1. Laboratory of Enteric Pathogens, Health Protection Agency Centre of Infections, 61 Colindale Avenue, London NW9 5EQ, United Kingdom
2. Department of Food and Environmental Safety, Veterinary Laboratories Agency, Weybridge New Haw, Addlestone Surrey KT15 3NB, United Kingdom
3. Department of Infectious, Parasitic and Immune-mediated Diseases, Istituto Superiore di Sanità, Rome, Italy