1. Institute of Virology, Medical School Hannover, D-30623 Hannover, Germany.
2. National Institute of Public Health, Departments of Veterinary Services, Srobérova 48, CZ-10042 Prague 10, Czech Republic.
3. Electron Microscopy, Srobérova 48, CZ-10042 Prague 10, Czech Republic.
4. Central Laboratory Animal Facility and Laboratory Animal Science Unit, Medical School Hannover, D-30623 Hannover, Germany.