are limitations to the use of alphaviruses in the field of gene therapy due to their lack of targeting, however, through the introduction of variable antibody domains in a non-conserved loop in the structure of e2, specific populations of cells have been targeted. furthermore, the use of whole alphaviruses for gene therapy is of limited efficacy both because several internal alphaviral proteins are involved in the induction of apoptosis upon infection and also because the alphaviral capsid mediates only the transient introduction of mrna into host cells. neither of these limitations extend to alphaviral envelope pseudotypes of retroviruses or lentiviruses. however, the expression of sindbis virus envelopes may lead to apoptosis, and their introduction into host cells upon infection by sindbis virus envelope pseudotyped retroviruses may also lead to cell death. the toxicity of sindbis viral envelopes may be the cause of the very low production titers realized from packaging cells constructed to produce sindbis pseudotypes. another branch of research involving alphaviruses is in vaccination. alphaviruses are apt to be engineered to create replicon vectors which efficiently induce humoral and t-cell immune responses. they could therefore be used to vaccinate against viral, bacterial, protozoan, and tumor antigens. history 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. alphavirus is now the sole genus in the family. 1930 – western equine encephalitis virus is first isolated in the united states (the first alphavirus ever isolated) 1933 – eastern equine encephalitis virus is first isolated in the united states. 1938 – venezuelan equine encephalitis is isolated. 1941 – western equine encephalitis epidemic is seen in the united states. it affects 300,000 horses and 3,336 humans. 1941 – norman gregg notices large number of children with cataracts following a rubella outbreak. this and other defects are then categorized under the congenital rubella syndrome. 1942 – semliki forest virus is isolated in buliyama, bwamba county, uganda. 1952 – sindbis virus is isolated in the sindbis health district, 40 mi (64 km) north of cairo, egypt. 1959 – ross river virus is isolated from aedes vigilax mosquitoes (now known as ochlerotatus vigilax) which were trapped at the ross river in australia. 1963 – ross river virus, which causes epidemic polyarthritis (mostly seen in australia), is isolated by doherty and colleagues. 1971 – last epidemic of venezuelan equine encephalitis is seen in horses in southern texas. 1986 – barmah forest virus is identified as causing human disease in australia. 2001 – scientists solved the crystal structure of the glycoprotein shell of the semliki forest virus. 2005–2006 – large epidemic of the chikungunya virus on the island of la réunion and the surrounding islands in the indian ocean 2006 – major epidemic of the chikungunya virus in india with over 1.5 million cases reported