1. Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK 2Department of Microbiology, Faculty of Medicine, Kuwait University, PO Box 24923, Safat 13110, Kuwait 3Hospital for Tropical Diseases, University College London Hospitals NHS Trust, Mortimer Market, London WC1E 6AU, UK