In a national outbreak of bird flu that has already seen Wisconsin declare a state of emergency, a lethal strain of the virus has been found in a flock of millions of egg-laying hens at a facility in Iowa, officials say.

The confirmation of the infectious virus means every single hen on the farm in the state's northwest Osceola County will have to be destroyed, the U.S. Department of Agriculture said.

Iowa produces nearly one in every five eggs consumed in the United States, and almost 10 percent of the egg-laying hens in the state are at the farm where the avian flu was detected.

Since the virus, which is affecting both chickens and turkeys, was first detected in Minnesota in early March; it has been found in Missouri, Arkansas, Wisconsin, North and South Dakota and now Iowa.

Before it was found in the Osceola Country chicken farm, it has previously been confirmed at a turkey farm in the state.

The chicken farm is operated by Sunrise Farms, a division of the Sonstegard Foods Company, and provides eggs to food manufacturers, retailers and government agencies.

"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 Foods said in a statement.

A quarantine of the farm is in effect and the birds will be killed to keep the disease, which can completely wipe out a flock in 48 hours, from spreading, said the USDA, which has spent $45 million to date responding to the outbreak. Iowa is among 12 states that have detected bird flu in poultry since the beginning of the year. The other states are Arkansas, Idaho, Kansas, Minnesota, Missouri, Montana, North Dakota, Oregon, South Dakota, Washington and Wisconsin.

Officials also are concerned about infections on turkey farms and in other poultry flocks along the central flyway.

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 reported, health officials said.