1. Division of Ruminant Medicine and Veterinary Epidemiology, Department of Clinical Sciences, Swedish University of Agricultural Sciences, Box 7054, SE-750 07 Uppsala, Sweden
2. Department of Microbiology and Molecular Cell Biology, Eastern Virginia Medical School, 700 West Olney Road, Norfolk, VA 23507-1696, USA
3. Department of Biomedical Sciences and Veterinary Public Health, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden
4. Department of Virology, Immunobiology and Parasitology, National Veterinary Institute, Box 7028, SE-751 89 Uppsala, Sweden
5. Department of Immunology, The Pirbright Institute, Ash Road, Pirbright GU24 0NF, UK