1. Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario, Canada.
2. Laboratory for Foodborne Zoonoses, Public Health Agency of Canada, Guelph, Ontario.
3. Cooperative State Research, Education, and Extension Service, United States Department of Agriculture, Washington, DC.
4. Department of Veterinary Diagnostic and Production Animal Medicine, Iowa State University, Ames, Iowa.