1 Institute of Veterinary Virology, University of Bern, PO Box 8466, CH-3001 Bern, Switzerland. Electronic address: Ernst.Peterhans@vetsuisse.unibe.ch.
2 Institute of Veterinary Virology, University of Bern, PO Box 8466, CH-3001 Bern, Switzerland. Electronic address: Matthias.Schweizer@vetsuisse.unibe.ch.