1. 1Foodborne and Diarrheal Diseases Branch, Division of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, 1600 Clifton Road N.E. (M/S A-38), Atlanta, Geoorgia 30333
2. 2Center for Food Safety and Applied Nutrition, Office of Scientific Assessment and Support, Division of Market Studies, Epidemiology Branch, Food and Drug Administration, 200 C Street S.W., Washington, D.C. 20204, USA