Robert G. Wallace

Geographic spread of highly pathogenic influenza A H5N1, the bird flu strain, appears a necessary condition for accelerating the evolution of a related human-to-human infection. As H5N1 spreads the virus diversifies in response to the variety of socioecological environments encountered, increasing the chance a human infection emerges. Genetic phylogenies have for the most part provided only qualitative evidence that localities differ in H5N1 diversity. For the first time H5N1 variation is quantified across geographic space. We constructed a statistical phylogeography of 481 H5N1 hemagglutinin genetic sequences from samples collected across 28 Eurasian and African localities through 2006. The MigraPhyla protocol showed southern China was a source of multiple H5N1 strains. Nested clade analysis indicated H5N1 was widely dispersed across southern China by both limited dispersal and long distance colonization. The UniFrac metric, a measure of shared phylogenetic history, grouped H5N1 from Indonesia, Japan, Thailand and Vietnam with those from southeastern Chinese provinces engaged in intensive international trade. Finally, H5N1's accumulative phylogenetic diversity was greatest in southern China and declined beyond. The gradient was interrupted by areas of greater and lesser phylogenetic dispersion, indicating H5N1 migration was restricted at some geopolitical borders. Thailand and Vietnam, just south of China, showed significant phylogenetic clustering, suggesting newly invasive H5N1 strains have been repeatedly filtered out at their northern borders even as both countries suffered recurring outbreaks of endemic strains. In contrast, Japan, while successful in controlling outbreaks, has been subjected to multiple introductions of the virus. The analysis demonstrates phylogenies can provide local health officials with more than hypotheses about relatedness. Pathogen dispersal, the functional relationships among disease ecologies across localities, and the efficacy of control efforts can also be inferred, all from viral genetic sequences alone.

The geographic diffusion of highly pathogenic influenza A H5N1 has largely been traced from the perspective of the virus's victims. Birds of a variety of avian orders have been sampled across localities, and their infection has been identified by a general genetic test. Another approach tracks the migration from the perspective of the virus alone, by way of a phylogeography of H5N1 genetic sequences. Although several phylogenies in the literature have labeled H5N1 clades by geographic region, none has analytically inferred the history of the virus's migration. With a statistical phylogeography of 192 hemagglutinin and neuraminidase isolates, we show that the Chinese province of Guangdong is the source of multiple H5N1 strains spreading at both regional and international scales. In contrast, Indochina appears to be a regional sink, at the same time demonstrating bidirectional dispersal among localities within the region. An evolutionary trace of HA₁across the phylogeography suggests a mechanism by which H5N1 is able to infect repeated cycles of host species across localities, regardless of the host species first infected in each locale. The trace also hypothesizes amino acid replacements that preceded the first recorded outbreak of pathogenic H5N1 in Hong Kong, 1997.