1. Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, Colorado 80523, USA
2. Present address: Oregon State University, College of Veterinary Medicine, Magruder 105, Corvallis, Oregon 97331-4802, USA
3. Division of Vector-Borne Infectious Diseases, Center for Infectious Diseases, Centers for Disease Control, Public Health Service, P.O. Box 2087, Fort Collins, Colorado 80522, USA