The Food and Drug Administration plans to restrict a family of antibiotics commonly used to treat livestock, citing concerns that overuse might promote the development of drug-resistant bacteria that can infect people.

On Wednesday, the FDA said it would limit the use of cephalosporin in cattle, swine, chicken and turkey. The antibiotics can no longer be used to prevent diseases in livestock starting April 5, though they can still be used to treat illnesses, the FDA said.

Consumer advocates support the FDA’s decision. But they say the move is long overdue and deals with only one small part of a much larger public health issue tied to the overuse of antibiotics in animals. They want the FDA to also curtail the use of antibiotics in animal feed, an issue the agency has been grappling with since 1977.

The problem, public health advocates say, is that some antibiotics given to treat illnesses in people are widely used on animals to promote disease prevention and weight gain, as well as compensate for crowded conditions on ranches and farms. The prevalence of those antibiotics in livestock has been linked in several studies to the creation of drug-resistant “superbugs” that can spread to humans who work with or eat the animals.

Resistance to cephalosporin can be especially devastating because the antibiotics are regularly prescribed to humans. The antibiotics, which include brands such as Keflex and the generic cephalexin, are used to treat many illnesses, including pneumonia and salmonella infections in children.

“It’s one of the big guns,” said David Wallinga, a physician with the Institute for Agriculture and Trade Policy who also works with the Keep Antibiotics Working coalition. “When someone comes into an emergency room and you don’t know what they’re sick with, you try to treat them with one of the big guns. When your big guns start being ineffective, . . . you’re really in trouble.”

In 2008, the FDA issued an outright ban of cephalosporin for livestock. But the agency withdrew the plan after strong opposition from industry groups that wanted to preserve some uses of the antibiotics.

On Wednesday, the FDA responded by carving out some exceptions. For example, veterinarians will still be able to use or prescribe cephalosporins for limited use in livestock, if they follow approved doses.

The industry has long said the FDA does not have the science to back the ban. But Wednesday, the National Pork Producers Council said the agency’s more narrowly focused ban makes better sense. “We don’t support the need for [a ban], but we do believe that our pigs will not suffer endlessly because of it” because disease-treatment will be permitted, said Liz Wagstrom, the group’s chief veterinarian.

Others who have been tracking the issue are less pleased with the FDA. Rep. Louise M. Slaughter (D-N.Y.) described the FDA’s actions as a “modest” step in the fight to avert a massive public health threat.

“We’re really just looking at the tip of the iceberg,” said Slaughter, who has written legislation since 2007 that would restrict the use of certain antibiotics.

She also criticized the FDA for not taking action sooner. In 2009, the Centers for Disease Control and Prevention found that nearly 3 percent of salmonella tested was cephalosporin-resistant. “How many Americans have needlessly been sickened in the meantime?” she said.

In 2010, the FDA issued voluntarily guidelines urging the judicious use of these drugs. But those have yet to be finalized. Last year, several environmental and public health groups sued the FDA to force it to stop the industry from adding certain antibiotics to the feed of healthy animals.

“We don’t believe the voluntary guidelines will make a dent in this problem,” said Avinash Kar, an attorney at the Natural Resources Defense Council, one of the plaintiffs. “For the last 30 years or so, we’ve been working on a voluntary system and that hasn’t done much.”