1. Veterinary Teaching Hospital; Department of Clinical Sciences; Hebrew University of Jerusalem; PO Box 12 Rehovot 76100 Israel
2. Section of Biometrics and Preventive Medicine; Department of Population Health and Reproduction; School of Veterinary Medicine; University of California; Davis California 95616-8746 USA
3. Israel Institute for Biological Research; PO Box 19 Ness Ziona Israel