During the second outbreak of avian influenza H5N1 in Thailand, probable horizontal transmission among tigers was demonstrated in the tiger zoo. Sequencing and phylogenetic analysis of those viruses showed no differences from the first isolate obtained in January 2004. This finding has implications for influenza virus epidemiology and pathogenicity in mammals.

Outbreaks of H5N1 highly pathogenic avian influenza (HPAI) occurred in various types of domestic poultry in Thailand during 2004-05. H5N1 viruses were also detected in humans and other mammalian species. Infections were mainly detected in backyard chickens and domestic ducks. The geographic distribution of the 2004 outbreaks was widespread throughout Thailand; most outbreaks occurred in the Central Region, the southern part of the Northern Region, and the Eastern Region. In 2005, the H5N1 outbreaks continued and showed a clustered pattern in four provinces in the southern part of the Northern Region and in one province in the Central Region. H5N1 HPAI outbreaks caused serious socioeconomic consequences to the poultry industry, the social community, farmers' livelihood, and human health. After key measures were implemented, the incidence of the outbreaks declined remarkably in 2005.

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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.

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