1. Field Station for Epidemiology, University of Veterinary Medicine, Hannover Foundation, Büscheler Straße 9, D-49456 Bakum, Germany
2. Institute for Animal Hygiene, Animal Welfare and Farm Animal Behaviour, University of Veterinary Medicine, Hannover Foundation, Bischofsholer Damm 15, D-30173 Hannover, Germany
3. Institute for Biometry, Epidemiology and Information Processing, University of Veterinary Medicine, Hannover Foundation, Bünteweg 2, D-30559 Hannover, Germany
4. Statistical Consulting and Analysis, Center for Higher Education, TU Dortmund University, Vogelpothsweg 78, D-44227 Dortmund, Germany