1. Virus and Prion Diseases Research Unit, National Animal Disease Center, USDA, Agricultural Research Service, Ames, Iowa, USA
2. Blood Systems Research Institute and Department of Laboratory Medicine, University of California at San Francisco, San Francisco, California, USA
3. Infectious Bacterial Diseases Research Unit, National Animal Disease Center, USDA, Agricultural Research Service, Ames, Iowa, USA
4. Indiana Animal Disease Diagnostic Laboratory and Department of Comparative Pathobiology, Purdue University, West Lafayette, Indiana, USA