1. Department of Population Health, College of Veterinary Medicine, University of Georgia, Athens, Georgia, USA
2. Virus and Prion Diseases Research Unit, ARS-USDA, Ames, Iowa, USA
3. Department of Veterinary and Biomedical Sciences, Penn State University, University Park, Pennsylvania, USA
4. Veterinary Diagnostic & Production Animal Medicine, College of Veterinary Medicine, Iowa State University, Ames, Iowa, USA