Five-week-old female outbred CD-1 mice were purchased from Charles River Laboratories (Wilmington, MA). The mice were housed, 5 mice per cage, in micro-isolator cages, in a BSL-4 biological containment laboratory with a 12-h day-night photoperiod. Virus inocula were prepared in HBSS, pH 7.4, with no supplements. Each mouse was inoculated i.p. with 0.1 ml of inoculum in a tuberculin syringe fitted with a 26-gauge, 3/8-inch needle. Moribund mice or mice pre-selected for euthanasia were anesthetized with isoflurane, whole blood was collected via cardiac puncture, and the mice were euthanatized by cervical dislocation. Liver, spleen, and brain tissues were aseptically collected immediately following euthanasia or as soon as dead mice were discovered. All experiments were performed in accordance with guidelines of the Institutional Animal Care and Use Committee of the University of Texas Medical Branch and the recommendations in the Guide for the Care and Use of Laboratory Animals (Institute of Laboratory Animal Resources, National Research Council, National Academy of Sciences, 1996). The facilities used are fully accredited by the American Association for Accreditation of Laboratory Animal Care.