USDA report finds transmission likely happening when workers, cows, vehicles and equipment go to multiple farms

Bird flu cases among animals continue to rise as US officials warn about the role of humans in spreading the virus.

There are now 102 herds in the US with H5N1 cases, though given the prevalence of inactivated virus in commercially available pasteurized milk, experts believe the true number may be even higher.

Poultry operations have reported cases likely linked to dairy farms, indicating the virus may be spilling back from cows into birds. Nearly 97m domesticated birds in the US have been affected by the highly pathogenic influenza since 2022.

Although officials believe this outbreak was originally sparked by a single introduction from migratory birds into cows – for instance, via feed contaminated by bird feces – late last year, genomic analyses now suggest it is being spread between animals and farms by human activity, according a report released by the US Department of Agriculture (USDA) on Thursday.