1. * Parasitology Unit, Department of Experimental Medicine, Faculty of Medicine, Division South, University of Chile, P.O. Box 427-3, Santiago 3, Chile
2. † Department of Medicine, Hospital “Paula Jaraquemada,” Santiago, Chile
3. ‡ Wolfson Unit/Department of Medical Protozoology, London School of Hygiene and Tropical Medicine, Keppel Street, London WCJ E 7 HT, United Kingdom