CHICAGO, April 20 (Reuters) - Iowa, the top U.S. egg-producing state, found a lethal strain of bird flu in a flock of millions of hens at an egg-laying facility on Monday, the worst case so far in a national outbreak that has now prompted Wisconsin to declare a state of emergency.

The U.S. Department of Agriculture said the Iowa flock numbered 5.3 million birds while the company that operates the farm said it was 3.8 million. It was unclear why there was a discrepancy.

Iowa was already among the 12 U.S. states to have detected bird flu infections since the beginning of the year. The other states with infected poultry flocks are Arkansas, Idaho, Kansas, Minnesota, Missouri, Montana, North Dakota, Oregon, South Dakota, Washington and Wisconsin.

Bird flu, also called avian influenza or AI, is a viral disease of birds. The U.S. Centers for Disease Control and Prevention considers the risk for human infections to be low, and no human cases have been found yet.

The infected Iowa birds were being raised near Harris, Iowa, by Sunrise Farms, an affiliate of Sonstegard Foods Company, the company said. The farm houses 3.8 million hens, according to the company, which sells eggs to food manufacturers, government agencies and retailers.

"We went to great lengths to prevent our birds from contracting AI, but despite best efforts we now confirm many of our birds are testing positive," Sonstegard said in a statement.

The flock has been quarantined, and birds on the property will be culled to prevent the spread of the disease, the USDA said. The virus can kill nearly an entire flock within 48 hours.

In Wisconsin, Governor Scott Walker on Monday declared a state of emergency after three poultry flocks became infected in the past week, according to a statement from his office. The infected birds were chickens at an egg-laying facility, turkeys and a 

Bird flu outbreak could set US record with 'probable' cases in Iowa

Updated yesterday at 11:13am

PHOTO: If the virus is confirmed at all five farms, the number of sites where H5 has been found in Iowa would rise to 17.(AFP: Sam Yeh)

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MAP: United States

The highly pathogenic H5 avian flu has turned up in initial tests at five farms in Iowa, including a commercial egg operation housing up to 5.5 million birds, Iowa's agriculture department says.

If the virus is confirmed at the farms in additional tests under way at a US department of agriculture laboratory, the total number of American cases could surpass 20 million birds and result in the biggest death toll in a bird flu outbreak in US history.

Avian flu at the egg farm in Buena Vista County, where workers saw an unexpected jump in bird deaths before the flock was tested, could be the largest single farm operation to be hit in the current outbreak.

The farm's owner, Rembrandt Foods, one of the top US egg producers, confirmed the outbreak but disputed the number of birds affected.

The state did not name the affected farm.

Avian flu was "probable" at four other commercial farms in Buena Vista, Sioux and Clay counties, the Iowa department of agriculture and land stewardship said.

If the virus is confirmed at all five farms in the coming days, the number of sites where H5 has been found in Iowa would rise to 17.

Broiler farm hit, poultry meat industry could be affected

The news comes only hours after state officials confirmed on Thursday that an Iowa-based chicken broiler breeding farm had initially tested positive for the virus.

The facility in Kossuth County houses an estimated 19,000 birds, the state agriculture department said.

Birds were dying in greater than normal numbers at the breeding farm, which is a typical sign of influenza infection in a flock.

This is thought to be first time the virus has affected a broiler breeding farm in this outbreak.

Such breeding farms are traditionally known for having extremely tight biosecurity systems.

Though the operation is small compared with some of the other poultry farm sites in the Midwest that have been affected by the current outbreak, the probable breach of a chicken broiler breeder's biosecurity underscores the potential for the country's poultry meat industry supply chain to be affected.

Typically, such facilities' chickens lay fertile eggs, which are sent to a hatchery to produce chicks that are later raised and slaughtered for meat.

Infections with the H5N2 strain have picked up pace in the past few weeks.

Only a million birds had been infected at the beginning of April after the strain was first found in January.

Two bird flu strains have been found in the United States this year.

The H5N2 strain has been reported in Arkansas, Idaho, Iowa, Kansas, Kentucky, Minnesota, Missouri, Montana, North Dakota, Oregon, South Dakota, Washington and Wisconsin.

It has also been identified on farms in Ontario, Canada.

The H5N8 strain has been identified in California and Idaho, according to the agriculture department.

More than 15 million commercial birds nationwide have died or are expected to be killed in the current outbreak.

Reuters