1. Respiratory and Neurologic Disease Unit, National Animal Disease Center, USDA, Agricultural Research Service, PO Box 70, Ames, IA 50010
2. National Veterinary Services Laboratories, USDA, Animal and Plant Health Inspection Service, Veterinary Services, PO Box 844, Ames, IA 50010
3. South Dakota Animal Industry Board, Area Veterinary Medical Officer, 5524 Canyon Trail, Piedmont, SD 57769
4. Department of Veterinary Pathology, University of Georgia, College of Veterinary Medicine, Athens, GA 30602