1. National Wildlife Research Center, Veterinary Services, Animal and Plant Health Inspection Service, US Department of Agriculture, 4101 LaPorte Avenue, Fort Collins, Colorado 80521, USA
2. Department of Biomedical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, 3107 Rampart Road, Fort Collins, Colorado 80523, USA
3. National Veterinary Services Laboratories, Veterinary Services, Animal and Plant Health Inspection Service, US Department of Agriculture, 1920 Dayton Avenue, Ames, Iowa 50010, USA