1. Department of Animal Supply, National Institute of Public Health, PO Box 1, 3720 BA Bilthoven, The Netherlands
2. Laboratory for Zoonoses and Food Microbiology, National Institute of Public Health, PO Box 1, 3720 BA Bilthoven, The Netherlands
3. Laboratory for Pathology, National Institute of Public Health, PO Box 1, 3720 BA Bilthoven, The Netherlands