1. Department of Reproduction, Obstetrics and Herd Health, Ghent University, Belgium.
2. Department of Pathology, Bacteriology, and Poultry Diseases, Faculty of Veterinary Medicine, Ghent University, Belgium.
3. Flanders Animal Health Service, Torhout, Belgium.
4. Institut für Tierzucht, Bundesforschungsanstalt für Landwirtschaft (FAL), 31535 Neustadt-Mariensee, Germany.