for National Geographic News
Since late 2003, tens of millions of chickens have either died or been slaughtered in East and Southeast Asia in an attempt to contain the spread of a deadly strain of the avian flu virus.
But there is another possible carrier of the virus far more difficult to control than domesticated chickens: migratory birds.
Migrating birds may have caused the outbreak of avian flu that killed thousands of domestic fowl in Siberia this summer. Scientists have also found birds on Lake Qinghai in China, where many birds come to migrate, to be infected with the strain.
Fearing that the virus may be spreading from Asia to other parts of the world, bird experts in Alaska have in recent weeks been testing migratory birds there for avian influenza.
If avian flu is introduced to North America by migrating birds, "Alaska is the most likely state where it would first arrive, because that's where the flyways intersect," said Hon Ip, director of the U.S. Geological Survey's (USGS) virology lab in Madison, Wisconsin. His lab is handling some of the tests.
Highway or Flyway?
There are several avian flu strains. The most dangerous is the HPAI H5N1 strain, which first appeared in Asia in 1997 and since 2003 has caused mass mortality in chickens.
Although the strain does not spread easily to humans, more than a hundred people have been infected with the virus. At least 55 people have died.
Scientists worry that H5N1 will combine with a human flu virus strain to create an exceptionally dangerous influenza that could spread easily from person to person and potentially kill millions of people.
Migratory waterfowlparticularly wild ducksare the natural reservoir of avian influenza viruses. The birds typically do not become ill when infected with the virus. Domestic poultry, on the other hand, are highly susceptible to epidemics of avian flu viruses.
Ip notes that some bird flu epidemics arose after domestic bird flocks came in contact with wild migratory waterfowl. The USGS virologist adds that scientists suspect that H5N1 first arose in the poultry industry in China and that "movement of chickens and poultry workers contributed to spread the virus in the region."
"The $64,000 question is whether the spread [of avian flu] is by the highway or the flyway," he said.
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