1. Institute of Animal Nutrition, Department of Veterinary Medicine, Freie Universität Berlin, D-14195 Berlin, Germany
2. Institute of Microbiology and Epizootics, Department of Veterinary Medicine, Freie Universität Berlin, D-10115 Berlin, Germany
3. Institute of Medical Biometry and Clinical Epidemiology, Charité Universitätsmedizin Berlin, D-12203 Berlin, Germany