1. Department of Pathology, Bacteriology and Avian Diseases; Faculty of Veterinary Medicine; Ghent University; Salisburylaan 133 B-9820 Merelbeke Belgium
2. Veterinary and Agrochemical Research Centre; Groeselenberg 99 B-1180 Brussels Belgium
3. Department of Veterinary Pathology; Utrecht University; Yalelaan 1 3584 CL Utrecht The Netherlands