1. Cantonal Institute of Microbiology, 6500 Bellinzona, Switzerland.
2. Department of Epidemiology and Public Health, Swiss Tropical and Public Health Institute, 4051 Basel, Switzerland.
3. Faculty of Science, University of Basel, 4051 Basel, Switzerland.
4. Veterinary Public Health Institute, Vetsuisse Faculty, University of Berne, 3097 Berne, Switzerland.