1. Department of Pathology and Infectious Disease, Royal Veterinary College, Hatfield, Hertfordshire, UK
2. Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York
3. Current address: College of Veterinary Medicine, Oregon State University, Corvallis, Oregon
4. Current address: Animal and Plant Health Laboratories Division, Agri-Food and Veterinary Authority, Animal and Plant Health Center, Republic of Singapore
5. Current address: Department of Pathology and Infectious Diseases, Royal Veterinary College, Hatfield, Hertfordshire, UK