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Nine-day-old chicks congregate at a Foster Farms chicken ranch near Turlock, California, June 16, 2014. Credit: Reuters/Max Whittaker

Major U.S. poultry firms are administering antibiotics to their flocks far more pervasively than regulators realize, posing a potential risk to human health.

Internal records examined by Reuters reveal that some of the nation’s largest poultry producers routinely feed chickens an array of antibiotics – not just when sickness strikes, but as a standard practice over most of the birds’ lives.

In every instance of antibiotic use identified by Reuters, the doses were at the low levels that scientists say are especially conducive to the growth of so-called superbugs, bacteria that gain resistance to conventional medicines used to treat people. Some of the antibiotics belong to categories considered medically important to humans.

The internal documents contain details on how five major companies – Tyson Foods, Pilgrim’s Pride, Perdue Farms, George’s and Koch Foods – medicate some of their flocks.

The documented evidence of routine use of antibiotics for long durations was “astonishing,” said Donald Kennedy, a former U.S. Food and Drug Administration commissioner.

Kennedy, president emeritus of Stanford University, said such widespread use of the drugs for extended periods can create a “systematic source of antibiotic resistance” in bacteria, the risks of which are not fully understood. “This could be an even larger piece of the antibiotic-resistance problem than I had thought,” Kennedy said.