1. Department of Infectious Diseases and Immunology; Utrecht University; Yalelaan 1 3485 CL Utrecht The Netherlands
2. Department of Herd Health and Reproduction; Utrecht University; Yalelaan 7 3584 CL Utrecht The Netherlands
3. University of Veterinary Science; Department of Animal Nutrition; Rottenbiller u. 50 Budapest H-1077 Hungary
4. II. Medizinische Tierklinik der Universität München; Veterinärstrasse 13 80539 Munchen Germany
5. Laboratory for Public Health; Department of Pathology and Microbiology; Jelsumerstraat 6 8917 EN Leeuwarden The Netherlands