As expected, mice infected with parental demyelinating strain MHV-A59 at days 5 to 7 pi had optic nerve inflammation ( Fig. 1C) , whereas optic nerves from mice infected with parental nondemyelinating strain MHV-2 did not (Fig. 1B) ; their optic nerves appeared similar to those of mock-infected mice (Fig. 1A) (8) . The inability of MHV-2 to induce optic neuritis was expected because MHV-2 does not infect the brain parenchyma and is unable to induce encephalitis (13, 14) . By contrast, the RSMHV-2 strain induces encephalitis (2, 4, 5) . RSA59 induced optic neuritis similar to the parental MHV-A59 strain at days 5 to 7 pi (Fig. 1F) , whereas most eyes of RSMHV2-infected mice exhibited no optic nerve inflamma-tion (Fig. 1D) ; however, there was mild inflammation in a few optic nerves (Fig. 1E) .