1. Department of Virology, University of Helsinki, Helsinki 00290, Finland
2. Institute of Biodiversity, Animal Health and Comparative Medicine, University of Glasgow, Glasgow G12 8QQ, UK
3. The Moredun Research Institute, Pentlands Science Park, Penicuik, Midlothian EH26 0PZ, UK
4. Center for Infectious Disease Dynamics and Department of Biology, The Pennsylvania State University, University Park, PA 16082, USA