1. The Wistar Institute of Anatomy and Biology, 3601 Spruce Street, Philadelphia, Pennsylvania 19104, USA
2. School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, Pennsylvania 19438, USA
3. U.S. Department of Agriculture, Agricultural Research Service, Animal Parasite Research Laboratory, P.O. Box 952, Auburn, Alabama 36831, USA
4. Virginia Department of Health, Office of Epidemiology, Richmond, Virginia 23219, USA
5. Author to whom reprint requests should be sent