1. Departments of Anatomy and Physiology, College of Veterinary Medicine, Kansas State University, Manhattan, KS, USA
2. USDA, Agricultural Research Service, National Animal Disease Center, Virus and Prion Research Unit, 1920 Dayton Avenue, Ames, IA 50010, USA
3. Present address: Department of Agricultural and Environmental Sciences, College of Agriculture, Human and Natural Sciences, Tennessee State University, 3500 John A. Merritt Boulevard, Nashville, TN, USA.