1. Department of Biomedical Sciences, College of Veterinary Medicine, Oregon State University, PO Box 429, Corvallis, OR 97339-0429
2. Animal Health Laboratory, Laboratory Services Division, University of Guelph, Guelph, Ontario N1G 2W0, Canada
3. Department of Veterinary Microbiology, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5B4, Canada