1. Department of Microbiology, Faculty of Medicine, University of Geneva, 1 rue-Michel Servet, 1211 Geneva 4, Switzerland.
2. Division of Parasitology, Medical Research Council National Institute for Medical Research, Mill Hill, London NW7 1AA, UK.
3. Nuffield Department of Clinical Laboratory Science, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, UK.