1. USDA/APHIS National Wildlife Research Center, Fort Collins, Colorado 80521, USA
2. Department of Clinical Sciences, Colorado State University, Fort Collins, Colorado 80523, USA
3. USDA/ARS National Animal Disease Center, Ames, Iowa 50010, USA
4. Department of Microbiology and Immunology, University of Otago, P.O. Box 56, Dunedin, New Zealand
5. AgResearch, Hopkirk Research Institute, Private Bag 1108, Palmerston North 4442, New Zealand