1. Department of Clinical Veterinary Medicine, College of Veterinary Medicine, Northeast Agricultural University, Harbin, Heilongjiang 150000, P.R. China
2. College of Veterinary Medicine, Henan University of Animal Husbandry and Economy, Zhengzhou, Henan 450046, P.R. China
3. College of Animal Science and Veterinary Medicine, Heilongjiang Bayi Agricultural University, Daqing, Heilongjiang 163000, P.R. China