1 Viral Special Pathogens Branch, Division of High Consequence Pathogens & Pathology, CDC, 1600 Clifton Road, Atlanta, GA 30329, USA.
2 Department of Microbiology & Immunology, University of Texas Medical Branch, Galveston, TX 77555, USA ; Galveston National Laboratory, University of Texas Medical Branch, Galveston, TX 77555, USA.