A federal analysis of 30 antibiotics used in animal feed found that the majority of them were likely to be contributing to the growing problem of bacterial infections that are resistant to treatment in people, according to documents released Monday by a health advocacy group.
The analysis, conducted by the Food and Drug Administration and covering the years 2001 to 2010, was detailed in internal records that the nonprofit group, the Natural Resources Defense Council, obtained through a Freedom of Information Act request and subsequent litigation.
In the documents, some of which were reviewed by The New York Times, scientists from the F.D.A. studied 30 penicillin and tetracycline additives in animal feed. They found that 18 of them posed a high risk of exposing humans to antibiotic-resistant bacteria through food.
Resistant bacteria make it difficult and sometimes impossible to treat infections with ordinary antibiotics. The scientists did not have enough data to judge the other 12 drugs.
At least two million Americans fall sick every year and about 23,000 die from antibiotic-resistant infections, the Centers for Disease Control and Prevention estimates. Representatives of the food industry largely blame hospitals and treatments given to people for the rise of deadly superbugs. But many scientists believe that indiscriminate use of antibiotics in animal feed is a major contributor.
Farmers and ranchers feed small amounts of the drugs to animals over their lifetimes to keep them healthy in crowded conditions, causing bacteria to develop a resistance passed on to people through the environment and eating meat from the animals.
In a statement, the F.D.A. said the drugs under review had been “older, approved penicillin and tetracycline products,” and that the agency had issued letters to their producers asking for additional safety data. It said those efforts had been part of a broader assessment of antibiotics, also called antimicrobials, given to animals raised for food, and that it has since made major policy changes to address them.
The F.D.A. has tried repeatedly to rein in the use of the drugs in animals. It adopted regulations in 1973 that required companies to submit studies showing that a drug’s use in animal feed did not promote resistance in people.
In 1977, the agency proposed withdrawing approvals for animal feed additives containing penicillin and most tetracyclines, but it never followed through, said Avinash Kar, a lawyer for the Natural Resources Defense Council. The group sued the agency in 2012 to try to force it to carry out the 1977 proposal.
“This is an agency that has repeatedly found, since the 1970s, that these drugs pose a risk to human health, but it has not done anything meaningful with those conclusions,” Mr. Kar said.
But the agency has taken some steps in recent years. In 2012, it restricted the use of cephalosporins in animals. They are drugs that are also prescribed to treat pneumonia and strep throat in people.
Last year, the F.D.A. moved to phase out the indiscriminate use of antibiotics in cows, pigs and chickens, though it continued to allow them for the treatment of illness, a policy decision that consumer health advocates said weakened the new rule.
“In December 2013, the F.D.A. began formal implementation of a strategy to phase out the use of all medically important antimicrobials,” the agency said. “The F.D.A. is confident that its current strategy to protect the effectiveness of medically important antimicrobials, including penicillins and tetracyclines, is the most efficient and effective way to change the use of these products in animal agriculture.”
Mr. Kar said that only one of the 30 drugs studied ever met the government’s safety criteria. And it only satisfied the 1973 regulations that required companies to submit studies showing that a drug’s use in animal feed did not lead to drug resistance in bacteria that can affect human health.
In 2003, the agency issued guidelines that divided drugs into three categories according to how risky they were for human health. Under the guidelines, a high-risk drug should be used for no longer than 21 days at a time, and only for individual animals, not entire herds or flocks.
High-risk drugs are often allowed to be used much more broadly, said Carmen Cordova, a microbiologist for the Natural Resources Defense Council. The 2003 guidelines apply only to drugs that were approved after 2003, and the consumer group said none of the 30 drugs fell into that category. Mr. Kar said the F.D.A. analysis was still relevant.