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Bird Flu Strain Diversified, May Be Harder to Conquer

Brian Handwerk
for National Geographic News
February 7, 2006
 
Scientists may need to cast a much wider net to track and curb the spread of bird flu, a new study suggests.

That's because the deadly H5N1 avian influenza strain has several distinct genetic branches, or sublineages, spread across several geographic regions, the research shows.

Robert Webster, a virologist at St. Jude Children's Research Hospital in Memphis, Tennessee, and in the department of microbiology at the University of Hong Kong, co-authored the study.

He says the variations may pose a troubling puzzle for scientists hoping to develop effective, strain-specific human vaccines to battle a possible pandemic.

"The virus in Turkey is different from the one in Indonesia, which is different from the one in Vietnam, and so on," Webster said. "We have no idea which might be the one that takes off—if any of them do."

Webster and colleagues report their findings in this week's edition of the journal Proceedings of the National Academy of Sciences.

Experts warn that if any sublineage of H5N1 mutates into a form that can be easily transferred between people, a global pandemic could be imminent and result in tens of millions of deaths.

The World Health Organization (WHO) estimates that H5N1 has so far infected about 160 people in 7 countries, killing at least 85 worldwide since the first bird-to-human transmission in 2003.

(Read an excerpt from a National Geographic magazine feature about bird flu.)

More Complex Than Thought

While tracking poultry around Southeast Asia, Webster and his team found that different geographical locales feature distinct sublineages of the disease. They have identified at least four branches so far.

"This is the first clear indication that the H5N1 situation is more complicated than we think," said Hon Ip, diagnostic virologist at the U.S. Geological Survey's National Wildlife Health Center in Madison, Wisconsin.

"[It is] getting more complicated, and any pandemic-vaccination planning had better take some of this complexity into account."

"Webster has shown that the different H5N1 sublineages are starting to diversify," Ip continued.

"And if we choose one particular strain of H5N1 to make a pandemic vaccine with, say the Dk/HN/5806/03, will the vaccine confer protection when the pandemic virus is Vietnam/1194/04?"

No one can be sure, but scientists do hope that despite the newfound diversity, vaccines in development could prove at least partially effective.

Karen Lacourciere is an influenza program officer at the National Institute of Allergy and Infectious Disease (NIAID) at the National Institutes of Health in Bethesda, Maryland.

"Even a vaccine made using a strain that's not a perfect match for a virus that may emerge, but based on a similar strain, could lessen the impact," she said. "You'd expect some benefit."

Healthy Birds More Dangerous Than Dead Ones

In addition to their diversity findings, the team detected the virus among apparently healthy birds in areas where deadly flu outbreaks have not appeared.

This is an especially troubling development, Webster notes.

"Don't worry about the [birds] that died, it's those that are alive and apparently healthy and hosting this virus [that are of greatest concern]," he said.

Authorities must fight the temptation to conclude that there is no bird flu problem among populations that appear healthy, he said.

"When the birds appear healthy but they are [carrying] this virus, then you've got a serious problem," he said.

"So you have to look at doing sampling of healthy birds, as Thailand has done. … But to do that sort of thing in China would be a huge amount of work."

Infected wild birds further complicate surveillance of the disease. Migratory birds were found to carry the deadly strain when tested prior to their lengthy flights.

"In the past a lot of people have blamed migratory birds for continually reinfecting the domestic poultry," NIAID's Lacourciere said.

"But these data show that H5N1 can be perpetuated by a cycle between domestic and migratory birds. Poultry may sometimes be responsible for reinfecting migratory birds," Ip said.

Most humans afflicted by bird flu likely caught the disease from proximity to domestic animals.

The paper's authors stress the need for surveillance in a broad geographic target area to include as many different genetic variations of the deadly strain as possible.

"This paper points the way that monitoring has to be done," said USGS's Ip.

"We need to look beyond the presence of H5N1, and we have to look at the virus's genetic sequence in detail. It has to be done in different avian populations—terrestrial domestic birds, domestic ducks and geese, and wild migratory birds.

"And more of the kind of analysis like that in this paper is needed in order to assess the ways that the virus is evolving and spreading."

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