E.A.J. Fischer1, Cindy Dierikx1, Alieda van Essen-Zandbergen1, H.J.W. van Roermund1, Dik Mevius1, Arjan Stegeman2, Don Klinkenberg2
1Central Veterinary Institute, part of Wageningen UR, P.O. Box 65, 8200, AB Lelystad, the Netherlands
2Department of Farm Animal Health, Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80163, 3508, TD Utrecht, the Netherlands