1. Ministerio de Sanidad y Consumo, Instituto de Salud Carlos III, Centro Nacional de Microbiologia, 28220 Majadahonda, Madrid, Spain1;
2. University of Edinburgh, Centre for Tropical Veterinary Medicine, Easter Bush, Roslin, Midlothian, Scotland EH25 9RG2; and
3. Institute for Animal Health, Pirbright Laboratories, Pirbright, Woking, Surrey, England GU24 0NF3