1. Central Institute Disease Control (CIDC-Lelystad), P.O. Box 2004, 8203 AA Lelystad, The Netherlands
2. Department of Pathology, Faculty of Veterinary Medicine, University of Utrecht, P.O. Box 80158, 3508 TD, Utrecht, The Netherlands
3. Department of Poultry Diseases, Faculty of Veterinary Medicine, Al-Fateh University, P.O. Box 13662 Tripoli, Libya
4. Institute for Animal Science and Health (ID-Lelystad), P.O. Box 65, 8200 AB, Lelystad, The Netherlands