FIG. 2.

Histopathologic examination of SJL/J mice infected with either parent or M2 virus. At least 10 cross sections of each spinal cord from three mice per group were examined for histopathology 136 days after viral infection. A representative micrograph for each group is shown. One-micrometer-thick, Epon-embedded sections were stained with toluidine blue. Bar = 50 μm. (A) Spinal cord section from a mouse infected with the pathogenic parent virus showing severe white matter inflammation accompanied by axonal and myelin degeneration. Numerous macrophages are seen in the field. (B) Spinal cord section from a clinically healthy mouse after infection with 10⁸ PFU of M2 variant virus demonstrates normal white matter. No inflammation or demyelination is seen in the field. (C) Spinal cord section from a clinically affected mouse after infection with 10⁸ PFU of M2 variant virus shows mild to moderate white matter involvement by inflammation and demyelination. This field represents the maximum severity observed in animals infected with the variant virus.