CHICAGO—Bird flu has wiped out 7.4 million chickens in Pennsylvania in the past month, a swift and devastating loss that veterinarians and industry members suspect may be linked to an unusually cold winter.
Infections of flocks raised to produce eggs and meat extend a U.S. outbreak that began four years ago and has eliminated 196 million birds nationwide. The virus, often spread by wild birds, has also infected U.S. farm workers and poultry and mammals across the planet.
