Togaviruses: General Features

The Togaviridae is a family of enveloped, single-stranded, plus-strand RNA viruses that occur nearly worldwide. Viruses in this family include those of the genus Alphavirus, a group of arthropod- (principally mosquito) borne, zoonotic viruses maintained primarily in rodents, primates, and birds by mosquito vectors. The alphaviruses can cause a febrile illness accompanied by one of two disease syndromes: encephalitis, or arthritis accompanied with rash. Human infections generally occur when people intrude on enzootic alphavirus transmission habitats and are fed upon by infected mosquitoes, or when alphaviruses emerge to cause epizootics and epidemics via host range changes. The other genus in the family Togaviridae is the distantly related Rubivirus, with Rubella virus, the etiologic agent of 3-day measles (German measles), as its only member. Most alphavirus complexes are restricted in their distribution to either the Old or New World, and host mobility appears to regulate the genetic and antigenic diversity within alphaviruses. The evolution of alphaviruses is dominated by purifying selection, presumably reflecting strong constraints imposed by their transmission cycles, which require alternate replication in arthropods and vertebrates. There is currently no licensed vaccine or effective treatment for alphaviral infections, but a live vaccine has dramatically reduced Rubella virus infections in many regions of the world.