Alphaviruses Causing Encephalitis (Togaviridae)

Encephalitic alphaviruses are maintained in natural cycles between vertebrate (birds or small mammals) and invertebrate (mosquito) hosts in the Americas. North American eastern equine encephalitis virus (EEEV) causes severe encephalitis, while South American EEEV (Madariaga), western equine encephalitis and Highlands J viruses cause encephalitis that is less severe. Enzootic strains of Venezuelan equine encephalitis virus (VEEV) cause little disease, but epizootic strains cause equine encephalitis and human disease. These alphaviruses replicate in muscle or lymphoid tissue with spread to the nervous system where neurons are infected. VEEV also causes respiratory and reticuloendothelial infection and can be transmitted by aerosol, as well as mosquitoes.