Viruses of the H5 and H7 subtype are the "bad guys of the flu world," said Richard Webby, a virologist at St. Jude Children's Research Hospital. "We don't want H5 or H7 viruses in humans." These viruses have the capacity to become highly pathogenic and cause a much more pervasive disease than the influenza viruses currently circulating in the human population.
Until 1997 scientists believed that it was impossible for birds to directly infect humans with H5N1. But an outbreak in Hong Kong in 1997 that killed six of 18 people infected with the virus proved the contrary.
- Deadly New Virus Draws Experts to "Hot Zones"
- Cats Can Catch and Spread Bird Flu, Study Says
- Health Officials Struggle to Understand SARS
- China Is Perfect Breeding Ground for Viruses Like SARS, Expert Says
- Disastrous 1918 Flu Linked to Birds, Study Says
- The Mystery of the SARS Virus: How Is It Spread?
Though H1, H2, and H3 influenza viruses now circulate within the human population every flu season, each one triggered a pandemic upon its debut.
New viruses, to which the people have never been exposed, cause pandemics because humans lack immunitywhich only results from exposure. This situation occurred three times in the last century. In 1918 an H1N1 virus triggered the notorious Spanish flu pandemic that killed at least 20 million worldwide, 500,000 in the U.S. alone. In 1957 and 1958 another new influenza virus, H2N2, dubbed the Asian flu, killed 70,000 Americans. In 1968 and 1969 the Hong Kong flu, H3N2, killed 34,000 in the U.S.
H5N1 is becoming more lethal and may be better suited to infect mammalian hosts. Studies in mice published earlier this year show that H5N1 is evolving rapidly. The H5N1 virus isolated from ducks in mainland China in 2002 was more deadly than that from 1999.
One approach to preventing a pandemic has been to eliminate the virus from its animal reservoir by killing millions of chickens. But this is economically devastating to countries like Thailand, which is the fourth largest producer and exporter of chickens. Almost a hundred million chickens have died or been culled since the 2004 outbreak.
In many places ducks and chickens mingle freely on family farms. They wander in and out of the home, and children are frequently the ones who tend the animals. Although no transmission of H5N1 from duck to human has been reported so far, this now merits consideration in light of human cases that cannot be tied to poultry outbreaks.
"How do you identify infected ducks? What cautionary measures do you recommend if you can't tell if ducks are sick or not?" Stöhr said. "This is going to complicate our strategy in controlling this virus."
Virus Gets Tougher
Outbreaks of H5N1 in poultry have been reported in Laos, Cambodia, Vietnam, Thailand, Indonesia, Malaysia, and China. And new results from St. Jude suggest that the virus is getting tougher. The 2004 H5N1 can survive at 37 degrees Celsius (98 degrees Fahrenheit) for six days, compared with just two days for the H5N1 virus from the 1997 outbreak.
"We can't eradicate this virus any time soon," said Tim Uyeki, a medical epidemiologist in the influenza branch of the Centers for Disease Control and Prevention in Atlanta, Georgia. "Even if infected poultry are culled, the virus persists in the feces and soil under certain environmental conditions. Unless you could disinfect the entire areas with H5N1 contamination, the virus could continue to spread to healthy poultry."
"As H5N1 continues to circulate in poultry and evolve, the virus will become an increasing public health threat. It's a ticking time bomb," Uyeki said.
Robert Cook, chief veterinarian at the Bronx-based Wildlife Conservation Society in New York, believes that a global surveillance program would be a vital early warning system for spotting new potential pandemics. Such a system should bring together disease data from wildlife, livestock, and humans.
The WHO will hold an informal meeting in Geneva on November 11 for vaccine makers and representatives from government and regulatory agencies to discuss a pandemic vaccine project.
There is currently no approved human vaccine for H5N1. One potential vaccine was "very effective in ferrets," said Webby, who co-developed and tested it at St. Jude. Webby's vaccine is scheduled for clinical trials that will gauge safety and efficacy.
All pandemic viruses in the last century arose quickly and traveled around the world in less than a year. It takes about eight months of R&D before vaccine production can begin. "We now have a window of opportunity to get this done," Stöhr said.
Bijal P. Trivedi is a freelance science and health journalist based in Washington, D.C.
Don't Miss a Discovery
Sign up for the free Inside National Geographic newsletter. Every two weeks we'll send you our top news stories by e-mail.