DES MOINES, Iowa (AP) — Bird flu that’s devastated Midwestern farms likely spread by several means, including on machinery and workers, by rodents and possibly even by the wind. That’s according to a U.S. Department of Agriculture report released Monday. The agency studied genetic properties of virus samples on more than 80 turkey and chicken farms.

USDA scientists say wild birds introduced the virus onto farms, but it appears the virus spread in other ways once there. The scientists found lapses in biosecurity on farms and environmental factors likely contributed to the spread of the disease.

More than 49 million birds died or were euthanized in 15 states this spring as the virus spread from the Pacific Northwest into Midwest farms. It’s the nation’s worst outbreak of bird flu.