1. The Departments of Pathobiology and Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, Pennsylvania 19348, USA
2. Department of Toxicology, Diagnostic Laboratory, Cornell University, Ithaca, New York 14850, USA
3. Department of Microbiology & Immunology, Center for Neurovirology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA