1. The Roslin Institute and R(D)SVS University of Edinburgh; Easter Bush Midlothian EH25 9RG UK
2. Animal and Plant Health Agency Weybridge; Woodham Lane Addlestone Surrey KT15 3NB UK
3. Central Veterinary Institute part of Wageningen UR (CVI); Department of Infection Biology; P.O. Box 65 8200 AB Lelystad The Netherlands