For the measurement of virus replication in the brain and liver, at 5 days postinfection, mice were sacrificed and perfused with 10 ml of PBS, and brains and livers were removed. The left half of the brain and the liver were placed directly into 2 ml of isotonic saline with 0.167% gelatin (gel saline). The right half of the brain was used for histology and viral antigen staining as described below. All organs were weighed and stored frozen at −80°C until determination of virus titers. Organs were homogenized, and virus titers were determined by plaque assay on L2 cell monolayers (6).