1. The Wistar Institute of Anatomy and Biology, 3601 Spruce Street, Philadelphia, Pennsylvania 19104, USA
2. New address: Thomas Jefferson University, Department of Microbiology, 1020 Locust Street, Philadelphia, Pennsylvania 19107, USA
3. Department of Microbiology, Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada M5S 1A8
4. Animal Diseases Research Institute, Agriculture Canada, 3851 Fallowfield Road, Nepean, Ontario, Canada K2H 8P9