One stool sample was randomly selected from each of the 17 farms. The samples were suspended in PBS (0.01 M phosphate, pH7.2-7.4, 0.15NaCl), vortexed, centrifuged at 15000 g, and filtered through a 0.22-mm filter to remove eukaryotic-and bacterial-cell-sized particles [45, 46, 54, 55] . The filtrates were then treated with benzonase, DNase and RNase to digest non-particleprotected nucleic acid as reported [54, 55] . Viral nucleic acids were then extracted using the TIANamp virus DNA/RNA Kit (TIANGEN BIOTECH, BEIJING, CO., Ltd.). Viral cDNA synthesis was performed by incubation of the extracted viral RNA/DNA with 100 pmol of primer K-8N [56] with a degenerate 39 end and the use of Superscript reverse transcriptase, and the opposite strand of the cDNA was generated after melting and reannealing and primer extension using Klenow DNA polymerase [45, 46, [54] [55] [56] . PCR of extension products was performed as reported previously using the K-8 primer (K-8N without the degenerate 39 end) [54] . This protocol amplifies both viral RNA and DNA genomes [54, 55] .