We would like to extend our gratitude to the platform and funding provided by the National Key Research and Development Program of China (Grant No. 2016YFD0501200 ), Scientific Research Foundation of Hunan Provincial Education Department (Grant No. 17B125 ), China Postdoctoral Science Foundation (Grant No. 2017M620346 ), College of Veterinary Medicine, Hunan Agricultural University , and Hunan Collaborative Innovation Center of Animal Production Safety . We would like to thank Editage [ www.editage.cn ] for English language editing.