Virus isolation in pigs
During the surveillance period from 2012 to 2016, a total of 8,954 nasal swabs (NS), 18,026 tracheal swabs (TS) and 6,532 pig sera were collected from pigs originated from 505 pig farms in 11 provinces in Mainland China and Hong Kong. The median number of swab samples collected per visit from the Sheung Shui slaughterhouse was 226 (range 102-291). H1N1 (n = 278), H3N2 (n = 174) and H1N2 (n = 51) subtype viruses were isolated. Only H3N2 viruses are further analysed in this study. There was a significantly higher influenza A H3-subtype virus isolation rate in nasal swabs, compared with tracheal swabs (p = 0.014).