The largest-ever U.S. outbreak of avian influenza, which has devastated Midwestern poultry and egg producers in recent weeks, could be felt at Thanksgiving tables across the nation come November, farmers and some trade groups say. The virulent H5N2 strain has already spread to 14 states and led to the deaths or scheduled euthanizations of more than 21 million birds, including 3.3 million turkeys in Minnesota, the nation’s top turkey producer.