Comparative neurovirulence and tissue tropism of wild-type and attenuated strains of Venezuelan equine encephalitis virus administered by aerosol in C3H/HeN and BALB/c mice

To assess the potential for aerosol administration of vaccines for Venezuel an equine encephalitis virus (VEE), we compared the neurovirulence and tiss ue tropism of the wild-type Trinidad donkey (TrD) strain to those of the at tenuated TC83 and V3526 strains of VEE in mice. Six to 8-week-old female C3 H/HeN and BALB/c mice were aerosol exposed to one of the three VEE strains. Three mice of each strain were euthanatized at different times and their t issues were processed and stained using hematoxylin and eosin, immunohistoc hemistry, and in situ hybridization. All three viral strains infected the b rains of mice and induced encephalitis. TrD spread caudally from the olfact ory bulbs to all regions of the brain, caused widespread necrotizing panenc ephalitis by day 5, and resulted in 100% mortality (geometric mean = 7 days ) in both mouse strains. By comparison, TC83 relatively spared the caudal r egions of the brain but still caused 100% mortality in the C3H/HeN mice (ge ometric mean = 12 days), yet it did not kill any BALB/c mice. V3526 infecti vity of the brain was the most limited, mainly affecting the neocortex and diencephalon. This virus was not lethal in either mouse strain. The TrD str ain also infected the olfactory neuroepithelium, local lymphoid tissues, te eth, and vomeronasal organs, whereas the affinity of TC83 and V3526 outside the brain was essentially limited to the olfactory neuroepithelium. Attenu ated VEE strains administered to mice by aerosol have restricted tissue tro pism as compared with wild-type virus; however, even attenuated strains can infect the brain and induce encephalitis.