1. Centers for Disease Control and Prevention, Atlanta, GA 30329, USA.
2. Rocky Mountain Veterinary Branch, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT 59840, USA.
3. Gilead Sciences, Foster City, CA 94404, USA.
4. Laboratory of Virology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT 59840, USA.