Molecular evidence for the origin of the widespread Venezuelan equine encephalitis epizootic of 1969 to 1972.

Venezuelan equine encephalitis (VEE) virus is a mosquito-borne pathogen that has caused encephalitis in equine species and humans during sporadic outbreaks in the western hemisphere. The last, and most widespread, VEE outbreak occurred in South America, Central America, Mexico and the U.S.A. (Texas) during 1969 to 1972. We have cloned and sequenced the genome of a virulent VEE subtype I-AB virus, strain 71-180, isolated in Texas in 1971. Thirty-four nucleotide differences were detected between the genome of 71-180 virus and that of the subtype I-AB Trinidad donkey (TRD) virus isolated during the 1943 VEE epizootic in Trinidad. Fifteen nucleotide changes occurred in the non-structural genes, 16 in the structural genes and three in the 3' non-coding region. Only six of the nucleotide differences resulted in amino acid substitutions: one change in each of non-structural proteins nsP1 and nsP3, two in the E2 envelope glycoprotein, one in the 6K polypeptide and one in the E1 envelope glycoprotein. The close genetic relationship between 71-180 virus and TRD virus, commonly used for production of formalin-inactivated VEE vaccines, suggests that incompletely inactivated virulent vaccine virus may have been the source of this and other VEE outbreaks. Use of formalized virulent virus was discontinued during the 1969 to 1972 panzootic. No VEE epizootics have been reported since the introduction of the live attenuated TC-83 vaccine virus.