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

The infected Iowa birds were being raised near the city of Harris by Sunrise Farms, an affiliate of Sonstegard Foods Co., the company said.

The facility houses 3.8 million hens, according to the company, which sells eggs to food manufacturers, government agencies and retailers.