1. National Animal Disease Center, Agricultural Research Service, US Department of Agriculture, Ames, IA 50010, USA
2. Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, WA 99164, USA
3. Departments of Veterinary Population Medicine and Veterinary and Biomedical Sciences, University of Minnesota, St Paul, MN 55108, USA
4. Molecular Biology and Molecular Virology, Department of Animal Science, Michigan State University, East Lansing, MI 48824, USA
5. Animal Bioscience Centre, Teagasc, Grange, BT55&GE Co. Meath, Ireland
6. Livestock Diseases Programme, Institute for Animal Health, Compton, Near Newbury RG20 7NN, UK
7. Department of Microbiology and Immunology, University of Texas Medical Branch, Galveston, TX 77555-1070, USA