1. Department of Veterinary Microbiology and Preventive Medicine1, Department of Veterinary Diagnostic and Production Animal Medicine2, and Veterinary Medical Research Institute (VMRI)3, Iowa State University, 1802 Elwood Drive, Bldg 6, Ames, IA 50011-1240, USA
2. Viral and Prion Diseases of Livestock Research Unit, National Animal Disease Center, ARS, USDA, Ames, Iowa, USA4