Kun Li1, Muhammad Shahzad2. 1. College of Veterinary Medicine, Huazhong Agricultural University, Wuhan, 430070, People's Republic of China; College of Animal Nutrition and Feed Science, Huazhong Agricultural University, Wuhan, 430070, Hubei, People's Republic of China; Department of Pathobiology, College of Veterinary Medicine, University of Illinois At, Urbana-Champaign, USA. Electronic address: lik2014@sina.com. 2. University College of Veterinary & Animal Sciences, The Islamia University of Bahawalpur, 63100, Pakistan.