1. Imperial College London, Faculty of Medicine, Division of Infectious Disease, 8 Norfolk Place, London W2 1PG, UK
2. Department of Surgery, Oncology and Gastroenterology, University of Padua, Padua, Italy
3. OIE/FAO and National Reference Laboratory for Avian Influenza A Virus, Istituto Zooprofilattico Sperimentale delle Venezie, Padua, Italy