1. Companion Animal Hospital ‘t Heike, P.O. Box 244, Veldhoven, 5500 AE, the Netherlands, , ,
2. Research Laboratory for Infectious Diseases, National Institute of Public Health and Environment, P.O. Box 1, Bilthoven, 3720 BA, the Netherlands
3. Centre for Biostatistics, Padualaan 14, Utrecht, 3584 CH, the Netherlands
4. Department of Bacteriology, Institute of Infectious Diseases and Immunology, Faculty of Veterinary Medicine, Universiteit Utrecht, P.O. box 80165, Utrecht, 3508 TD, the Netherlands