1. Office of the Director, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
2. Department of Veterinary Medicine, VA-MD College of Veterinary Medicine, University of Maryland, College Park, MD 20742, USA.
3. Viral Pathogenesis and Evolution Section, Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.