1. Department of Farm Animal Health, Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80151, 3508 TD Utrecht, The Netherlands
2. Department of Primary Industries and Fisheries (Queensland), Animal Research Institute, Yeerongpilly, Australia
3. Department of Infectious Disease and Immunology, Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80165, 3508 TD Utrecht, The Netherlands
4. Rikilt, Institute of Food Safety, Wageningen UR, P.O. Box 230, 6700 AE Wageningen, The Netherlands