1. Farm Animal Health Department; Faculty of Veterinary Medicine; Utrecht University; PO Box 80151 3508 TD Utrecht The Netherlands
2. Oosterwolde Veterinary Practice; PO Box 93 8430 AB Oosterwolde The Netherlands
3. Quantitative Veterinary Epidemiology Group; Wageningen Institute of Animal Sciences; Wageningen University and Research Centre; 6700 AH Wageningen The Netherlands