By Tom Polansek

CHICAGO (Reuters) - A flock of 200,000 hens in Wisconsin is the first commercial U.S. chicken operation to become infected with a lethal strain of bird flu, the U.S. Department of Agriculture said, widening the impact of a virus that has killed hundreds of thousands of turkeys this year.

The chickens were at an egg-laying facility in Jefferson County, Wisconsin, between Madison and Milwaukee, and represented the state's first case of the H5N2 flu, according to the Agriculture Department.

Wisconsin authorities did not identify the owner of the chickens, which will be quarantined and culled to prevent the disease from spreading.

Since the beginning of the year the flu, which can kill nearly an entire flock within 48 hours, has been found in commercial poultry operations and backyard poultry flocks in 11 states stretching from Oregon to Arkansas.

The discoveries have prompted major overseas buyers, including China and Mexico, to restrict imports of U.S. poultry and eggs in the $5.7 billion export market. Producers such as Tyson Foods Inc have strengthened biosecurity measures to keep the disease off farms.

Migratory ducks are believed to be spreading the virus as they travel to northern states after spending the winter farther south, veterinarians have said.

Epidemiologists and veterinarians said on Monday they were concerned more about the geographic spread of the disease than the first infection in a commercial chicken operation. Chickens had been expected to become infected, just as turkeys became infected.

"The big deal is it's in another state," said John Glisson, vice president of research for the U.S. Poultry and Egg Association.

Neither chickens nor eggs from the Wisconsin facility will enter the food system, officials said.

Investigators were trying to determine how the flu entered the facility but may never be able to pinpoint it, said Wisconsin's assistant state veterinarian Darlene Konkle.

Teams in other states with infections have been exploring the possibility that farm workers or vehicles may have spread the virus by tracking contaminated feces from wild birds into barns.

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"We've been on heightened alert with our biosecurity measures," said Tom Super, spokesman for the National Chicken Council. "I don't see that changing anytime soon."

Last week, the Agriculture Department reduced its forecasts for poultry exports by nearly 6 percent from March due partly to bird flu outbreaks.

No human cases of the flu have been detected.

(Reporting by Tom Polansek; Editing by Dan Grebler, Toni Reinhold)