According to CIDRAP, the petitions were filed in 1999 and 2005 by the Center for Science in the Public Interest (CSPI) along with several environmental and medical organisations. They asked the FDA to cancel its approval of the 'herdwide and flockwide' uses of several classes of antibiotics for promoting growth and preventing disease (in the absence of existing disease) in chickens, swine and beef cattle, out of concern that such use will spur resistance and reduce the drugs' effectiveness in humans.

The petitioners have voiced disappointment with the FDA action, saying the agency's voluntary approach will not work.

In separate letters denying the two petitions, the FDA said the legal process necessary to withdraw approval of a drug is expensive and slow and that the voluntary strategy the agency unveiled in 2010 should lead to improved use of the antibiotics in food animals.

"FDA believes that the strategy set out in draft guidance #209 is a pathway to achieving the same goals as those advanced by your organization, i.e., judicious use of medically-important antimicrobials," wrote Leslie Kux, FDA acting assistant commissioner for policy, in the denial of the 2005 petition.

"Additionally, given the considerable amount of agency resources that are required to pursue withdrawal proceedings, we believe the current proposed approach will accomplish these goals in a more timely and resource-efficient manner than would otherwise be the case," she added.

To cancel approval of a drug, the FDA must follow a number of statutory requirements, including holding a formal evidentiary hearing, the letter says. It took seven years to withdraw diethylstilbestrol in the 1970s, and the recent withdrawal of enrofloxacin use in poultry took five years and cost the FDA $3.3 million, Ms Kux wrote.

The agency proposed draft guidance for achieving 'judicious use' in June 2010. The guidance essentially says that antibiotic use in food animals should be limited to what is necessary for ensuring animal health and should take place under veterinary oversight, the letter notes.

"Based on feedback this Agency has received following the issuance of draft GFI #209, FDA believes the animal pharmaceutical industry is generally responsive to the prospect of working cooperatively with the agency to implement the principles recommended in draft GFI #209," Ms Kux wrote. She said the FDA hopes that with this approach, it will manage to "transition medically important antimicrobials currently approved for over the counter use in food-producing animals to a marketing status that involves veterinary oversight".

The drugs and classes cited in the denial of the 2005 petition are penicillins, tetracyclines, aminoglycosides, streptogramins, macrolides, lincomycin and sulphonamides.

CSPI expressed disappointment in the FDA's action. Michael Jacobson, CSPI's executive director, said in a statement last week that the industry's use of antibiotics in livestock is irresponsible and increases the prevalence of antibiotic-resistant pathogens that can be difficult or impossible to treat in humans.

"The industry has long failed to cooperate voluntarily, and the FDA should take binding action," Mr Jacobson said. "Consumers cannot afford another decade of delay."

Other groups that had signed onto the petition included the Environmental Defense Fund, the American Academy of Pediatrics, the American Public Health Association, Food Animal Concerns Trust (FACT), and the Union of Concerned Scientists.

In a statement responding to the FDA's denial, FACT said the agency did not challenge the need to reduce antibiotic use but argued that withdrawing them would be too expensive. Steven Roach, public health programme director at FACT, said in the statement that the FDA is waiting for the drug companies to do voluntarily what the agency is legally mandated to do.

"There is absolutely no reason to believe that drug companies will voluntarily reduce sales of antibiotics and act against their own financial self-interest," he said. "Without reductions in antibiotics used, it is impossible for there to be any public health benefit."

FACT said it supports proposed federal legislation that would require the FDA to address non-therapeutic antibiotic use and make it easier for the FDA to withdraw animal antibiotics that are unsafe from an antibiotic resistance vantage point.

Rep. Louise Slaughter (D-N.Y.), a microbiologist who authored the Preservation of Antibiotics for Medical Treatment Act cited by FACT, said last week that new FDA numbers on the rate of antibiotic use in agriculture – up 6.7 per cent from 2009 to 2010 – show the problem is getting worse, reports CIDRAP.

"The FDA needs to take common sense steps to reduce the needless use of antibiotics in healthy animals, and protect human beings," she said in the statement.