1. Friedrich-Loeffler-Institut, Federal Research Institute for Animal Health, Institute of Epidemiology, Seestrasse 55, D-16868 Wusterhausen, Germany
2. Ernst-Moritz-Arndt University Greifswald, Institute for Mathematics and Informatics, Friedrich-Ludwig-Jahn-Strasse 15a, D-17489 Greifswald, Germany