1. Infectious Disease Research Group, Department of Biological Sciences, University of Warwick, Coventry CV4 7AL
2. Institute for Animal Health, Compton, Newbury RG20 7NN
3. Veterinary Laboratories Agency, Rougham Hill, Bury Saint Edmunds IP33 2RX, United Kingdom
4. Departamento Patolgía Infecciosa, Campus Universitario de Rabanales, 14071 Córdoba, Spain