1. Department of Medicine, Faculty of Medicine, University of Khartoum
2. Department of Parasitology, National Health Laboratory, Ministry of Health, Khartoum, Sudan
3. National Institutes of Health, Building 5, Bethesda, Maryland 20205
4. Department of Pharmacology and Toxicology, Michigan State University, East Lansing, Michigan 48824
5. Medical Research Council, Institute of Tropical Medicine