1. Centers for Disease Control and Prevention, Viral Special Pathogens Branch, 1600 Clifton Rd. NE, Atlanta, Georgia 30333, USA
2. University of Georgia, College of Veterinary Medicine, 501 D. W. Brooks Drive, Athens, Georgia 30602, USA
3. Current address: San Diego Zoo Institute for Conservation Research, 15600 San Pasqual Valley Road, Escondido, California 92027, USA
4. Current address: Oregon Health and Science University, Vaccine and Gene Therapy Institute, 505 NW 185th Ave, Beaverton, Oregon 97006, USA