Estimated phylogenetic relationship of isolates close to south China border.

<p>Maximum Likelihood Trees for (a) dataset 4 and (b) dataset 5 containing sequences isolated in countries adjacent, or close to, the South China border. Outside bar indicates country of original according to colour and height of bar. In both trees, isolates form four major clades with strong bootstrap support. The majority of non Chinese isolates are placed in the China VI/SEA 3 and China II/SEA 2 clades. The China I/SEA 1 clade, which is the primary variant strain associated with the current epidemic in China, is exclusively composed of Chinese samples, with the exception of a small group of Vietnamese sequences (marked in yellow) in the left hand tree. These sequences form a separate branch with strong bootstrap support and the early branch point indicates these strains are distinct from the Chinese strains. The arrow marks a human sample from Viet Nam that is placed in the Chinese clade and which was isolated on the Vietnamese side of the principal border crossing into China. See main text for details.