1. USDA, Agricultural Research Service, National Animal Disease Center, Virus and Prion Research Unit, 1920 Dayton Ave, Ames, IA 50010, USA
2. USDA, Agricultural Research Service, United States Meat Animal Research Center, Animal Health Research Unit, State Spur 18D, Clay Center, NE 68933, USA
3. Department of Pathobiology, College of Veterinary Medicine, University of Illinois, 2001 South Lincoln Avenue, Urbana, IL 61802, USA