1. RIKILT Institute of Food Safety; Bornsesteeg 45 6708 PD Wageningen The Netherlands
2. Ruralia Institute, Seinäjoki Unit; University of Helsinki; Kampusranta 9C FI-60320 Seinäjoki Finland
3. Department of Infectious Diseases and Immunology, Faculty of Veterinary Medicine; Utrecht University; P.O. Box 80165 3508 TD Utrecht The Netherlands