By Maggie Fox, Health and Science Editor

WASHINGTON, June 5 (Reuters) - The feared H5N1 avian influenza has yet to make it to North America in the bodies of migrating birds, researchers said on Thursday.

Testing of more than 16,000 migratory birds between May 2006 and March 2007 showed no evidence of the H5N1 bird flu virus, which has become entrenched in many parts of Asia and which regularly pops up in flocks in Europe, the Middle East and Africa.

The birds are infected with virtually every other known strain of influenza, said Hon Ip of the U.S. Geological Survey, National Wildlife Health Center in Madison, Wisconsin. But not the highly pathogenic H5N1 virus.

"Maybe the Pacific Ocean is a nice, big biological barrier, for which I am forever grateful," Ip said.

"The general avian influenza infection rate is not really different in Alaska or North America than pretty much anywhere else. In spite of H5N1's spread through most of Asia and into Africa and Europe, that spread has not come into North America," Ip added in a telephone interview

About 1.7 percent of the birds were infected, but all with low-pathogenic strains of influenza viruses, which typically do not cause disease, Ip's team reported in the Virology Journal.

Highly pathogenic H5N1 avian influenza has forced the death or destruction of an estimated 300 million birds, according to the world animal health organization OIE.

SOURCE OF VIRUS

Birds are considered the original source of all influenza viruses. While H5N1 rarely infects people, it has killed 241 out of 383 infected in 15 countries.

Experts say the danger is that the virus will evolve just slightly into a form that people can easily catch and pass to one another, in which case the transmission rate could soar, causing a pandemic in which millions of people could die.

U.S. government officials have said it is inevitable that migratory birds will carry H5N1 to the Americas at some time.

An estimated that 1.5 million to 2.3 million birds migrate from Asia to Alaska each year.

But Ip, who worked with teams at the U.S. Department of Agriculture, the Fish and Wildlife Service and the Alaska Department of Fish and Game, says it has not happened yet.

The researchers are sampling birds that ornithologists say are the most likely to have migrated recently from Asia.

"Some of these viruses contain a mix of genes from both North American and Asian viruses," Ip said.

"We have direct evidence that the birds are carrying back at least a relative or descendant of viruses from Asia," he added. "This confirms we are sampling from those birds that are most likely to bring H5N1 back if H5N1 was to be brought back from Asia."

It also confirms that the viruses swap genes inside the birds -- a process that scientists believe gives rise to new and sometimes more dangerous strains.

The researchers have been testing birds since 2005 for H5N1, concentrating on Alaska but looking in all regions.

The birds most likely to be infected with any kind of flu virus are the dabbling ducks -- species such as mallards, Ip said. This reinforces the theory that the virus spreads as birds feed in the same water in which they are defecating.

Just this week U.S. chicken producer Tyson Foods Inc <TSN.N> said it would eradicate about 15,000 chickens in Arkansas that carried antibodies to a mild H7N3 strain of bird flu, even though the birds were never sick and there was no risk to human health.

An outbreak this week of H7N7 flu forced the slaughter of all the chickens at a farm in Oxfordshire in Britain.

(Editing by Will Dunham and Cynthia Osterman)

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