Ochlerotatus vigilax' prior to 2000, was known as 'Aedes vigilax'