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.
Although five people in China have been infected by the strain, 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 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 was ineffective against the strain and may 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 a virus and vaccine research company in Pittsburgh called Recombinomics Inc. who was not involved with the study.
The report, based on China's ongoing flu surveillance program, found that H5N1 became more prevalent from July 2005 to June 2006 compared with the previous 12 months. The researchers tested 53,220 birds in live poultry markets and found that 2.4% of them tested positive for any strain of H5N1, up from 0.9% a year earlier.
They found that ducks and geese were the most common carriers, and they were susceptible to bird flu year-round. Chickens tend to succumb only in the winter, but the researchers discovered cases in 11 out of the 12 months of their study, up from four out of 12 months the previous year. Overall, the peak flu season of October to March has been extended until June, the researchers found.
"There seems to be a lot more of the virus around in 2006 than in 2005," said Richard Webby, an influenza researcher at St. Jude Research Hospital who was not connected with the study.
The Fujian-like strain, named after the coastal Chinese province where it was first identified, has steadily risen in prevalence over the last year, accounting for 103 of the 108 samples tested from April to June, according to the report.
The emergence of a dominant strain has some advantages.
"In the short term it's easier to control a single dominant lineage than a number of smaller ones," Webby said.
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 continues to spread.
Now, said lead author Dr. Yi Guan, a Hong Kong University professor, "we believe it is likely a third wave has already started."
The quick rise of the Fujian-like strain should prompt China to rethink its vaccination program, several experts said. Official policy dictates that all Chinese poultry be vaccinated.
Some countries have balked at vaccinations out of fear that it would suppress the virus in birds but not really eliminate it.
A mandatory vaccine program in Vietnam has effectively stopped H5N1 transmission in that country, but those results were not repeated in China.
"We don't know if the vaccine was wrong or if some birds were missed," Webster said. "That's the $60-million question."
Vaccinating poultry against H5N1 amounts to "a huge natural experiment," said Dr. Scott P. Layne, an epidemiologist at the UCLA School of Public Health. "By vaccinating we're manipulating the virus' evolution. Is that good, neutral or bad for us? Who knows?"
Though H5N1 is widespread in birds, none of the strains that have emerged so far are adept at crossing the species boundary to infect humans. Since 2003, 256 people have been infected with H5N1, resulting in 151 deaths, according to the World Health Organization.
More rare are cases of the virus jumping directly from person to person -- a necessary ingredient for a pandemic.
But the chances of such a strain emerging increases with each new infection. The worst-case scenario is that a person will be simultaneously infected with bird flu and a human flu, giving the two viruses a chance to reshuffle their genetic elements to create a deadly strain that spreads easily from person to person.
Dr. David Nabarro, who coordinates the United Nations' efforts against human and avian influenza, said the new data were a reminder that H5N1 was 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."