1. Department of Biology, Montana State University, Bozeman, Montana 59717-0346, USA
2. Department of Pathology, School of Veterinary Medicine, University of Tennessee, Knoxville, Tennessee 37901, USA
3. Viral and Rickettsial Zoonoses Branch, Centers for Disease Control, 1600 Clifton Road, N.E., Atlanta, Georgia 30333, USA
4. James A. Baker Institute for Animal Health, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853, USA