1. From the National Animal Disease Center, USDA, ARS, PO Box 70, Ames, IA 50010 (Briggs, Frank, Zehr); the Conservation and Production Research Laboratory, USDA, ARS, Bushland, TX 79012 (Purdy); and the Department of Veterinary Pathobiology, Texas A&M University, College Station, TX 77843 (Loan).