1. Veterinary Epidemiology Unit, Department of Obstetrics, Reproduction and Herd Health, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, Merelbeke, Belgium
2. Institute for Risk Assessment Sciences, Utrecht University, Yalelaan 2, CM Utrecht, The Netherlands
3. Department of Infectious Diseases and Immunology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, CL Utrecht, The Netherlands
4. Wageningen Bioveterinary Research, Houtribweg 39, RA, Lelystad, The Netherlands