1. Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, USA
2. University of Minnesota Veterinary Diagnostic Laboratory, St Paul, MN 55108, USA
3. Virus and Prion Diseases Research Unit, National Animal Disease Center, USDA-ARS, Ames, IA 50010, USA
4. Center for Infectious Disease Dynamics, Department of Biology, The Pennsylvania State University, University Park, PA 16802, USA