he-antibiotics9 (Bob Chamberlin / Los Angeles Times / February 2, 2010)

At a minimum, any effort to move forward at this point would have to consider other classes of antibiotics that have become popular since the 1970s and include a rationale for which ones should be targeted, according to the agency.

The FDA's move may have been prompted by a lawsuit filed by advocacy groups including the Natural Resources Defense Council and the Food Animal Concerns Trust. Convinced that feeding antibiotics to animals is a major public health problem, they were trying to force the FDA to move forward with its plans — not abandon them altogether.

"It's the FDA's overcautious attitude," said Steven Roach, public health program director for FACT. "If anything we have more evidence now than we did in 1977. And they had enough evidence to proceed back then."

The drugs that were affected by the FDA's decision are among the oldest around. Tetracycline tops the list of most popular antibiotics, with more than 4 million kilograms used in food animals per year, according to 2009 data collected by the FDA. Penicillin comes in fourth, with about 610,000 kilograms of the drug used each year. (Both figures include drugs used on healthy animals as well as to treat those that are sick.)

Both drugs still are widely used by pork producers but in very specific ways, Wagstrom says. For instance, penicillin (usually in combination with other antibiotics) is fed to weanling pigs because it stimulates growth at this early stage in life.

How antibiotics promote growth is not entirely understood. The drugs may affect gut bacteria in a way that permits food nutrients to be better absorbed, or they may suppress low-level disease, according to Dr. H. Morgan Scott, a professor at Kansas State University's College of Veterinary Medicine in Manhattan, Kan. "If they're growing faster, some people would argue they must be healthier."

The FDA has made other moves to restrict antibiotic use in animals. In 2003, the agency began requiring drug companies to do a risk assessment of drugs given to animals as part of the overall approval process. However, those rules don't apply to older drugs like penicillin, Roach said.

The FDA is also developing guidelines for the "judicious use" of antibiotics in livestock. These guidelines, first made public in July 2010, are still in draft form. The agency hasn't set a timeline to finalize them, although it's considered a priority, according to FDA spokeswoman Stephanie Yao.

But those guidelines will only be voluntary, which is why watchdog groups tried to force the FDA to move forward with the decades-old rules about penicillin and tetracycline, Roach said.

The lawsuit brought by the NRDC, FACT and others to force the FDA to take regulatory action against antibiotics in animal feed is ongoing.

Meanwhile, Rep. Louise Slaughter (D-N.Y.) has introduced legislation to restrict the use of medically important antibiotics in agriculture. The Preservation of Antibiotics for Medical Treatment Act would require the FDA to re-review approvals for drugs currently allowed in animal feed. Slaughter re-introduced the bill last March.