1. Veterinary Sciences Division, Bacteriology Branch, Agri-Food and Biosciences Institute, Stormont, Stoney Road, Belfast BT4 3SD, UK
2. Parco Tecnologico Padano, Via Einstein, Polo Universitario, Lodi 26900, Italy
3. The Roslin Institute and R (D) SVS, University of Edinburgh, Roslin, Midlothian EH25 9PS, UK