1. Laboratory of Infectious Diseases, College of Veterinary Medicine, China Agricultural University, Beijing 100193, People's Republic of China
2. The Shandong Animal Disease Control Center, Jinan 250022, Shandong Province, People's Republic of China
3. Department of Microbiology, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Pokfulam, Hong Kong SAR, China