1. Moredun Research Institute, Edinburgh EH26 0PZ, United Kingdom
2. Department of Microbiology, Immunology and Pathology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523
3. Department of Animal Science, Texas A&M University, College Station, Texas 77843-2471
4. Department of Medical Microbiology, College of Veterinary Medicine, University of Georgia, Athens, Georgia