Surveillance and sample collection
The Sheung Shui Slaughterhouse is the largest slaughterhouse in Asia, able to accommodate 12,000 pigs and slaughter up to 5,000 pigs per day. From 5th January 2012 to 15thDecember 2016, our surveillance team collected paired serum and nasal swabs from randomly selected pigs soon after slaughter, 50 paired pig sera and nasal swabs being collected at each visit, which was increased to 75 paired samples since 19th November 2015. Tracheal swabs were also collected at post-mortem inspection. Information of the originating province and farm was derived from pig tattoo identification numbers. We also recorded the arrival date of the slaughtered pigs, from which the length of stay at slaughterhouse was derived. Sampling was conducted twice per month to allow identification of any potential seasonal pattern.