TOGAviruses are named for their thick lipid coat. They are generally spread by insects (primarily mosquitos), and have therefore been grouped within the Arbovirus (Arthropod-borne Virus) classification. It should be noted, however, that not all Togaviruses are spread in this way.
The Togavirus 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. As previously mentioned, the vector for Togaviridae is primarily the mosquito, where replication of the virus occurs.
There are 3 genera in the Togavirus group:
Some Alphavirus species produce a severe equine encephalitis that has a mortality rate of up to 90% in horses and 10% in humans.
Eastern Equine Encephalitis (EEE). This disease is mosquito-borne and has been reported from eastern Canada to South America. Infection can cause severe illness with high mortality/ morbidity and residual neurological damage.
Western Equine Encephalitis (WEE). This is also mosquito-borne and exists in western Canada, USA, Mexico, Argentina and Brazil. Birds function as the intermediate host; mammals are dead-end hosts in most cases. WEE infection tends to be milder but more common than EEE infection. It does not produce clinical manifestations in most mammals, but can cause illness in humans and horses, the latter of which may be severely affected.
Venezuelan Equine Encephalitis (VEE). This species of Alphavirus is found mostly in South America but has recently spread to Texas and Florida (via mosquitoes).Sindbis Virus. Like most other Togaviruses, Sindbis Virus is spread by mosquitoes but does not tend to result in clinical disease in most mammals (including humans). This species is commonly used in research.
Semliki Forest Virus (SFV). This species produces mild infection and is only used as a model for research studies.
Rubella virus, also known as German measles, is the only member of the Rubivirus group. Unlike the Alphaviral species, Rubella has no arthropod vector.
This genus causes Equine Arteritis Virus. It also has no arthropod vector.