Genetic diversity of swine influenza viruses isolated from pigs during 2000 to 2005 in Thailand

Background Recent studies have revealed the existence of genetic diversity in swine influenza viruses (SIVs) in the world. In Thailand, there has been a little information on the molecular characteristics of the SIVs since the first isolation of viruses of H1N1 and H3N2 subtypes in the late 1970s. Our previous study demonstrated that Thai H1N1 SIVs possessed the classical swine H1 and avian-like swine N1 genes (Takemae et al., Proceedings of the Options for the Control of Influenza VI.2007;350-353). Objectives In the present study, we genetically characterized 12 SIVs including those of H1N1, H1N2 and H3N2 subtypes isolated between 2000 and 2005. Methods We determined the entire nucleotide sequences of the eight gene segments of those isolates. Results Phylogenetic analysis revealed the existence of nine distinct genotypes amongst the Thai SIVs. These genotypes arose from multiple introductions of classical swine, avian-like swine and human viruses. The existence of two distinct sublineages within classical swine H1 and NS, avian-like swine PA and M and human H3 and N2 genes of the Thai SIVs suggested that introduction of viruses of classical swine, avian-like swine and human origins occurred twice respectively into the Thai pig population. The predominance of avian-like swine genes amongst the Thai SIVs was evident. In particular, three polymerase (PB1, PB2 and PA) and matrix genes of avian-like swine origin were retained in all the Thai SIVs examined. Conclusions These observations may suggest that genes of avian-like swine lineages have some advantages to be maintained in pigs as seen in the SIVs established through multiple introductions in other regions.