1. Division of Viral Products, Center for Biologics Evaluation and Research, U.S. Food and Drug Administration, Bethesda, MD 20892, USA.
2. Virus and Prion Research Unit, National Animal Disease Center, Agricultural Research Service, U.S. Department of Agriculture, Ames, IA 50010, USA.
3. Department of Veterinary Diagnostic and Production Animal Medicine, Ames, IA 50010, USA.
4. Kansas State Veterinary Diagnostic Laboratory, Manhattan, KS 66506, USA.