Major US poultry firms are administering antibiotics to their flocks far more pervasively than regulators realise, posing a potential risk to human health, says a Reuters report.

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 US 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.

Reuters reviewed more than 320 documents generated by six major poultry companies during the past two years. Called "feed tickets," the documents are issued to chicken growers by the mills that make feed to poultry companies' specifications. They list the names and grams per ton of each "active drug ingredient" in a batch of feed. They disclose the FDA-approved purpose of each medication. And they specify which stage in a chicken's roughly six-week life the feed is meant for.

The feed tickets examined represent a fraction of the tens of thousands issued annually to poultry farms run by or for major producers. The confidential information they contain nonetheless extends well beyond what the US government knows. Veterinary use of antibiotics is legal and has been rising for decades. But US regulators don't monitor how the drugs are administered on the farm - in what doses, for what purposes, or for how long. Made public here for the first time, the feed documents thus provide unique insight into how some major players use antibiotics.

The tickets indicate that two of the poultry producers - George's and Koch Foods - have administered drugs belonging to the same classes of antibiotics used to treat infections in humans. The practice is legal. But many medical scientists deem it particularly dangerous, because it runs the risk of promoting superbugs that can defeat the life-saving human antibiotics.

In interviews, another major producer, Foster Poultry Farms, acknowledged that it too has used drugs that are in the same classes as antibiotics considered medically important to humans by the FDA.

About 10 per cent of the feed tickets reviewed by Reuters list antibiotics belonging to medically important drug classes. But in recent presentations, scientists with the US Centers for Disease Control and Prevention said the use of any type of antibiotic, not just medically important ones, contributes to resistance. They said that whenever an antibiotic is administered, it kills weaker bacteria and enables the strongest to survive and multiply.

Frequent, sub-therapeutic use of antibiotics in low doses intensifies that effect, scientists and public health experts say. The risk: Any resulting superbugs might also develop cross-resistance to medically important antibiotics.

According to the feed tickets reviewed, low doses of antibiotics were part of the standard diet for some flocks at five companies: Tyson, Pilgrim's, Perdue, George's and Koch.

"These are not targeted uses aimed at specific bugs for defined duration. They're multiple, repeat shotgun blasts that will certainly kill off weaker bugs and promote the stronger, more resistant ones," said Keeve Nachman, director of the food production and public health program at Johns Hopkins University's Bloomberg School of Public Health.

The poultry industry's lobby takes issue with the concerns of government and academic scientists, saying there is little evidence that bacteria which do become resistant also infect people.

"Several scientific, peer reviewed risk assessments demonstrate that resistance emerging in animals and transferring to humans does not happen in measurable amounts, if at all," said Tom Super, spokesman for the National Chicken Council. He said using antibiotics to prevent diseases in flocks "is good, prudent veterinary medicine. Prevention of the disease prevents unnecessary suffering and prevents the overuse of potentially medically important antibiotics in treatment of sick birds."

Poultry producers began using antibiotics in the 1940s, not long after scientists discovered that penicillin, streptomycin and chlortetracycline helped control outbreaks of disease in chickens. The drugs offered an added benefit: They kept the birds' digestive tracts healthy, and chickens were able to gain more weight without eating more food.

Today, 80 percent of all antibiotics used in America are given not to people, but to livestock.

David Acheson, a former senior medical officer for the USDA and the FDA, now serves on a food safety advisory board for Foster Farms. He said the board never examined Foster's use of antibiotics and whether its practices could have spawned superbugs.

"Does anyone know that it happened? No. Is it possible? Could it have happened? Yes," Acheson said. "We know that antibiotic use, irrelevant of what you are treating, whether it be human or animal, can increase the likelihood of resistance. It's biology at work." - Reuters