1. Department of Mammalian Virology; Institute for Animal Science and Health (ID-Lelystad); PO Box 65 8200 AB Lelystad The Netherlands
2. Department of Avian Virology; Institute for Animal Science and Health (ID-Lelystad); PO Box 65 8200 AB Lelystad The Netherlands
3. Department of Herd Health and Reproduction; Faculty of Veterinary Medicine; Utrecht University; PO Box 80151 3508 TD Utrecht The Netherlands