The Chinese press is reporting that the outbreak started as early as April of this year. (http://www.chinanews.com/sh/2018/08-15/8600530.shtml) A partial translation is below.
The first ASF case was officially confirmed on Aug 3, 2018 on a small farm (owner Zhang) located in the Shenbei District, Shenyang city, Liaoning province. Further investigations indicated that the owner of the ASF index farm purchased 45 pigs on July 5, 2018 from a farm (owner Wang) located in the Hunnan District, Shenyang city, Liaoning province. Fecal samples collected from Wang’s farm were confirmed ASFV RNA positive by PCR.
Wang said that the last entry of pigs onto the farm occurred on March 24, 2018. There were 100 piglets purchased from Chuanying District, Jilin city, Jilin province. In April, some pigs started to become sick and then died on the Wang farm. Wang did not report the abnormal pig death; instead the remaining live pigs were sold to the Zhang farm in Shenyang and other people. So far, all of the pigs initially from the Wang farm have been tracked and culled. Wang was held in a detention center due to violation of the laws.