[1] Ctr Dis Control & Prevent, Special Pathogens Branch, Div Viral & Rickettsial Dis, Atlanta, GA 30333 USA
[2] Univ Michigan, Dept Epidemiol, Ann Arbor, MI 48103 USA
[3] Univ Texas, Ctr Biodef & Emerging Infect Dis, Med Branch, Galveston, TX 77555 USA
[4] Univ Texas, Dept Pathol, Med Branch, Galveston, TX 77555 USA