CHICAGO—Iowa reported a highly lethal form of bird flu in a backyard poultry flock on Wednesday, expanding a U.S. outbreak of the disease to the top egg-producing state. The flock in Pottawattamie County, which was not being raised for commercial production, will be culled to prevent the spread of the disease, Iowa said. Over the past month, highly lethal bird flu cases have been confirmed at commercial farms in Indiana, Kentucky, and Delaware, triggering export restrictions for U.S. poultry products. In 2015, Iowa was at the center of the biggest-ever U.S. outbreak of avian flu, which killed about 50 million birds. By Tom Polansek