Suppliers of beef to McDonald’s, Taco Bell and Walmart are sourcing meat from U.S. farms that use antibiotics linked to the spread of dangerous superbugs, an investigation has found.

Unpublished U.S. government records obtained by the Bureau of Investigative Journalism and the Guardian show farms producing beef for meat packing firms Cargill, JBS, and Green Bay are risking public health by still using antibiotics classed as the “highest priority critically important,” to human health (HP-CIAs).

Such drugs are so essential to human medicine that their use in livestock farming should be stopped, the World Health Organization has warned.

The findings have prompted condemnation from public health experts and campaigners.

There is no ban on using antibiotics for treatment or to prevent disease, although farmers now need a veterinary prescription for many medically important antibiotics that were previously available over the counter and added to water and feed. There is a ban in the U.S. on using antibiotics for growth promotion, which has been in place since 2017.

But many U.S. cattle farmers still routinely use antibiotics often for months on end. Their use — and overuse — risks enabling bacteria to develop resistance, meaning the drugs stop working.

Antibiotic resistance is one of the gravest global public health threats. According to the Centers for Disease Control and Prevention, it is responsible for more than 35,000 deaths in the U.S. each year, and 1.3 million globally.