1. Laboratory of Virology, Rocky Mountain Laboratories, Division of Intramural Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, Montana 59840, USA;
2. Department of Medical Microbiology and Infectious Diseases, University of Manitoba, Winnipeg, Manitoba 93E 0J9, Canada
3. Rocky Mountain Veterinary Branch, Rocky Mountain Laboratories, Division of Intramural Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, Montana 59840, USA