Western equine encephalitis virus (WEEV) is a naturally occurring recombinant virus derived from ancestral Sindbis and
Eastern equine encephalitis viruses. We previously showed that
infection by WEEV isolates McMillan (McM) and IMP-181 (
IMP) results in high (∼90-100%) and low (0%) mortality, respectively, in outbred CD-1 mice when virus is delivered by either subcutaneous or
aerosol routes. However, relatively little is known about specific virulence determinants of WEEV. We previously observed that
IMP infected Culex tarsalis mosquitoes at a high rate (app. 80%) following ingestion of an infected bloodmeal but these mosquitoes were infected by McM at a much lower rate (10%). To understand the viral role in these phenotypic differences, we characterized the pathogenic phenotypes of McM/
IMP chimeras. Chimeras encoding the E2 of McM on an
IMP backbone (or the reciprocal) had the most significant effect on
infection phenotypes in mice or mosquitoes. Furthermore, exchanging the
arginine, present on
IMP E2
glycoprotein at position 214, for the
glutamine present at the same position on McM, ablated mouse mortality. Curiously, the reciprocal exchange did not confer mouse virulence to the
IMP virus. Mosquito infectivity was also determined and significantly, one of the important loci was the same as the mouse virulence determinant identified above. Replacing either
IMP E2
amino acid 181 or 214 with the corresponding McM
amino acid lowered mosquito
infection rates to McM-like levels. As with the mouse neurovirulence, reciprocal exchange of
amino acids did not confer mosquito infectivity. The identification of WEEV E2
amino acid 214 as necessary for both
IMP mosquito infectivity and McM mouse virulence indicates that they are mutually exclusive phenotypes and suggests an explanation for the lack of human or equine WEE cases even in the presence of active transmission.