1. Arthropod-borne and Infectious Diseases Laboratory, Foothills Campus, Colorado State University, Fort Collins, Colorado 80523, USA
2. Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, M/S G-14, Atlanta, Georgia 30333, USA
3. Current address: William P. Sweeney, Comcast IP Services, 3 Executive Campus, Cherry Hill, New Jersey 08002, USA
4. Current address: National Wildlife Research Center, US Department of Agriculture, 4101 LaPorte Ave., Fort Collins, Colorado 80521, USA