Spreading Its Wings
It's only a matter of time before Bird flu reaches the United States. Can we stop the killer virus?
The need for surveillance, a time-honored technique in infectious disease control, is particularly acute because no one knows exactly how the H5N1 virus has spread so far, so fast. Avian influenza has been around for at least a century in both wild and domesticated birds. But until recently, outbreaks of virulent strains in poultry were exceedingly rare. The H5N1 virus first appeared in 1997, when thousands of chickens in Hong Kong suddenly sickened and died. That outbreak was halted when public-health officials ordered the slaughter of all the poultry in Hong Kong. The virus re-emerged in Southeast Asia in 2003 and proliferated rapidly. That outbreak continues, and efforts to contain it by culling flocks or vaccinating birds, measures that had worked in the past, have failed. At least 97 people are known to have died from bird flu since 2003. Almost all of them lived in households with backyard poultry flocks.
But in the past year, avian influenza has started to kill wild birds, which had long been able to harbor the disease without getting sick. In April 2005, more than 6,000 bar-headed geese died at Qinghai Lake in central China, a congregating point for migratory fowl. That was a wake-up call to wildlife biologists; the last time avian influenza afflicted large numbers of wildfowl was in 1961. "That's the really surprising part of it, that wild birds are now being killed by this virus as well," says Leslie Dierauf, director of the USGS National Wildlife Health Center in Madison, Wis. "It's changed somehow, and we're not sure how."
Indeed, three months after the Qinghai Lake die-off, poultry in Western Siberia started dying of H5N1. The virus has spread like wildfire since then, first moving through Central Asia into Turkey and eastern Europe. Those viruses, when tested, proved almost identical to those recovered from the birds at Qinghai Lake.
In the past three months, the H5N1 virus has gone ballistic, infecting birds in 21 countries in Africa, Europe, and the Middle East. It's unclear if the outbreaks are the result of bird migrations, poultry shipments, or other human activities. In some countries, like Nigeria, only poultry has been infected. In others, like Germany, only wild birds are dying. The uncertainty has set off a fierce battle between some wildlife conservationists, who feel that wild birds are being unfairly maligned, and agricultural interests.
"Who is the spreader?" asks Robert Webster, a bird flu authority at St. Jude's Children's Research Hospital in Memphis. Webster, working with colleagues in Asia, reported last month that H5N1 is widespread in migratory birds in southern China, suggesting that they could carry the virus long distances. Webster says he's less concerned about the dead birds found in recent weeks in Europe than he is about those still aloft. "Dead birds don't migrate."
Testing flocks. Commercial poultry growers in the United States are acutely aware of the risk posed by H5N1, not the least because they've been dealing with less lethal and more common forms of bird flu for decades. The highly pathogenic H5 and H7 strains are both rare and more lethal, killing 80 to 100 percent of the birds infected. The United States has had three outbreaks of these strains, in 1924, 1983, and 2004; slaughtering the birds and disinfecting the farms contained them. In 2003, the Netherlands killed more than 30 million birds to arrest an outbreak of H7N7 bird flu. Other countries, notably China, have used vaccines, but the United States and most European nations eschew that method, arguing that vaccinated birds can act as carriers and infect others. If H5N1 attacked U.S. poultry flocks, producers would use the slaughter, quarantine, and sanitation techniques that have worked in the past, says Richard Lobb, a spokesman for the National Chicken Council. Almost all commercial poultry is raised indoors, and in January the large producers, which account for 95 percent of the chicken sold in the United States, started testing all flocks for H5N1.