1. Institute for Medical Microbiology, Infectious and Epidemic Diseases, University of Munich, Veterinaerst. 13; 80539 Munich, Germany
2. James A. Baker Institute for Animal Health, New York State College of Veterinary Medicine, Cornell University, Ithaca, New York 14853, USA
3. Laboratory of Persistent Viral Diseases, Rocky Mountain Laboratories, National Institutes of Health, National Institutes of Allergy and Infectious Diseases, Hamilton, Montana 59840, USA