Jeon Young-Han/AFP/Getty Images
South Korean officials inspect a rice field near Seoul in November after South Korea reported its second bird-flu outbreak. Since 2003, the virus strain has spread from Southeast Asia to Europe and Africa.
Melody Kokoszka/M. Kilpatrick, Proceedings of the National Academy of Sciences.
By studying genetic fingerprints, wild-bird flyways and the global chicken trade, scientists have traced the spread of bird flu through Southeast Asia to Europe and Africa. Arrows give the month of the outbreak and spread of bird flu from 2003 to January 2006.
By studying genetic fingerprints, wild-bird flyways and the global chicken trade, scientists have traced the spread of bird flu through Southeast Asia to Europe and Africa. Arrows give the month of the outbreak and spread of bird flu from 2003 to January 2006. Melody Kokoszka/M. Kilpatrick, Proceedings of the National Academy of Sciences.
Many experts predicted the bird flu would spread to North America this year. But the virus, which has already migrated from Southeast Asia to three other continents since 2003, has not yet reached the United States.
Some scientists now say it's not likely to happen the way many expected —- through wild migratory birds —- but instead through the global trade in live poultry.
Marm Kilpatrick, a biologist at the Consortium for Conservation Medicine, examined previous migration pathways of the bird-flu strain H5N1 in order to predict its possible path to the Western Hemisphere.
"The two main transmission routes for bird flu are through [feces] and then taking the virus back in when they feed," says Kilpatrick. "And then [it can] also [be transmitted] through infections in the throat."
Kilpatrick thinks the virus might get to American waterways through a two-step process, starting with infected poultry shipped to another country in this hemisphere.
"If migratory birds were to mingle with backyard poultry in Mexico or Brazil or Central America or even in Canada and then become infected, then they might migrate north or south to the U.S.," Kilpatrick says. "And then the virus might infect one of the geese or ducks in here simply by foraging in the same pond with these birds."
Kilpatrick tracked bird-flu migration through genetic fingerprints, wild-bird flyways and the global chicken trade.
"In Asia, it appeared that the poultry trade was responsible for about half the introductions [of bird flu]," Kilpatrick says. "Whereas in Europe, both the original introduction [of avian flu] into Europe — as well as the subsequent spread [of the flu] throughout Europe — appeared to be through migratory birds."
Kilpatrick says the flu then reached Africa through both migratory birds and poultry shipments.
Some in the poultry industry remain skeptical that the virus could reach the United States. Elizabeth Krushinskie, an expert on the poultry industry and a technical adviser for the U.S. Agency for International Development, says it is not likely that avian flu could reach the United States through poultry and wild birds.
"Yes, [bird flu] could come to Central and South America," Krushinskie says. "Would it be a problem? It could be a problem. Could it get into the United States that way? It probably could. Is it likely to? No."
If the avian flu did reach the United States through wild birds, some say the virus wouldn't necessarily devastate the poultry industry, because chickens are usually raised in sealed barns. But growing numbers of chickens are now raised as free-range poultry. By law, free-range birds must spend part of their lives outside, where they can mingle with wild chickens.
The interactions could potentially spread the bird flu, which worries scientists. Leslie Dierauf, the chair of the National Wildlife Health Center in Madison, Wisconsin, believes that it's just a matter of time before the strain makes its way to the United States.
"You know, I don't say 'if' anymore," she says. "I go with the majority of scientists who say 'when.'"
Dierauf says the new study, which appeared in the Proceedings of the National Academy of Sciences, underscores there are still big gaps in scientists' knowledge about how the bird-flu virus spreads.