1. U.S. Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, U.S. Department of Agriculture, Agricultural Research Service, Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, Maryland 20705, and U.S. Food and Drug Administration, Center for Veterinary Medicine, Division of Animal Research, 8401 Muirkirk Road, Laurel, Maryland 20708