2.2 Sample collection
In total, 571 serum samples were collected from apparently healthy pigeons from 39 racing pigeon clubs between November 2016 and August 2019 in 11 provinces or municipalities of China including Beijing, Hebei, Liaoning, Shanxi, Gansu, Qinghai, Shandong, Shaanxi, the Xinjiang Uygur Autonomous Region, Ningxia Hui Autonomous Region and Inner Mongolia Autonomous Region. Moreover, fifty-one fatally diseased racing pigeons from 7 racing pigeon club of the 5 provinces or municipalities were submitted to our laboratory in College of Veterinary Medicine, Northwest A&F University to determine the pathogen of the disease. The 0.2 g sections of internal organs (the heart, liver, spleen, lung, and intestine) were obtained during the postmortem examination of all mentioned above dead pigeons. All samples were stored at −80°C before DNA extraction.