Widespread use of antibiotics to stimulate growth of food animals is a major source of serious, sometimes fatal, disease in humans, according to researchers from the federal Centers for Disease Control and two state health departments.
Their new study has demonstrated conclusively for the first time that feeding antibiotics to beef and dairy cattle, hogs and poultry breeds a novel form of microbe that can later infect humans.
Such organisms create a new public-health problem because they are resistant to antibiotics crucial to treatment of many human diseases. Studies estimate that about half of the 35 million pounds of U.S.-produced antibiotics are given to animals and the other half prescribed for humans.
Scientists have long suspected that indiscriminate use of antibiotics in animals and humans could increase growth of drug-resistant bacteria, but the chain of events between the farm and physician has been difficult to prove.
In an unusual study combining medical detective work with the latest in computer- and genetic-engineering technology, CDC's Dr. Scott D. Holmberg and colleagues found that bacteria resistant to antibiotic drugs caused serious intestinal illnesses in persons who had eaten hamburger made from farm animals in South Dakota.
An editorial accompanying their report in this week's New England Journal of Medicine praised the study as a "missing link" in the longstanding debate. Calling the new evidence "compelling," Dr. Stuart Levy of Tufts University School of Medicine called for restrictions on use of antibiotics to promote animal growth.
"Surely the time has come to stop gambling with antibiotics," he said. "Although their use as feed additives had a major role in advancing livestock production in the past, the consequences of this practice are now too evident to overlook."
Levy noted that several European nations had restricted such use. The Food and Drug Administration, which sought in 1977 to ban or restrict use of such antibiotics as penicillin, was later overruled by Congress, which pressed for more data.
The debate in this country is as much economic as scientific. For more than 30 years, small amounts of antibiotics have been added to feed to make animals grow more efficiently and quickly.
"These drugs reduce the cost ultimately of the meat that the consumer buys . . . because the cost of feeding these animals has been reduced," said Fred Holt of the Animal Health Institute, which represents pharmaceutical companies that produce animal drugs.
Dr. Gerald Guest of the FDA's Center for Veterinary Medicine said yesterday that before reporting to Congress early next year, the agency would review the new study and a separate one it commissioned.
The CDC study involved a painstaking effort to track an outbreak of serious intestinal illness caused by salmonella bacteria, a common cause of food poisoning thought to be responsible for as many as 4 million U.S. cases of human infection, the CDC's Holmberg said.
The year-old investigation, begun in Minnesota, involved 18 persons infected in four midwestern states early last year with a particular form of salmonella resistant to three antibiotic drugs. Eleven were hospitalized for an average of eight days, and another died after being infected in the hospital.
Minnesota health officials noticed an unusual increase in such infections and requested CDC aid. Many of the patients became ill after taking antibiotics to treat minor upper-respiratory infections, so researchers initially ruled out contamination of the drugs, then focused on the bacterial source, Holmberg said.
The patients' only common characteristic was that they had eaten hamburger meat in the week before becoming ill. A survey of state health officials also disclosed four unusual cases of salmonella illness in South Dakota.
Investigators found that three of the four South Dakota victims were related by marriage but had not seen each other recently. A relative, however, had given all four beef made from animals fed low levels of antibiotics, Holmberg said.
Genetic examination of salmonella found in both states' cases revealed that their "molecular fingerprints matched," he said.
Using newly available computer records following cattle from slaughter to market, "we were able to trace the beef from South Dakota to the very supermarkets used by the Minnesota patients," he said.
While the cattle appeared to be the ultimate source of the dangerous organisms, in some cases the organisms were spread between humans in a household or, in the case of the dead victim, from a medical instrument contaminated after being used with another salmonella-stricken patient, the study found.
Holmberg emphasized that the problem is exacerbated because humans use antibiotics for relatively minor medical problems.
In his study, he said, some patients were infected with antibiotic-resistant organisms after eating contaminated hamburger but did not show symptoms of disease until after taking antibiotics for other purposes.
While these drugs killed some germs, they fostered growth of others resistant to antibiotics and created a far more serious illness, he said.
"As more and more bacteria become resistant to more and more antibiotics, ultimately this limits our use of one of our most important group of drugs," he said.