1. Comparative Medicine Research Institute, Yangzhou University; Yangzhou 225009 People's Republic of China
2. College Veterinary Medicine, Yangzhou University; Yangzhou 225009 People's Republic of China
3. Joint International Research Laboratory of Agriculture and Agri-Product Safety, Ministry of Education; Yangzhou 225009 People's Republic of China
4. Center for Infectious Diseases, Biosciences Division; SRI International; Harrisonburg VA 22802