1. Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20205
2. Department of Microbiology, Division of Parasitology, New York University School of Medicine, New York, New York 10016
3. Department of Entomology, London School of Hygiene and Tropical Medicine, University of London, Keppel Street, London WC1E 7HT, United Kingdom