1. Department of Animal Science, Colorado State University, Fort Collins 80523-1171
2. USDA-ARS-Livestock Issues Research Unit, Lubbock, TX 79403
3. Immunology and Disease Resistance Laboratory, USDA-ARS, Beltsville, MD 20705
4. Intervet-Schering Plough Animal Health, De Soto, KS 66018
5. Department of Microbiology, Immunology, and Pathology, Colorado State University, Fort Collins 80523-1619