1. State Key Laboratory of Veterinary Etiological Biology, Lanzhou Veterinary Research Institute, Chinese Academy of Agricultural Sciences, Lanzhou, Gansu Province, P.R. China.
2. College of Veterinary Medicine, Hunan Agricultural University, Changsha, Hunan Province, P.R. China.
3. College of Animal Science and Technology, Jilin Agricultural University, Changchun, Jilin Province, P.R. China.
4. College of Animal Science and Technology, Anhui Agricultural University, Hefei, Anhui Province, P.R. China.