1. Hygiene Institut, Abteilung Parasitologie, Universitätsklinikum Heidelberg, Im Neuenheimer Feld 324, 69120 Heidelberg, Germany, Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, New York 10461, and Biological Chemistry, Silberman Institute of Life Sciences, Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel