1. Institute of Zoology, Zoological Society of London, Regent's Park, London NW1 4RY, UK
2. Agri-food and Biosciences Institute, Veterinary Sciences Division, Stoney Road, Stormont, Belfast BT4 3SD, Northern Ireland
3. Royal Veterinary College, Department of Pathology and Infectious Disease Hawkshead Lane, North Mymms, Hatfield, Herts AL9 7TA, UK
4. Veterinary Medical Research Institute, Hungarian Academy of Science, PO Box 18, H-1581 Budapest, Hungary
5. School of Biological and Chemical Sciences, Queen Mary, University of London, London E1 4NS, UK