1. Division of Microbiology, Institute for Animal Health, Compton Laboratory, Berkshire RG20 7NN, UK
2. Immunology Unit, Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, Keppel St, London WC1E 7HT, UK
3. Defence Science and Technology Laboratory, Porton Down, Salisbury, Wiltshire SP4 0JQ, UK