A strain of H5N1 avian influenza identified last year has become dominant in southern China and is the source of a new wave of bird flu in Southeast Asia, scientists reported today.
The so-called Fujian-like strain has infected poultry in Hong Kong, Laos, Malaysia and Thailand and sickened people in China and Thailand.
Five people in China have been infected by the strain, but the virus does not appear to have improved its ability to spread among humans, according to the study published in the Proceedings of the National Academy of Sciences.
"There's no evidence in this paper of additional human-to-human transmission, which is the real bottom line we're all worried about," said Robert Webster, a virologist at St. Jude Children's Research Hospital in Memphis, Tenn., and co-author of the study. "But so long as the virus is out there in these numbers, it's going to be a continuing pandemic threat."
The researchers also said the vaccine administered to birds in China is ineffective against the strain and might have facilitated its proliferation among the country's 15 billon chickens, geese and ducks by eliminating weaker flu strains.
"With China immunizing all of these birds, they're basically driving the evolution" of H5N1, said Henry Niman, president of Recombinomics Inc., a virus and vaccine research company in Pittsburgh. He was not involved with the study.
The report, based on China's flu surveillance program, found that H5N1 became more prevalent between July 2005 and June of this year than it had been the previous year. The researchers tested 53,220 birds in live poultry markets and found that 2.4 percent tested positive for any strain of H5N1, up from 0.9 percent a year earlier.
They found that ducks and geese were the most common carriers and were susceptible to bird flu year-round. Chickens tend to succumb only in the winter, but the researchers found cases in 11 of the 12 months of their study, up from four of 12 months the previous year. Overall, the peak flu season, October through March, has been extended until June, the researchers found.
The Fujian-like strain, named for the coastal Chinese province where it was first identified, has steadily risen in prevalence over the past year, accounting for 103 of the 108 samples tested from April to June this year, the report said.
The emergence of a dominant strain offers advantages.
"In the short term it's easier to control a single dominant lineage than a number of smaller ones," said Richard Webby, an influenza researcher at St. Jude's Research Hospital who was not connected with the study.
But a dominant strain is also more likely to spread widely. That has happened twice since H5N1 was first identified in China in 1996.
The first wave of outbreaks were limited to Asia, but the second wave traveled from western China's Qinghai Lake in 2005 to Europe and Africa, and it continues to spread.
Now, said lead author Dr. Yi Guan, a professor at Hong Kong University, "we believe it is likely a third wave has already started."
Although H5N1 is widespread in birds, no strain that has emerged is adept at infecting humans. Since 2003, 256 people have been infected with H5N1, resulting in 151 deaths, according to the World Health Organization.
Even more rare are cases of the virus jumping directly from person to person, without which there can be no pandemic.
But the chances of such a strain emerging increase with each new infection. At worst, a person would be infected simultaneously with bird flu and a regular human flu, giving the two viruses a chance to reshuffle their genetic elements to create a deadly strain that could spread easily from person to person.
Dr. David Nabarro, who coordinates the United Nations' efforts to combat human and avian influenza, said the new data are a reminder that H5N1 is constantly evolving.
"I don't think it's a sign that we're getting any closer to pandemic flu," Nabarro said. "Frankly, I don't know how we're going to know when pandemic flu gets close. We're just going to get hit by it."
Karen Kaplan writes for the Los Angeles Times.