By Tom Polansek CHICAGO (Reuters) -U.S. poultry producers are tightening safety measures for their flocks as disease experts warn that wild birds are likely spreading a highly lethal form of avian flu across the country. Indiana on Wednesday reported highly pathogenic bird flu on a commercial turkey farm, leading China, South Korea and Mexico to ban poultry imports from the state. The outbreak put the U.S. industry on edge at a time that labor shortages are fueling food inflation. The disease is already widespread in Europe and affecting Africa, Asia and Canada, but the outbreak in Indiana, wh…