1. Department of Animal Science, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
2. Department of Diagnostic Medicine/Pathobiology, College of Veterinary Medicine, Kansas State University, Manhattan 66506
3. Department of Animal Science, University of Vermont, Burlington 05405
4. Food and Feed Safety Research Unit, Southern Plains Agricultural Research Center, Agricultural Research Service, USDA, College Station, TX 77845