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Phylogenetics Indicate Bird Flu Migration Curtailed at Some Borders

NEW YORK (GenomeWeb News) – Bird flu virus migration is not uniform and seems to be checked at some international borders, according to a new study that mapped the virus’ genetic variation.
 
Two University of California, Irvine, researchers used statistical phylogeography to track bird flu’s spread from and within China. Their work, published in the journal PLoS ONE online today, suggests the virus’ advance is not always uniform. Instead, some regions appear to be more difficult for the virus to permeate. By understanding the yet-unknown factors that influence viral migration, scientists may ultimately be able to predict or control the virus’ march across continents.
 
“You can think of it as a type of evolutionary forensics,” UC Irvine evolutionary biologist Robert Wallace, lead author on the study, said in a statement. “When a bomb explodes, investigators can determine how many charges went off and the strength and direction of the blast, all from the resulting damage alone. Here we can determine the way H5N1 has spread and evolved by the resulting viral diversity.”
 
Bird flu, influenza A H5N1, emerged in China in 1996. Despite intermittent scares, though, the virus hasn’t achieved the ability to efficiently transmit to humans in the last dozen years. Since the first human infections were detected in 2003, roughly 350 human cases of H5N1 have been reported.
 
Even so, given its high mortality rate (more than 60 percent of people who get the virus die from it), bird flu remains a source of consternation for scientists, governments, and the public, alike. To date, the virus has been detected as far away as the UK and West Africa. And the more the virus travels, the more likely it is that it will evolve and acquire the ability to efficiently infect humans. That’s because it becomes adept at constantly evolving to survive in different hosts, environments, and socioeconomic settings.
 
In an effort to understand this bird flu migration, Wallace and senior author Walter Fitch created a gene tree based on 481 H5N1 hemagglutinin genetic sequences from GenBank and the Influenza Sequence Database. The samples had been collected from 28 Eurasian and African spots between 1996 and 2006. They then used the MigraPhyla protocol to measure the total viral migration events as well as the UniFrac metric to try to pinpoint areas in which H5N1 viruses shared an evolutionary history.
 
As expected, southern China was the source of several different H5N1 strains. Based on this study, the virus seems to have spread over short and long distances from the Chinese province of Guangdong. Southern China was apparently home to the most phylogenetically diverse bird flu strains. This diversity dwindled with increasing distance from this region.
 
But the virus does not appear to have spread in a uniform gradient. Within China itself, though, different migration patterns existed. For instance, a strain in the southern province of Hunan, China, closely resembles those found in Indonesia, Japan, Thailand, and Vietnam and appears to be the result of heavy trade between the regions.
 
Even so, some regions seem to have been difficult for the virus to migrate into. Wallace and Fitch found that in parts of Thailand and Vietnam just south of China there were tightly clustered bird flu strains but not some of the newer H5N1 strains. This indicates that several invasive strains were somehow bounced back from the northern borders of Thailand and Vietnam.
 
The next step will be trying to understand why some countries or regions are predisposed to bird flu invasion, while others remain relatively protected. This could not only assist in tracking the virus’ evolution and ecology, the authors suggest, but could also help health officials evaluate the effectiveness of their viral control efforts.
 
“Some localities, Thailand and Vietnam for example, appear more resistant to H5N1 introduction than others,” the authors wrote. “Identifying these localities — and their relevant borders — may aid efforts at learning what socioecological factors block (or enable) H5N1 migration at a variety of spatial scales.”

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