1. a Animal Sciences Group , Wageningen University and Research Centre , P.O. Box 65, 8200 , AB , Lelystad , The Netherlands
2. b Faculty of Veterinary Medicine, Department of Farm Animal Health , Utrecht University , P.O. Box 80151, 3508 , TD , Utrecht , The Netherlands
3. c Animal Health Service , Arnsbergstraat 7, P.O. Box 9, 7400 , AA , Deventer , The Netherlands
4. d Department of Virology , Central Institute for Animal Disease Control, Wageningen University and Research Centre , P.O. Box 2004, 8203 , AA , Lelystad , The Netherlands