By Tom Polansek
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 backyard flock of mixed-breed birds, comprising more than 326,000 birds in all.
He has authorized the state's National Guard to help contain the disease, citing "thin" resources available from the federal government. The Guard will disinfect trucks exiting infected premises, a state spokeswoman said.
The USDA has spent $45 million so far responding to the U.S. bird flu outbreak and has deployed about 60 people to Minnesota, the top U.S. turkey-producing state, which has found more infected flocks than any other state.
The infections have hurt trade in the $5.7 billion U.S. export market for poultry and eggs.
(Additional reporting by P.J. Huffstutter in Chicago; Editing by Alan Crosby and Matthew Lewis)