1. 1Iowa State University, College of Veterinary Medicine, Ames, Iowa 50011
2. 2Pre-Harvest Food Safety and Enteric Diseases Research Unit, National Animal Disease Center, U.S. Department of Agriculture, Agricultural Research Service, P.O. Box 70, Ames, Iowa 50010, USA
3. 3Federal University of Lavras, Department of Veterinary Medicine, Lavras, Minas Gerais, Brazil