1. Institute for Zoo Biology and Wildlife Research Berlin, Alfred-Kowalke-Strasse 17, P.O. Box 601103, 10252 Berlin, Germany (e-mail: )
2. Animal Disease Research Unit, Agricultural Research Service, USDA, and Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, WA 99164-7030, USA
3. Institute of Virology, Faculty of Veterinary Medicine, University of Zurich, Winterthurerstrasse 266a, 8057 Zürich, Switzerland