1. College of Veterinary Medicine, Sichuan Agricultural University, Chengdu, Sichuan, P. R. China
2. Laboratory Animal Center, Southwest Medical University, LuZhou, Sichuan, P. R. China
3. College of Animal Science and Technology, Northwest A&F University, Yangling, Shaanxi, P.R. China
4. College of Animal Science and Technology, Sichuan Agricultural University, Chengdu, Sichuan, P. R. China