1. Center for Infection and Immunity, Mailman School of Public Health, Columbia University, New York, New York, USA;
2. 454 Life Sciences, Branford, Connecticut, USA;
3. Division of Viral and Rickettsial Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, USA; and
4. Department of Veterinary Surgery and Medicine, Ahmadu Bello University, Zaria, Nigeria